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If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: The Story of a Boulder; or, Gleanings from the Note-book of a - Field Geologist - -Author: Sir Archibald Geikie - -Release Date: November 10, 2021 [eBook #66703] - -Language: English - -Character set encoding: UTF-8 - -Produced by: Tom Cosmas from materials made available on The Internet - Archive and placed in the Public Domain. - -*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF A BOULDER; OR, -GLEANINGS FROM THE NOTE-BOOK OF A FIELD GEOLOGIST *** - - - - - THE STORY OF A BOULDER. - - - - - THE STORY OF A BOULDER - - OR - - GLEANINGS FROM THE NOTE-BOOK OF A FIELD GEOLOGIST - - - - BY - - ARCHIBALD GEIKIE - - OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. - - - Illustrated with Woodcuts. - - - EDINBURGH: THOMAS CONSTABLE AND CO. HAMILTON, ADAMS, AND CO., LONDON. - - MDCCCLVIII. - - - EDINBURGH: T. CONSTABLE, PRINTER TO HER MAJESTY. - - - TO - - GEORGE WILSON, M.D., F.R.S.E. - - REGIUS PROFESSOR OF TECHNOLOGY IN THE UNIVERSITY OF EDINBURGH, - - THESE PAGES - - ARE - - AFFECTIONATELY INSCRIBED. - - - - -PREFACE - - -The present Volume has been written among the rocks which it seeks -to describe, during the intervals of leisure of a field-geologist. -Its composition has been carried on by snatches, often short and far -apart, some of the descriptions having been jotted down on the spot -by streamlet and hill-side, or in the quiet of old quarries; others, -again, in railway-carriage or stage-coach. By much the larger portion, -however, has been written by the village fireside, after the field-work -of the day was over--a season not the most favourable to any mental -exercise, for weariness of body is apt to beget lassitude of mind. In -short, were I to say that these Chapters have been as often thrown -aside and resumed again as they contain paragraphs, the statement would -probably not exceed the truth. But the erratic life of an itinerant -student of science is attended with yet greater disadvantages. It -entails an absence from all libraries, more especially scientific -ones, and the number of works of reference admissible into his _parva -supellex_ must ever be few indeed. With these hindrances, can the -writer venture to hope that what has thus been so disjointed and -unconnected to him, will not seem equally so to his readers? Yet if his -descriptions, written, as it were, face to face with Nature, are found -to have caught some tinge of Nature's freshness, and please the reader -well enough to set him in the way of becoming a geologist, he shall -have accomplished all his design. - -It cannot be too widely known, or too often pressed on the attention, -especially of the young, that a true acquaintance with science, so -delightful to its possessors, is not to be acquired at second-hand. -Text-books and manuals are valuable only so far as they supplement -and direct our own observations. A man whose knowledge of Nature is -derived solely from these sources, differs about as much from one who -betakes himself to Nature herself, as a dusty, desiccated mummy does -from a living man. You have the same bones and sinews in both; but -in the one they are hard and dry, wholly incapable of action; in the -other they are instinct with freshness and life. He who would know what -physical science really is, must go out into the fields and learn it -for himself: and whatever branch he may choose, he will not be long in -discovering that a forenoon intelligently spent there must be deemed -of far more worth than days and weeks passed among books. He sees the -objects of his study with his own eyes, and not through "the spectacles -of books;" facts come home to him with a vividness and reality they -never can possess in the closet; the free buoyant air brightens his -spirits and invigorates his mind, and he returns again to his desk or -his workshop with a store of new health and pleasure and knowledge. -Geology is peculiarly rich in these advantages, and lies in a manner -open to all. No matter what may be the season of the year, it offers -always some material for observation. In the depth of winter we have -the effects of ice and frost to fall back upon, though the country -should lie buried in snow; and then when the longer and brighter -days of spring and summer come round, how easily may the hammer be -buckled round the waist, and the student emerge from the dust of town -into the joyous air of the country, for a few delightful hours among -the rocks; or when autumn returns with its long anticipated holidays, -and preparations are made for a scamper in some distant locality, -hammer and note-book will not occupy much room in the portmanteau, -and will certainly be found most entertaining company. The following -pages--forming a digest of the Carboniferous rocks--may, perhaps, in -some measure, guide the explorations of the observer, by indicating -to him the scope of geological research, the principles on which the -science rests, and the mode in which it is pursued. But I repeat, no -book, no lecture-room, no museum, will make a geologist of him. He must -away to the fields and study for himself, and the more he can learn -there he will become the better geologist. - -He need not burden himself with accoutrements. A hammer, pretty stout -in its dimensions, with a round blunt face and a flat sharp tail; a -note-book and a good pocket-lens, are all he needs to begin with. -Having these, let him seek to learn the general characters' of the -more common rocks, aiding himself, where he can, by a comparison with -the specimens of a museum, or, failing that, with the descriptions of -a text-book. Let him then endeavour to become acquainted with some of -the more characteristic fossils of the district in which he resides, -so as to be able to recognise them wherever they occur. Private -collections and local museums are now becoming comparatively common, -and these, where accessible, will aid him vastly in his studies. Having -at length mastered the more abundant rocks and organic remains of his -neighbourhood, let him try to trace out the connexion of the different -strata across the country, so as to understand its structure. For -this purpose it will be necessary to examine every ravine and natural -exposure of the rocks, along with quarries, ditches, railway-cuttings, -and, in short, the whole surface of the district. A general notion -of the geology of the place will, not perhaps be of very difficult -attainment; and this done, the observer should attempt to put down -the connexion of the rocks on paper, for till this is accomplished he -will have at the best but an imperfect, and perhaps incorrect notion -of the subject. The best map of the district should be obtained, also -a clinometer, or instrument for ascertaining the angle at which rocks -_dip_ with the horizon, and a pocket-compass with which to mark the -direction of the _dip_ and _strike_ of strata, that is, the _outcrop_, -or line which they form when they come to the surface. Thus armed, he -may commence a geological survey of his neighbourhood. Wherever he -sees a bed of rock exposed, it should be marked down on his map with -an arrow pointing to the direction in which the stratum is dipping, -the angle of dip, ascertained by the clinometer, being put alongside. -The nature of the rock, whether sandstone, shale, limestone, or -greenstone, must be set down at the same place, and, to save room, -a system of marks for the different rocks may be conveniently used. -When a sufficient area of ground has been thus traversed, the student -may find, say a row of arrows on his map all pointing due west, and -indicating a set of quarries about a quarter of a mile distant from one -another, the rock in each of them dipping to the west. If there be at -the one end a limestone containing certain fossils, and at the other -end a stratum exactly similar, containing the same fossils, while the -quarries between display the same rock, he will infer, of course, that -the whole is one limestone, and will accordingly draw a line from the -last quarry on the north to the last on the south, connecting them all -together. If the bed dips steeply down, the line will be narrower,--if -but slightly inclined, it will be broader; the breadth of such a line -(which may be coloured to taste) always varying with the thickness -of the stratum and the angle which it makes with the horizon. In a -district where faults and curvatures along with trap-rocks abound, the -mapping becomes more complex, but the principle remains the same--a -curved stratum on the ground making a similarly curved band on the map, -and a fault or dislocation of a set of beds producing, in the same way, -a corresponding break in the lines traced. In short, a geological map -should be as far as possible a transcript of the surface rocks of a -country. The beginner should avoid, however, attempting too much; it -will be enough for him at first to have mastered the leading features -of the geology of his district; the details cannot be shown save on -a map of a large scale, and are better transferred to his note-book. -The use of such mapping is to enable us to gain a correct knowledge of -the geological structure of a country, and of the relation of rocks to -each other as regards age, origin, &c. Bacon tells us that "writing -makes an exact man;" we may say with equal truth that mapping makes -an exact geologist. It is sometimes easy enough to obtain a notion of -the general character of a district by taking a few rambles across it; -but we can never know it thoroughly until we have mapped it. And this -is done not as mere dry routine, or by a series of hard uninteresting -rules. In reading off the geological structure of a country, we -ascertain its history during many thousand ages long prior to that -of man. We become, as it were, interpreters of hieroglyphics, and -historians of long-perished dynasties. - -Those who have had experience of field-geology, know how vain it is -to attempt to compress into a page or two the results of years, and -that a few vague general directions are about the utmost that can be -attempted. The practice of the science cannot be taught in books, far -less in prefaces, neither can it be learned from them. And so I once -more repeat the advice: Get away to the fields. Seek to decipher the -geological records for yourself, and look with your own eyes into the -long series of ages whose annals lie inscribed among the rocks. If you -can secure the co-operation of a few companions, so much the better. -Half-a-dozen hammers zealously at work in a richly fossiliferous -stratum will soon pile up a tolerable collection of its treasures. But -whether singly or in company, use your eyes and your hammer, and even -though in the end you should never become a geologist, you will in the -meantime gain health and vigour, and a clearness of observation, that -will stand you in good stead through life. - - - - -CONTENTS. - - -CHAPTER I. - -PAGE - - Scene near Colinton in midsummer--A grey travelled Boulder--Its - aspect and contents--Its story of the past, - - 1 - - -CHAPTER II. - - Exterior of the boulder--Travelled stones a difficult problem--Once - referred to the Deluge--Other theories--Novelty of the true - solution--Icebergs formed in three ways--Progress and scenery - of an iceberg--Its effects--Size of icebergs--Boulder clay - had a glacial origin--This explanation confirmed by fossil - shells--Laws of the distribution of life--Deductions, - - 6 - - -CHAPTER III. - - How the boulder came to be one--"Crag and tail"--Scenery of - central Scotland: Edinburgh--"Crag and tail" formerly - associated in its origin with the boulder-clay--This - explanation erroneous--Denudation an old process--Its - results--Illustration from the Mid-Lothian coal-field--The - three Ross-shire hills--The Hebrides relics of an - ancient land--Scenery of the western coast--Effects of - the breakers--Denudation of the Secondary strata of the - Hebrides--Preservative influence of trap-rocks--Lost - species of the Hebrides--Illustration--Origin of the - general denudation of the country--Illustrative action - of streams--Denudation a very slow process--Many old - land-surfaces may have been effaced--Varied aspect of the - British Islands during a period of submergence--Illustration, - - 18 - - -CHAPTER IV. - - Interior of the boulder--Wide intervals of - Geology--Illustration--Long interval between the - formation of the boulder as part of a sand-bed, - and its striation by glacial action--Sketch of the - intervening ages--The boulder a Lower Carboniferous - rock--Cycles of the astronomer and the geologist - contrasted--Illustration--Plants shown by the boulder once - grew green on land--Traces of that ancient land Its seas, - shores, forests, and lakes, all productive of material - aids to our comfort and power--Plants of the Carboniferous - era--Ferns--Tree-ferns--Calamites--Asterophyllites-- - Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined - palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest - known flower--Grade of the Carboniferous flora--Its resemblance - to that of New Zealand, - - 30 - - -CHAPTER V. - - Scenery of the carboniferous forests--Contrast in the - appearance of coal districts at the present day--Abundance - of animal life in the Carboniferous era--Advantages - of palæontology over fossil-botany--Carboniferous - fauna--Actiniæ--Cup-corals--Architecture of the present - day might be improved by study of the architecture of the - Carboniferous period--Mode of propagation of corals--A - forenoon on the beach--Various stages in the decomposition of - shells--Sea-mat--Bryozoa--Fenestella--Retepora--Stone-lilies-- - Popular superstitions--Structure of the stone-lilies--Aspect - of the sea-bottom on which the stone-lilies - flourished--Sea-urchins--Crustacea, their high - antiquity--Cyprides--Architecture of the Crustacea and - mollusca contrasted--King-crabs, - - 59 - - -CHAPTER VI. - - Carboniferous fauna continued--George Herbert's ode on - "Man"--His idea of creation--What nature teaches on - this subject--Molluscous animals--Range of species in - time proportionate to their distribution in space--Two - principles of renovation and decay exhibited alike in the - physical world and the world of life--Their effects--The - mollusca--Abundantly represented in the carboniferous - rocks--Pteropods--Brachiopods--Productus--Its alliance with - Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods-- - Land-snail of Nova Scotia--Cephalopods--Structure of - orthoceras--Habits of living nautilus, - - 86 - - -CHAPTER VII. - - Classification of the naturalist not always correspondent with - the order of nature--Incongruous grouping of animals - in the invertebrate division--Rudimentary skeleton - of the cephalopods--Introduction of the vertebrate - type into creation--Ichthyolites of the carboniferous - rocks--Their state of keeping--Classification of fossil - fishes--Placoids--Ichthyodorulites--Ganoids--Their structure - exemplified in the megalichthys and holoptychius--Cranium of - megalichthys--Its armature of scales--Microscopic structure of - a scale--Skeleton of megalichthys--History of the discovery - of the holoptychius--Confounded with megalichthys--External - ornament of holoptychius--Its jaws and teeth--Microscopic - structure of the teeth--Paucity of terrestrial fauna in coal - measures--Insect remains--Relics of reptiles--Concluding - summary of the characters of the Carboniferous fauna--Results, - - 110 - - -CHAPTER VIII. - - Sand and gravel of the boulder--What they suggested--Their - consideration leads us among the more mechanical operations - of Nature--An endless succession of mutations in the - economy of the universe--Exhibited in plants In animals--In - the action of winds and oceanic currents--Beautifully - shown by the ceaseless passage of water from land to - sea, and sea to land--This interchange not an isolated - phenomenon--How aided in its effects by a universal process - of decay going on wherever a land surface is exposed to - the air--Complex mode of Nature's operations--Interlacing - of different causes in the production of an apparently - single and simple effect--Decay of rocks--Chemical - changes--Underground and surface decomposition--Carbonated - springs--The Spar Cave--Action of rain-water--Decay of - granite--Scene in Skye--Trap-dykes--Weathered cliffs of - sandstone--Of conglomerate--Of shale--Of limestone--Caverns - of Raasay--Incident--Causes of this waste of calcareous - rocks--Tombstones, - - 138 - - -CHAPTER IX. - - Mechanical forces at work in the disintegration of - rocks--Rains--Landslips--Effects of frosts--Glaciers and - icebergs--Abrading power of rivers--Suggested volume on the - geology of rivers--Some of its probable contents--Scene - in a woody ravine--First idea of the origin of the ravine - one of primeval cataclysms--Proved to be incorrect--Love - of the marvellous long the bane of geology--More careful - examination shows the operations of Nature to be singularly - uniform and gradual--The doctrine of slow and gradual change - not less poetic than that of sudden paroxysms--The origin - of the ravine may be sought among some of the quieter - processes of Nature--Features of the ravine Lessons of the - waterfall--Course of the stream through level ground--True - history of the ravine--Waves and currents--What becomes of the - waste of the land--The Rhone and the Leman Lake--Deltas on the - sea-margin--Reproductive effects of currents and waves--Usual - belief in the stability of the land and the mutability of the - ocean--The reverse true--Continual interchange of land and - sea part of the economy of Nature--The continuance of such a - condition of things in future ages rendered probable by its - continuance during the past, - - 157 - - -CHAPTER X. - - The structure of the stratified part of the earth's - crust conveniently studied by the examination of a - single formation--A coal-field selected for this - purpose--Illustration of the principles necessary to such - an investigation--The antiquities of a country of value in - compiling its pre-historic annals--Geological antiquities - equally valuable and more satisfactorily arranged--Order - of superposition of stratified formations--Each formation - contains its own suite of organic remains--The age of - the boulder defined by this test from fossils--Each - formation as a rule shades into the adjacent ones--Mineral - substances chiefly composing the stratified rocks few - in number--Not of much value in themselves as a test of - age--The Mid-Lothian coal-basin--Its subdivisions--The - limestone of Burdiehouse--Its fossil remains--Its probable - origin--Carboniferous limestone series of Mid-Lothian--Its - relation to that of England--Its organic remains totally - different from those of Burdiehouse--Structure and - scenery of Roman Camp Hill--Its quarries of the mountain - limestone--Fossils of these quarries indicative of an ancient - ocean-bed--Origin of the limestones--Similar formations still - in progress--Coral-reefs and their calcareous silt--Sunset - among the old quarries of Roman Camp Hill, - - 178 - - -CHAPTER XI. - - Intercalation of coal seams among mountain limestone beds of - Mid-Lothian--North Greens seam--Most of our coal seams - indicate former land-surfaces--Origin of coal a debated - question--Erect fossil trees in coal-measures--Deductions to - be drawn therefrom--Difference between the mountain limestone - of Scotland and that of England--Coal-bearing character of the - northern series--Divisions of the Mid-Lothian coal-field--The - Edge coals--Their origin illustrated by the growth of modern - deltas--Delta of the Nile--Of the Mississippi--Of the - Ganges--Progress of formation of the Edge coals--Scenery - of the period like that of modern deltas--Calculations of - the time required for the growth of a coal-field--Why of - doubtful value--Roslyn Sandstone group--Affords proofs of - a general and more rapid subsidence beneath the sea--Its - great continuity--Probable origin--Flat coals--Similar - in origin to the Edge coals below--Their series not now - complete--Recapitulation of the general changes indicated by - the Mid-Lothian coal-field, - - 204 - - -CHAPTER XII. - - Trap-pebbles of the boulder--Thickness of the earth's crust - unknown--Not of much consequence to the practical - geologist--Interior of the earth in a highly heated - condition--Proofs of this--Granite and hypogene - rocks--Trap-rocks: their identity with lavas and - ashes--Scenery of a trappean country--Subdivisions of - the trap-rocks--Intrusive traps--Trap-dykes--Intrusive - sheets--Salisbury Crags--Traps of the neighbourhood of - Edinburgh--Amorphous masses--Contemporaneous trap-rocks of - two kinds--Contemporaneous melted rocks--Tests for their age - and origin--Examples from neighbourhood of Edinburgh--Tufas - or volcanic ashes--Their structure and origin--Example - of contemporaneous trap-rocks--Mode of interpreting - them--Volcanoes of Carboniferous times--Conclusion, - - 235 - - - - -THE STORY OF A BOULDER. - - - - -CHAPTER I. - - - Scene near Colinton in midsummer--A grey travelled Boulder--Its - aspect and contents--Its story of the past. - -Three miles to the south-west of Edinburgh, and not many hundred yards -from the sequestered village of Colinton, there is a ravine, overshaded -by a thick growth of beech and elm, and traversed beneath by a stream, -which, rising far away among the southern hills, winds through the -rich champaign country of Mid-Lothian. It is, at all seasons of the -year, one of the most picturesque nooks in the county. I have seen it -in the depth of winter--the leafless boughs doddered and dripping, the -rocks dank and bare save where half-hidden by the rotting herbage, and -the stream, red and swollen, roaring angrily down the glen, while the -families, located along its banks, fleeing in terror to the higher -grounds, had left their cottages to the mercy of the torrent. The last -time I visited the place was in the heart of June, and surely never did -woodland scene appear more exquisitely beautiful. The beech trees were -in full leaf, and shot their silvery boughs in slender arches athwart -the dell, intertwining with the broader foliage and deeper green of -the elm, and the still darker spray of the stately fir. The rocks on -either side were tapestried with verdure; festoons of ivy, with here -and there a thread of honey-suckle interwoven, hung gracefully from the -cliffs overhead; each projecting ledge had its tuft of harebells, or -speedwell, or dog-violets, with their blue flowers peeping out of the -moss and lichens; the herb-robert trailed its red blossoms over crag -and stone; the wood-sorrel nestled its bright leaves and pale flowerets -among the gnarled roots of beech and elm; while high over all, alike on -the rocks above and among the ferns below, towered the gently drooping -stalks of the fox-glove. The stream, almost gone, scarcely broke the -stillness with a low drowsy murmur, as it sauntered on among the -_lapides adesos_ of its pebbly channel. Horace's beautiful lines found -again their realization:-- - - "Qua pinus ingens, albaque populus - Umbram hospitalem consociare amant - Ramis, et obliquo laborat - Lympha fugax trepidare rivo." [1] - -[Footnote 1: - - Where the tall pine and poplar pale - Delight to cast athwart the vale - A pleasing shade. - While the clear stream low murmuring bells. - And o'er its winding channel toils - Adown the glade.--A. G. -] - -It was noon, and the sun shone more brightly and with greater heat -than had been felt for years. The air, heavy and warm, induced a -feeling of listlessness and languor, and the day seemed one for which -the only appropriate employment would have been to read once again -the "Castle of Indolence." But failing that, I found it pleasant to -watch the flickering light shot in fitful gleams through the thick -canopy of leaves, and thus, in the coolness of the shade, to mark -these rays--sole messengers from the sweltering world around--as they -danced from rock to stream, now lighting up the ripples that curled -dreamily on, now chequering some huge boulder that lay smooth and -polished in mid-channel, anon glancing playfully among the thickets -of briar or honeysuckle and vanishing in the shade. Sometimes a -wagtail would alight at hand, or a bee drone lazily past, while even -an occasional butterfly would venture down into this shady covert. -But, with these exceptions, the animal creation seemed to have gone to -sleep, an example which it was somewhat difficult to avoid following. -While thus idly engaged, my eye rested on a large boulder on the -opposite side. It lay partly imbedded in a stiff clay, and partly -protruding from the surface of the bank some way above the stream. -A thick arbour of leafage overhung it, through which not even the -faintest ray of sunshine could force its way. The spot seemed cooler -and more picturesque than that which I occupied, and so, crossing the -well-nigh empty channel, I climbed the bank and was soon seated on the -boulder. A stout hammer is a constant companion in my rambles, and was -soon employed on this occasion in chipping almost unconsciously the -newly-acquired seat. The action was, perhaps, deserving of the satire -of Wordsworth's Solitary:-- - - "You may trace him oft - By scars, which his activity has left - Beside our roads and pathways, though, thank Heaven! - This covert nook reports not of his hand. - He, who with pocket-hammer smites the edge - Of luckless rock or prominent stone, disguised - In weather-stains, or crusted o'er by Nature - With her first growths, detaching by the stroke - A chip or splinter to resolve his doubts; - And, with that ready answer satisfied, - The substance classes by some barbarous name, - And hurries on; or from the fragments picks - His specimen; if but imply interveined - With sparkling mineral, or should crystal cube - Lurk in its cells and thinks himself enriched. - Wealthier, and doubtless wiser, than before!" - -There was nothing in the distant aspect of the boulder to attract -attention. It was just such a mass as dozens of others all round. Nor, -on closer inspection, might anything peculiar have been observed. It -had an irregularly oblong form, about two or three feet long, and half -as high. Ferns and herbage were grouped around it, the wood-sorrel -clustered up its sides, and little patches of moss and lichen nestled -in its crevices. And yet, withal, there was something about it that, -ere long, riveted my attention. I examined it minutely from one end to -the other, and from top to bottom. The more I looked the more did I see -to interest me; and when, after a little labour, some portions of its -upper surface were detached, my curiosity was abundantly gratified. -That grey lichened stone, half hid among foliage, and unheeded by any -human being, afforded me material for a pleasant forenoon's thought. -Will my reader accept an expanded narrative of my reverie? - -I can almost anticipate a smile. "What can there be remarkable in such -a grey stone, hidden in a wood, and of which nobody knows anything? -It never formed part of any ancient building; it marks the site of -no event in the olden time; it is linked with nothing in the history -of our country. What of interest, then, can it have for us?" Nay, I -reply, you are therein mistaken. It is, assuredly, linked with the -history of our country--it does mark the passing of many a historical -event long ere human history began; and, though no tool ever came upon -it, it did once form part of a building that rose under the finger of -the Almighty during the long ages of a bygone eternity. To change the -figure, this boulder seemed like a curious volume, regularly paged, -with a few extracts from older works. Bacon tells us that "some books -are to be tasted, others to be swallowed, and some few to be chewed and -digested." Of the last honour I think the boulder fully worthy, and if -the reader will accompany me, I shall endeavour to show him how the -process was attempted by me. - -The rock consisted of a hard grey sandstone finely laminated above, -and getting pebbly and conglomeritic below. The included pebbles were -well worn, and belonged to various kinds of rock. The upper part of -the block was all rounded, smoothed, and deeply grooved, and, when -split open, displayed numerous stems and leaflets of plants converted -into a black coaly substance. These plants were easily recognisable -as well-known organisms of the carboniferous strata, and it became -accordingly evident that the boulder was a block of carboniferous -sandstone. The pebbles below, however, must have been derived from -more ancient rocks, and they were thus seen to represent some older -geological formation. In this grey rock, therefore, there could at -once be detected well-marked traces of at least two widely-separated -ages. The evidence for each was indubitable, and the chronology of -the whole mass could not be mistaken. The surface striation bore -undoubted evidence of the glacial period, the embedded plants as -plainly indicated the far more ancient era of the coal-measures, while -the pebbles of the base pointed, though dimly, to some still more -primeval age. I had here, as it were, a quaint, old, black-letter -volume of the middle ages, giving an account of events that were -taking place at the time it was written, and containing on its earlier -pages numerous quotations from authors of antiquity. The scratched -surface, to complete the simile, may be compared to this old work -done up in a modern binding. Let us, then, first of all, look for a -little at the exterior of the volume, and inquire into the origin of -that strangely-striated surface, and of the clay in which the boulder -rested. - - - - -CHAPTER II. - - - Exterior of the boulder--Travelled stones a difficult problem--Once - referred to the Deluge--Other theories--Novelty of the true - solution--Icebergs formed in three ways--Progress and scenery - of an iceberg--Its effects--Size of icebergs--Boulder-clay - had a glacial origin--This explanation confirmed by fossil - shells--Laws of the distribution of life--Deductions. - -Has the reader, when wandering up the course of a stream, rod in hand -perhaps, ever paused at some huge rounded block of gneiss or granite -damming up the channel, and puzzled himself for a moment to conjecture -how it could get there? Or when rolling along in a railway carriage, -through some deep cutting of sand, clay, and gravel, did the question -ever obtrude itself how such masses of water-worn material came into -existence? Did he ever wonder at the odd position of some huge grey -boulder, far away among the hills, arrested as it were on the steep -slope of a deep glen, or perched on the edge of a precipitous cliff, as -though a push with the hand would hurl it down into the ravine below? -Or did he ever watch the operations of the quarryman, and mark, as each -spadeful of soil was removed, how the surface of the rock below was all -smoothed, and striated, and grooved? - -These questions, seemingly simple enough, involve what was wont to -be one of the greatest problems of geology, and not many years have -elapsed since it was solved. The whole surface of the country was -observed to be thickly covered with a series of clays, gravels, and -sands, often abounding in rounded masses of rock of all sizes up to -several yards in diameter. These deposits were seen to cover all -the harder rocks, and to occur in a very irregular manner, sometimes -heaped up into great mounds, and sometimes entirely wanting. They were -evidently the results of no agency visible now, either on the land -or around our coasts. They had an appearance rather of tumultuous -and violent action, and so it was wisely concluded that they must be -traces of the great deluge. The decision had at least this much in its -favour, it was thoroughly orthodox, and accordingly received marked -approbation, more especially from those who wished well to the young -science of geology, but were not altogether sure of its tendencies. -But, alas! this promising symptom very soon vanished. As observers -multiplied, and investigations were carried on in different countries, -the truth came out that these clays and gravels were peculiarly a -northern formation; that they did not appear to exist in the south of -France, Italy, Asia Minor, Syria, and the contiguous countries. If, -then, they originated from the rushing of the diluvian waters, these -southern lands must have escaped the catastrophe, and the site of the -plains of Eden would have to be sought somewhere between the Alps -and the North Pole. This, of course, shocked all previous ideas of -topography; it was accordingly agreed, at least among more thoughtful -men, that with these clays and sands the deluge could have had nothing -to do. - -Other theories speedily sprang up, endeavouring to account for the -phenomena by supposing great bodies of water rushing with terrific -force across whole continents, sweeping away the tops of hills, -tearing up and dispersing entire geological formations, and strewing -the ocean-bottom with scattered debris. But this explanation had the -disadvantage of being woefully unphilosophical and not very clearly -orthodox. Such debacles did not appear to have ever taken place in -any previous geologic era, and experience was against them. Besides, -they did not account for some of the most evident characteristics of -the phenomena, such as the northern character of the formation, the -long parallel striations of the rock surfaces, and the perching of -huge boulders on lofty hills, often hundreds of miles distant from the -parent rock. Geologists were completely at fault, and the boulder-clay -remained a mystery for years. - -When we consider the physical aspects of the countries where the -question was studied, we cannot much wonder that the truth was so hard -to find. In the midst of corn-fields and meadows, one cannot readily -realize the fact that the spot where they stand has been the site of -a wide-spread sea; and that where now villages and green lanes meet -the eye, there once swam the porpoise and the whale, or monsters of -a still earlier creation, unwieldy in bulk and uncouth in form. Such -changes, however, must have been, for their traces meet us on every -hand. We have the sea dashing against our shores, and there seems -nothing at all improbable in the assertion that once it dashed against -our hill-tops. No one, therefore, has any difficulty in giving such -statements his implicit belief. But who could have dreamed that these -fields, so warm and sunny, were once sealed in ice, and sunk beneath -a sea that was cumbered with many a wandering iceberg? Who could have -imagined, that down these glens, now carpeted with heath and harebell, -the glacier worked its slow way amid the stillness of perpetual snow? -And yet strange as it may seem, such is the true solution of the -problem. The boulder-clay was formed during the slow submergence of our -country beneath an icy sea, and the rock-surfaces owe their polished -and striated appearance to the grating across them of sand and stones -frozen into the bottom of vast icebergs, that drifted drearily from the -north. That we may the better see how these results have been effected, -let us glance for a little at the phenomena observable in northern -latitudes at the present day. - -Icebergs are formed in three principal ways:--1st, By glaciers -descending to the shore, and being borne seawards by land-winds; 2d, -By river-ice packed during spring, when the upper reaches of the rivers -begin to thaw; 3d, By coast-ice. - -I. There is an upper stratum of the atmosphere characterized by intense -cold, and called the region of perpetual snow. It covers the earth -like a great arch, the two ends resting, one on the arctic, the other -on the antarctic zone, while the centre, being about 16,000 feet above -the sea,[2] rises directly over the tropics. Wherever a mountain is -sufficiently lofty to pierce this upper stratum, its summit is covered -with snow, and, as the snow never melts, it is plain that, from the -accumulations of fresh snow-drifts, the mountain-tops, by gradually -increasing in height and width, would become the supporting columns -of vast hills of ice, which, breaking up at last from their weight -and width, would roll down the mountain-sides and cover vast areas -of country with a ruin and desolation more terrible than that of any -avalanche. Olympus would really be superposed upon Ossa. By a beautiful -arrangement this undue growth is prevented, so that the hill-tops -never vary much in height above the sea. The cone of ice and snow -which covers the higher part of the mountain, sends down into each of -the diverging valleys a long sluggish stream of ice, with a motion so -slow as to be almost imperceptible. These streams are called glaciers. -As they creep down the ravines and gorges, blocks of rock detached -by the frosts from the cliffs above, fall on the surface of the ice, -and are slowly carried along with it. The bottom also of the glaciers -is charged with sand, gravel, and mud, produced by the slow-crushing -movement; large rocky masses become eventually worn down into -fragments, and the whole surface of the hard rock below is traversed -by long parallel grooves and striæ in the direction of the glacier's -course. Among the Alps, the lowest point to which the glacier descends -is about 8500 feet. There the temperature gets too high to allow of its -further progress, and so it slowly melts away, choking up the valleys -with piles of rocky fragments called moraines, and 'giving rise to -numerous muddy streams that traverse the valleys, uniting at length -into great rivers such as the Rhone, which enters the Lake of Geneva -turbid and discoloured with glacial mud. - -[Footnote 2: The average height of the snow-line within the tropics -is 15,207 feet, but it varies according to the amount of land and sea -adjacent, and other causes. Thus, among the Bolivian Andes, owing to -the extensive radiation, and the ascending currents of air from the -neighbouring plains and valleys, the line stands at a level of 18,000 -feet, while, on mountains near Quito, that is, immediately on the -equatorial line, the lowest level is 15,795.--See Mrs. Somerville's -_Physical Geography_, 4th edit. p. 314.] - -In higher latitudes, where the lower limit of the snow-line descends to -the level of the sea, the glaciers are often seen protruding from the -shore, still laden with blocks that have been carried down from valleys -far in the interior. The action of storms and tides is sufficient to -detach large masses of the ice, which then floats off, and is often -wafted for hundreds of miles into temperate regions, where it gradually -melts away. Such floating islands are known as icebergs. - -II. In climates such as that of Canada, where the winters are very -severe, the rivers become solidly frozen over, and, if the frost be -intense enough, a cake of ice forms at the bottom. In this way sand, -mud, and rocky fragments strewing the banks or the channel of the -stream, are firmly enclosed. When spring sets in, and the upper parts -of the rivers begin to thaw, the swollen waters burst their wintry -integuments, and the ice is then said to _pack_. Layer is pushed -over layer, and mass heaped upon mass, until great floes are formed. -These have often the most fantastic shapes, and are borne down by the -current, dropping, as they go, the mud and boulders, with which they -are charged, until they are stranded along some coast line, or melt -away in mid-ocean. - -III. But icebergs are also produced by the freezing of the water of -the ocean. In high latitudes, this takes place when the temperature -falls to 28·5° of Fahrenheit. The surface of the sea then parts with its -saline ingredients, and takes the form of a sheet of ice, which, by -the addition of successive layers, augmented sometimes by snow-drifts, -often reaches a height of from thirty to forty feet. On the approach of -summer these ice-fields break up, crashing into fragments with a noise -like the thundering of cannon. The disparted portions are then carried -towards the equator by currents, and may be encountered by hundreds -floating in open sea. Their first form is flat, but, as they travel on, -they assume every variety of shape and size. - -On the shores of brackish seas, such as the Baltic, or along a coast -where the salt water is freshened by streams or snow-drifts from the -land, sheets of ice also frequently form during severe frosts. Sand and -boulders are thus frozen in, especially where a layer of ice has formed -upon the sea-bottom.[3] The action of gales or of tides is sufficient -to break up these masses, which are then either driven ashore and -frozen in a fresh cake of ice, or blown away to sea. The bergs formed -in this way have originally a low flat outline, and many extend as -ice-fields over an area of many miles, while, at a later time, they may -be seen towering precipitously as great hills, some 200 or 300 feet -high. - -[Footnote 3: I was informed by the late Mr. Hugh Miller, that a seam -of shale abounding in liassic fossils, had been found intercalated -among the boulder-clay beds in the vicinity of Eathie. He explained its -occurrence there by supposing that it had formed a reef along a shore -where ground-ice was forming; and so having been firmly frozen in, it -was torn up on the breaking of the ice, and deposited at a distance -among the mud at the sea-bottom.] - -Few sights in nature are more imposing than that of the huge, -solitary iceberg, as, regardless alike of wind and tide, it steers -its course across the face of the deep far away from land. Like one -of the "Hrim-thursar," or Frost-giants of Scandinavian mythology,[4] -it issues from the portals of the north armed with great blocks of -stone. Proudly it sails on. The waves that dash in foam against its -sides shake not the strength of its crystal walls, nor tarnish the -sheen of its emerald caves. Sleet and snow, storm and tempest, are its -congenial elements. Night falls around, and the stars are reflected -tremulously from a thousand peaks, and from the green depths of -"caverns measureless to man." Dawn again arises, and the slant rays of -the rising sun gleam brightly on every projecting crag and pinnacle, -as the berg still floats steadily on; yet, as it gains more southern -latitudes, what could not be accomplished by the united fury of the -waves, is slowly effected by the mildness of the climate. The floating -island becomes gradually shrouded in mist and spume, streamlets -everywhere trickle down its sides, and great crags ever and anon fall -with a sullen plunge into the deep. The mass becoming top-heavy, -reels over, exposing to light rocky fragments still firmly imbedded. -These, as the ice around them gives way, are dropped one by one into -the ocean, until at last the iceberg itself melts away, the mists are -dispelled, and sunshine once more rests upon the dimpled face of the -deep.[5] If, however, before this final dissipation, the wandering -island should be stranded on some coast, desolation and gloom are -spread over the country for leagues. The sun is obscured, and the air -chilled; the crops will not ripen; and, to avoid the horrors of famine, -the inhabitants are fain to seek some more genial locality until the -ice shall have melted away; and months may elapse before they can -return again to their villages. - -[Footnote 4: The account of the origin of these giants, as given in -the prose _Edda_, is very graphic, and may be not inaptly quoted -here:--"When the rivers that are called Elivagar had flowed far from -their sources," replied Har, "the venom which they rolled along; -hardened, as does dross that runs from a furnace, and became ice. -When the rivers flowed no longer, and the ice stood still, the vapour -arising from the venom gathered over it and froze to rime; and in this -manner were formed in Ginnungagap many layers of congealed vapour, -piled one over the other."--"That part of Ginnungagap," added Jafnhar, -"that lies towards the north, was thus filled with heavy masses of -gelid vapour and ice, whilst everywhere within were whirlwinds and -fleeting mists. But the southern part of Ginnungagap was lighted by -the sparks and flakes that flew into it from Muspellheim.... When the -heated blast met the gelid vapour, it melted into drops, and, by the -might of him who sent the heat, these drops quickened into life, and -took a human semblance. The being thus formed was named Ymir, from whom -descend the race of the Frost-giants (Hrim-thursar), as it is said in -the Völuspá, 'From Vidolph came all witches; from Vilmeith all wizards; -from Svarthöfdi all poison-seekers; and all giants from Ymir.'"--See -Mallet's _Northern Antiquities_, edit. Bohn, p. 402.] - -[Footnote 5: That beautiful expression of Æschylus occurs to me, so -impossible adequately to clothe in English: ἁνηριθμον γελασμα κυματων. -Who that has spent a calm summer day upon the sea, has not realized its -force and delicate beauty?] - -The iceberg melts away, but not without leaving well-marked traces of -its existence. If it disappear in mid-ocean, the mud and boulders, with -which it was charged, are scattered athwart the sea-bottom. Blocks -of stone may thus be carried across profound abysses, and deposited -hundreds of miles from the parent hill; and it should be noticed, that -this is the only way, so far as we know, in which such a thing could -be effected. Great currents could sweep masses of rock down into deep -gulfs, but could not sweep them up again, far less repeat this process -for hundreds of miles. Such blocks could only be transported by being -lifted up at the one place and set down at the other; and the only -agent we know of, capable of carrying such a freight, is the iceberg. -In this way, the bed of the sea in northern latitudes must be covered -with a thick stratum of mud and sand, plentifully interspersed with -boulders of all sizes, and its valleys must gradually be filled up as -year by year the deposit goes on. - -But this is not all. The visible portion of an iceberg is only about -one-ninth part of the real bulk of the whole mass, so that if one be -seen 100 feet high, its lowest peak may perhaps be away down 800 feet -below the waves. Now it is easy to see that such a moving island will -often grate across the summit and along the sides of submarine hills; -and when the lower part of the berg is roughened over with earth and -stones, the surface of the rock over which it passes will be torn up -and dispersed, or smoothed and striated, while the boulders imbedded in -the ice will be striated in turn. - -But some icebergs have been seen rising 300 feet over the sea; and -these, if their submarine portions sank to the maximum depth, -must have reached the enormous total height of 2700 feet--that is, -rather higher than the Cheviot Hills.[6] By such a mass, any rock or -mountain-top existing 2400 feet below the surface of the ocean would be -polished and grooved, and succeeding bergs depositing mud and boulders -upon it, this smoothed surface might be covered up and suffer no change -until the ocean-bed should be slowly upheaved to the light of day. -In this way, submarine rock surfaces at all depths, from the coast -line down to 2000 or 3000 feet, may be scratched and polished, and -eventually entombed in mud. - -[Footnote 6: In the _American Journal of Science_ for 1843, p. -155, mention is made of an iceberg aground on the Great Bank of -Newfoundland. The average depth of the water was about 500 feet, and -the visible portion of the berg from 50 to 70 feet high, so that its -total height must have been little short of 600 feet, of which only a -_tenth_ part remained above water.] - -[Illustration: Fig. 1. Iceberg grating along the sea-bottom and -depositing mud and boulders.] - -And such has been the origin of the deep clay, which, with its included -and accompanying boulders, covers so large a part of our country. When -this arctic condition of things began, the land must have been slowly -sinking beneath the sea; and so, as years rolled past, higher and yet -higher zones of land were brought down to the sea-level, where floating -ice, coming from the north-west, stranded upon the rocks, and scored -them all over as it grated along. This period of submergence may have -continued until even the highest peak of the Grampians disappeared, -and, after suffering from the grinding action of ice-freighted rocks, -eventually lay buried in mud far down beneath a wide expanse of sea, -over which there voyaged whole argosies of bergs. When the process of -elevation began, the action of waves and currents would tend greatly to -modify the surface of the glacial deposit of mud and boulders, as the -ocean-bed slowly rose to the level of the coast line. In some places -the muddy envelope was removed, and the subjacent rock laid bare, -all polished and grooved. In other localities, currents brought in a -continual supply of sand, or washed off the boulder mud and sand, and -then re-deposited them in irregular beds; hence resulted those local -deposits of stratified sand and gravel so frequently to be seen resting -over the boulder clay. At length, by degrees, the land emerged from the -sea, yet glaciers still capped its hills and choked its valleys; but -eventually a warmer and more genial climate arose, plants and animals, -such as those at present amongst us, and some, such as the wolf, no -longer extant, were ere long introduced; and eventually, as lord of the -whole, man took his place upon the scene.[7] - -[Footnote 7: The reader who wishes to enter more fully into the -geological effects of icebergs, should consult the suggestive section -on that subject in De la Beche's _Geological Observer_; also the -_Principles_ and _Visit to the United Stales_ of Sir Charles Lyell, -with the various authorities referred to by these writers.] - -It is pleasant to mark, when once the true solution of a difficulty is -obtained, how all the discordant elements fall one by one into order, -and how every new fact elicited tends to corroborate the conclusion. In -some parts of the glacial beds, there occur regular deposits of shells -which must have lived and died in the places where we find them. From -ten to fifteen per cent, of them belong to species which are extinct, -that is to say, have not been detected living in any sea. Some of them -are still inhabitants of the waters around our coasts, but the large -majority occur in the northern seas. They are emphatically northern -shells, and get smaller in size and fewer in number as they proceed -southward, till they disappear altogether. In like manner, the palm, -on the other hand, is characteristically a tropical plant. It attains -its fullest development in intertropical countries, getting stunted -in its progress towards either pole, and ceasing to grow in the open -air beyond the thirty-eighth parallel of latitude in the southern -hemisphere, and the forty-fifth in the northern. So, too, the ivy, -which in our country hangs out its glossy festoons in every woodland, -and around the crumbling walls of abbey, and castle, and tower, is -nursed in the drawing-rooms of St. Petersburg as a delicate and -favourite exotic. In short, the laws which regulate the habitat of a -plant or an animal are about as constant as those which determine its -form. There are, indeed, exceptions to both. We may sometimes find a -stray vulture from the shores of the Mediterranean gorging itself on -sheep or lambs among the wolds of England,[8] just as we often see - - "A double cherry seeming parted, - But yet an union in partition;" - -or as we hear of a sheep with five legs, and a kid with two heads. -But these exceptions, from their comparative rarity, only make the -laws more evident. When, therefore, we find, in various parts of our -country, beds of shells in such a state of preservation as to lead -us to believe that the animals must have lived and died where their -remains are now to be seen, we justly infer that the districts where -they occur must at one period have been submerged. If the shells -belong to fresh-water species, it is plain that they occur on the site -of an old lake. If they are marine, we conclude that the localities -where they are found no matter how high above the sea must formerly -have stood greatly lower, so as to form the ocean bed. To proceed one -step further. If the shells are of a southern type, that is, if they -belong to species[9] which are known to exist only in wanner seas than -our own, we pronounce that at a former period the latitudes of Great -Britain must have enjoyed a more temperate and genial climate, so as -to allow southern shells to have a wider range northwards. If, on the -other hand, they are of an arctic or boreal type, we in the same way -infer that our latitudes were once marked by a severer temperature than -they now possess, so as to permit northern shells to range farther -southwards. This reasoning is strictly correct, and the truth involved -forms the basis of all inquiries into the former condition of the earth -and its inhabitants. - -[Footnote 8: Two of these birds (_Neopron pecnopterus_) are stated to -have been seen near Kilve, in Somersetshire, in October 1825. One was -shot, the other escaped.] - -[Footnote 9: There is not a little difficulty in reasoning -satisfactorily as to climatal conditions, from the distribution of -kindred forms. Even in a single genus there may be a wide range of -geographical distribution, so that mere generic identity is not always -a safe guide. Thus, the elephant now flourishes in tropical countries, -but in the glacial period a long-haired species was abundant in the -frozen north. I have above restricted myself entirely to _species_ -whose habits and geographical distribution are already sufficiently -known.] - -The evidence furnished by the northern shells in the boulder-clay -series is, accordingly, of the most unmistakable kind. These organisms -tell us that at the time they lived our country lay sunk beneath a sea, -such as that of Iceland and the North Cape, over which many an iceberg -must have journeyed, and thus they corroborate our conclusions, derived -independently from the deep clay and boulder beds and the striated -rock-surfaces, as to the glacial origin of the boulder-clay. - - - - -CHAPTER III. - - - How the boulder came to be one "Crag and tail"--Scenery of - central Scotland: Edinburgh--"Crag and tail" formerly - associated in its origin with the boulder-clay--This - explanation erroneous--Denudation an old process--Its - results--Illustration from, the Mid-Lothian coal-field--The - three Ross-shire hills--The Hebrides relics of an - ancient land--Scenery of the western coast--Effects of - the breakers--Denudation of the Secondary strata of the - Hebrides--Preservative influence of trap-rocks--Lost - species of the Hebrides--Illustration--Origin of the - general denudation of the country--Illustrative action - of streams--Denudation a very slow process--Many old - land-surfaces may have been effaced--Varied aspect of the - British Islands during a period of submergence--Illustration. - -The scratched and grooved surface of the boulder was produced when -it was fast frozen in some iceberg, and driven gratingly across some -submarine summit, or stranded on some rocky coast-line. But, from its -rounded form, the stone had evidently undergone a long process of wear -and tear previous to its glacial journey. Probably it had hitherto lain -along a surf-beaten beach, where in the course of ages it had gradually -been worn into its present rounded shape. But how came it there? It -must originally have formed part of a flat sandstone bed, with many -other beds piled above it. By what agency, then, was this great pile -reduced to fragments? - -The answer to these questions must be a somewhat lengthened one, for -the subject relates not to a few beds of rock hastily broken up and -dispersed, but to the physical changes of an entire country, carried on -during a vast succession of geological periods. - -A phenomenon, known familiarly as "crag and tail," has long been -connected in its origin with the drift or boulder beds. Has my reader -ever travelled through central Scotland? If so, he must often have -noticed the abrupt isolated form of many of the hills, presenting a -mural front to the west, and a long sloping declivity to the east. -From the great number of isolated hard trap-rocks in this region, the -phenomenon is much better seen than in most other parts of the kingdom. -There is, for instance, the castle rock of Stirling, with its beetling -crag and castellated summit, which present so imposing a front to the -west. Many other examples are seen along the line of the Edinburgh and -Glasgow Railway. The range of hills south of Linlithgow, the singularly -abrupt basalt of Binny Craig, the long rounded ridge of Ratho, -the double-peaked crag of Dalmahoy, the broad undulation of woody -Corstorphine, are all examples more or less marked. Edinburgh itself -is an excellent illustration. The Calton Hill shows a steep front to -the town, while its eastern side slopes away down to the sea. Arthur's -Seat, in like manner, has a precipitous western face, and a gentle -declivity eastward. The Castle rock, too, shoots up perpendicularly -from the valley that girdles it on the north, west, and south, sinking -away to the east in a long slope-- - - "Whose ridgy back heaves to the sky, - Piled deep and massy, close and high." - -East-Lothian presents several well-marked instances; in particular, -North Berwick Law and Traprain. A phenomenon so general must have had -some general origin, and it was accordingly attributed to the same -agency which produced the drift-clays and the striated rock-surfaces, -when these were believed to be the results of great diluvial action. -It would seem, however, that the phenomenon of crag and tail should -not be associated with the boulder-clay. The latter is undoubtedly a -newer Tertiary formation,[10] but the denudation[11] which produced -crag and tail must have been going on long ere the Tertiary ages had -begun. There is satisfactory evidence that large areas of our country -were planed down at a greatly more ancient period than that of even the -oldest of the Tertiary series. Thus, the whole area of the county of -Sussex suffered a very extensive denudation during the later Secondary -ages. The Hebrides had undergone a similar process previous to the -deposition of the Lias and Oolite, and the Greywacke hills of south -Scotland, previous to the formation of the Old Red Sandstone. There -seems thus to have been a general and continuous process of degradation -at work during a long succession of geological ages. - -[Footnote 10: The reader is referred to the table of the geological -formations at the end of the volume for the relative position of the -beds described.] - -[Footnote 11: _Denudation_ is a geological term used to denote the -removal of rock by the wasting action of water, whereby the underlying -mineral masses are _denuded_ or laid bare.] - -The results of this long-continued action are of the most startling -kind. I have referred to the phenomenon of crag and tail as perhaps the -most readily observable. We must not fail to remember that the crag -which now stands up so prominently above the level of the surrounding -country, at one period lay buried beneath an accumulation of sandstone, -shale, or other strata, all of which have been carried away, so as -to leave the harder rock in bold relief, with a portion of the less -coherent strata sloping as a long tail from its eastern side. The crag, -too, is often breached in many places, worn down at one end, rounded -on the summit, and sometimes well-nigh ground away altogether, whilst -in front there is invariably a deep hollow scooped out by the current -when arrested by the abrupt cliff. In Fig. 2, _a_ represents a crag of -greenstone worn away and bared of the shales which once covered it; -_b_, the sloping "tail" of softer strata, protected from abrasion by -the resistance of the trap-rock, and covered by a deep layer of drift, -_d_; _c_ marks the hollow on the west side of the crag. - -[Illustration: Fig. 2 "Crag and tail."] - -But when we come to measure the actual amount of material that has been -carried away, we are lost in conjecture as to the vastness of the time -which such a process must have occupied. For instance, the coal-bearing -strata of Mid-Lothian must at one period have been connected with those -of Linlithgow and Stirling. At a subsequent date, the western area -subsided to form the Stirlingshire coal-basin, and the eastern area, in -like manner, sank down to form the coal-basin of Mid-Lothian, while the -intermediate portion stretched from east to west as a great arch, or, -as it is termed geologically, an _anticlinal axis_. Now, the whole of -this arch has been worn away, not a vestige of it remains, and yet its -upper or coal-bearing part was fully 3000 feet thick.[12] - -[Footnote 12: This remarkable example of denudation was first described -by Mr. M'Laren, in his _Sketch of the Geology of Fife and the -Lothians_, a work in which the author showed himself to be in advance -of the science of his time.] - -Let us take a small portion of this district, and endeavour to -calculate the amount of matter thus removed. The Pentland hills -form a chain stretching from near Edinburgh for some fourteen miles -southward, and having an average breadth of about two miles and a -half. They are formed chiefly of felspathic trap-rocks, resting upon -and interstratified with conglomerate apparently of Old Red age, which -in turn lies upon vertical Silurian slates. Before the Carboniferous -strata were thrown down by successive _faults_, they must have covered -these hills completely to a depth of not less than 6000 feet.[13] -From this small area, therefore, stratified sandstones, shales, -limestones, and coal, must have been removed to the enormous extent of -one billion, eight hundred and fifty-four thousand, four hundred and -sixty-four millions of cubic feet. - -[Footnote 13: The actual depth of the Mid-Lothian coal-field, to the -base of the carboniferous limestone, is rather more than 3000 feet. It -is, perhaps, rather under than over the truth to allow 3000 feet for -the total thickness of beds from the limestone to the conglomerate of -Liberton, though, owing to the curved and contorted position of the -strata from Edinburgh to Stirlingshire, it is impossible to obtain a -measurement of their real thickness. I have attributed the isolation -of the Falkirk and Mid-Lothian coal-fields to the effect of faults -and general depressions of their areas. This was assuredly the case -in the latter coal-field, and probably in the former also. The trap -which occurs between them, though in great abundance, has certainly -not acted as an elevating agent. It occurs in beds among the strata, -and, judging from the number of associated tufas, appears to have been -to a considerable extent erupted while the lower carboniferous series -was forming. Mr. M'Laren, in his excellent work, p. 100, states his -opinion that the traps may have materially contributed to push up the -coal strata. A careful and extended examination of the district has -convinced me that this view is incorrect.] - -But, perhaps, the most striking instances of denudation in the British -Islands are the three famous Ross-shire hills--Suil Veinn, Coul Mor, -and Coul Bheig. They are formed of piles of sandstone beds like tiers -of regular masonry, and reach a height of 3000 feet over the sea. The -sandstone of which they are composed must once have formed a bed or -set of beds fully 2000 feet thick, that covered the whole district for -many miles around. Yet of this extensive deposit there now exist only -a few isolated fragments. I have watched the sunshine and shadow of an -autumn sky resting alternately on these strange pyramidal hills, as -they towered in their giant proportions like the last remnants of a -mighty rampart that had stood the brunt of a long siege, and, breached -at last in many places, had been all but levelled to the ground. How -long-continued and how potent must that agency have been which could -cut down and disperse the massive barrier that flanked the western -coast of Ross-shire to a height of 2000 feet! - -The Hebrides are but the shattered relics of an old land that had its -mountain-peaks and its glens, its streams and lakes, and may have -nursed in its solitude the red-deer and the eagle, but was never -trodden by the foot of man. A glance at the map is enough to convince -us of this. We there see islands, and peninsulas, and promontories, -and deep bays, and long-retiring inlets, as though the country had -been submerged and only its higher points remained above water. The -conviction is impressed more strongly upon us by a visit to these -shores. We sail through the windings of one of the "sounds," and can -scarcely believe that we are on the bosom of the salt sea. Hills rise -on all sides, and the water, smooth as a polished mirror, shows so -pure and limpid that in the sunshine we can see the white pebbles that -strew its bed many fathoms down. The eastern shore is often abruptly -interrupted by long-receding lochs edged round with lofty mountains, -and thus, where we had looked to see a deep heathy glen, with, -perchance, a white tree-shaded mansion in the far distance, and a few -dun smoking cottages in front, we are surprised to catch a glimpse of -the white sails of a yacht, or the darker canvas of the herring-boats. -We sail on, and soon a sudden turn brings us abruptly to the mouth of -the sound. A bold headland, studded around with rocky islets, rises -perpendicularly from the sea, bleak and bare, without a bush or tree, -or the faintest trace of the proximity of man. The broad swell of the -Atlantic comes rolling in among these rocks, and breaks in foam against -the grey cliffs overhead. In tempests, such a scene must be of the most -terrific kind. Wo to the hapless vessel that is sucked into the vortex -of these breakers, whose roar is sometimes heard at the distance of -miles! Even in the calmest weather the white surf comes surging in, -and a low sullen boom is ever reverberating along the shore. We see -the harder rocks protruding far into the sea, and often pierced with -long twilight caves, while the softer ones are worn into deep clefts, -or hollowed out into open bays strewed over with shingle. The sunken -rocks and islets, scarcely showing their tops above water, were all -evidently at one time connected, for, as we recede from the shore, we -can mark how the process of demolition goes on. There is first the -projecting ness or promontory, well-nigh severed from the mainland, but -still connected by a rude arch, through which the swell ever gurgles -to and fro. Then, a little farther from the shore, a huge isolated -crag, washed on all sides by the surge, raises its grey lichen-clothed -summit. A short way beyond, there is the well-worn islet whose surface -shelters neither lichen nor sea-weed, but is ever wet with the dash of -the waves. Further to the sea, the white gleam of the breakers marks -the site of the sunken rock. Thus, in the space of a hundred yards, we -may sometimes behold the progress of change from land to sea, and see -before us a specimen of that action which slowly but yet steadily has -narrowed and breached the outline of our western shores.[14] - -[Footnote 14: I have endeavoured to illustrate the process of -denudation by a reference to breaker-action on the existing coast-line -of the Hebrides; but a strong current must have materially increased -the force of the ancient waves, and produced abrasion to some depth -below them.] - -If we attempt to trace the connexions of strata among the Hebrides, -we shall be more fully impressed with the magnitude of the changes -which have been effected. Thus the Lias and Oolite occur in patches -along the shores of Mull, Morven, Ardnamurchan, Eigg, Skye, Raasay, -and Applecross. But though now only in patches, these formations must -once have extended over a considerable area, for they seem to form the -under-rock of the whole of the northern part of Skye, and are seen in -almost every lone island from Ardnamurchan Point to the Shiant Isles. -These scattered portions, often many miles distant from each other, -are the remnants of a great sheet of liassic and oolitic strata, now -almost entirely swept away, and are extant from having been covered -over with hard trap-rocks. But for these it may be doubted whether we -should ever have known that corals once gleamed white along the shores -of Skye, that the many-chambered ammonite swam over the site of the -Coolin Hills, that the huge reptilian monsters of these ancient times, -icthyosaurs and plesiosaurs, careered through the waters that laved -the grey hills of Sleat, and that forests of zamia and cycas, and many -other plants indicative of a warm climate, bloomed green and luxuriant -along the site of that strange mist-clad cliff-line, that shoots up -into the pinnacles of the Storr and Quiraing. It is curious to reflect, -that the records of these peaceful scenes have been preserved to us by -the devastating eruptions of volcanic forces; that the old lava-streams -which spread death through the waters along whose bed they travelled, -have yet been the means of protecting the districts which they wasted, -while those parts where they did not reach have been long since swept -away. It is allowable to believe, that in the portions of liassic -strata which have been destroyed there existed the remains of not a -few species, perhaps some genera, to be found nowhere else, and of -whose former existence there is now, by consequence, no trace. In the -small island of Pabba--a relic of the Scottish Lias--I found thirty-one -species, of which Dr. Wright has pronounced four to be new.[15] A -subsequent visit to the adjacent island of Raasay has increased the -list. In short, every patch of these Secondary rocks, if thoroughly -explored, might be found to yield its peculiar organisms. And in the -far larger area that has been carried away there existed, doubtless, -many more. We are accustomed to see individuals perish and their -remains crumble away, but the species still holds on. In the stratified -portion of the earth's crust, however, we mark how not merely -individuals have perished, but whole genera and species; but of these -the remains are still before us in the rocks; we can study their forms, -and, from a comparison with recent species and genera, can arrive at -some idea of their nature and functions. In this way, we are able to -picture the various conditions of the earth when these organisms lived -in succession upon its surface. Yet, we may readily conjecture, that -in ancient eras many tribes and genera of plants and animals lived -for ages, and then passed away without leaving any record of their -existence. Many circumstances might concur to prevent the preservation -of their remains. The species of the Hebrides were preserved in the -usual manner, but the cemetery in which their remains were entombed has -been washed away, and they can be seen nowhere else. It is as if on -some isolated country there had lived a race of men, tall Patagonians, -or swarthy Hottentots, or diminutive Laplanders, with a civilisation of -their own; owing to some change of climate the race gradually dwindled -down until it died out; eventually, too, the land settled down beneath -the sea with all its ruined cities and villages, which, as they reached -in succession the level of the waves, were torn up and dispersed, and -other races at last voyaged over the site of that old land, dreaming -not, that in bygone years fellow-mortals of an extinct type had -pastured their herds where now there rolled a widespread sea. - -[Footnote 15: _Quart. Jour. Geol. Soc._, vol. xiv. p. 26.] - -But to return. We have seen that the long-continued action of the sea -has been sufficient to breach and waste away the existing coast-line -of western Scotland. When, therefore, such results are produced by so -ordinary a cause, need we go to seek the agency of great debacles to -explain the denudation of other parts of the country? It is known that -at great depths currents have little effect upon the rocks which they -traverse, and that their action is greater as it nears the surface. To -account for the phenomena of crag and tail, and the general denudation -of the country, we may suppose the land to have been often submerged -and re-elevated. As hill after hill rose towards or sank below the -sea-level, it would be assailed by a strong current that flowed -from the west and north-west, until, in its slow upward or downward -progress, it got beyond the reach of the denuding agencies. In this way -the general contour of the land would be greatly though very gradually -changed. Hills of sandstone, or other material of feeble resistance, -would be swept away, the harder trap-rocks would stand up bared of the -strata which once covered them, deep hollows would be excavated in -front of all the more prominent eminences, and long declivities would -be left behind them.--(See Fig. 2.) - -If my reader has ever visited the channel of a mountain-torrent-- - - "Imbres - Quern super notas aluere ripas"-- - -he must have noticed an exact counterpart to these appearances. When -the waters have subsided, the overflowed parts are seen to be covered -in many places with sand. Wherever a pebble occurs along the surface -of this sand, it has invariably a hollow before it on the side facing -the direction whence the stream is flowing, and a long tail of sand -pointing down the channel. If we watch the motion of the water along -its bed, the denuding agency may be seen actively at work. Every pebble -that protrudes above the shallow streamlet arrests the course of the -current, which is then diverted in three directions. One part turns -off to the right hand of the pebble, and cuts away the sand from its -flank; another part strikes off to the left, and removes the sand from -that side; while a middle part descends in front of the pebble, and, -by a kind of circular or gyratory movement, scoops out a hollow in the -sand in front. Behind the pebble the water is pretty still, so that the -sand remains undisturbed, and is further increased by the accumulation -above it of sediment swept round by the lateral currents. Now, in place -of the supposed stream, let us substitute the ocean with its westerly -current--for the pebble, a great trap-hill--for the sand, easily -friable shales and sandstones, and we have exactly the condition of -things which produced crag and tail. - -This process of destruction must have been in progress during many -geological ages. We may suppose, that in that time the land often -changed level, sometimes rising far above the sea, and sometimes -sinking deep below it. We can well believe that the surface would -often be covered with vegetation; that plants, widely differing from -those which are now indigenous, clothed its hill-sides and shaded -its valleys; and that animals of long extinct forms roamed over its -plains or prowled amid its forests. When the country, in the lapse of -centuries, sank beneath the sea-level, all trace of these scenes would -eventually be effaced. The westerly currents would soon recommence the -process of degradation, uprooting the forests, devastating the plains, -wearing down the hills, and scooping out the valleys; and so, when the -ocean-bed, in the course of ages, became again dry land, it would arise -"another and yet the same." The little valley, where once, perchance, -the mastodon used to rest his massive bulk amid a rich growth of ferns, -shaded by the thick umbrage of coniferous trees, would emerge a deep -glen with bare and barren rocks on either side; the site of the hill -whereon herds of the gazelle-like anoplothere were wont to browse, -might reappear a level plain; the low-browed rock, under whose shadow -the ungraceful palæothere used of old to rest from the heat of the -noon-tide sun, might emerge a beetling crag shooting up several hundred -feet over the valley. It is by this repeated elevation and submergence, -carried on for many ages, that our country has acquired its present -configuration. - -We can easily picture to ourselves the appearance which the British -Islands would thus at different periods present. At one time, nearly -the whole of England would be under water, with, however, a few islands -representing the higher peaks of Cornwall; others scattered over the -site of the West Riding of Yorkshire; and a hilly tract of land over -what is now Wales. Scotland must have existed in a sorely mutilated -state. A thick-set archipelago would represent the Cheviot Hills, and -the country south of the Forth and the Clyde; north of which there -would intervene a broad strait, with a comparatively large area of -undulating land beyond, stretching across what is now the area of -the Grampian Hills. A narrow fiord would run along the site of the -Caledonian Canal, cutting the country into two parts, and running -far into it on either side as deep lochs and bays. I have had such a -condition of things vividly recalled when on the summit of a lofty hill -in early morning, while the mists were still floating over the lower -grounds, and only the higher hill-tops, like so many islands, rose -above the sea of cloud. It was not a little interesting to cast the eye -athwart this changing scene, and mark how each well-known peak and -eminence looked when deprived of its broad sweep of base. What before -had always seemed an abrupt precipitous summit, now took the form of a -lonely rock or deep-sea stack, that might have served as a haunt for -the gull and the gannet. The long swelling hill rose above the mist as -a low undulating island, treeless and barren. It was easy to think of -that wide expanse of mist as the veritable domain of ocean, to picture -the time when these were veritable islands lashed by the surge, and -to conjure up visions of ice-floes drifting through the narrows, or -stranding on the rocks, amid a scene of wide-spread nakedness and -desolation. - - - - -CHAPTER IV. - - - Interior of the boulder Wide intervals of - Geology--Illustration--Long interval between the - formation of the boulder as part of a sand-bed, - and its striation by glacial action--Sketch of the - intervening ages--The boulder a Lower Carboniferous - rock--Cycles of the astronomer and the geologist - contrasted--Illustration--Plants shown by the boulder once - grew green on land--Traces of that ancient land--Its seas, - shores, forests, and lakes, all productive of material - aids to our comfort and power--Plants of the Carboniferous - era--Ferns--Tree-ferns--Calamites--Asterophyllites-- - Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined - palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest - known flower--Grade of the Carboniferous flora--Its resemblance - to that of New Zealand. - -I have likened the boulder to an old volume of the middle ages encased -in a modern binding. We have looked a little into the mechanism and -history of the boards; in other words, we have gone over the history of -the scratched surface of the boulder, of the clays and sands around it, -and of that still earlier cycle of denudation whereof the rock itself -is probably a relic. Before proceeding to open the volume itself, it -will be well that we clearly mark the wide interval in time between -the ages represented by the surface-striation and those indicated by -the interior of the boulder. When we proceed from the groovings on the -outside to the plants within, we pass, to be sure, over scarcely an -inch of space, but we make a leap over untold millenniums in point of -time. It is as if we had laid our hands on a volume of history which -had by some misfortune found its way into the nursery. The first page -that catches our eye relates the battle of the Reform Bill, and, on -turning the previous leaf, we find ourselves with Boadicea and her -woad-coloured soldiery. Now, if one utterly ignorant of the chronology -of the country were to be told that the volume related solely to one -people, he would at once see from the manners and customs delineated, -that the two pages referred to very different states of civilisation, -and consequently to widely-separated periods. But he could give no -account of how long an interval might have elapsed between the time -when London had its inhabitants massacred by Boadicea, and the time -when another generation of them was excited by the tardiness of King -William iv. He could form no conjecture as to what events might have -happened in the meanwhile. The interval might be a century or twenty -centuries, wherein the city might have been burnt down fifty times. -Clearly, if he wished to make himself acquainted with the intervening -history, he would have to betake himself to an unmutilated volume. - -And just so is it with our boulder. We can easily believe, merely from -looking at it as it lies on its clayey bed, that a long time must have -elapsed between the time of its formation as part of a sandstone bed, -and the period of its transportation and striation by an iceberg. The -sand of which it is formed must have been washed down by currents, and -other sediment would settle down over it. It would take some time to -acquire its present hardness and solidity, while, in long subsequent -times, after being broken up and well-rounded by breaker or current -action, it may have lain on some old coast-line for centuries before it -was finally frozen into an ice-floe, and so freighted to a distance. -But the stone, with all its stories of the olden time, can tell us -nothing of this intervening period. It leads us from a dreary frozen -sea at once into a land of tropical luxuriance, and so, if we desire to -know anything of the missing portion of the chronology, we must seek it -elsewhere. - -The Boulder-clay is one of the latest of geologic periods.[16] Beyond -it we get into Tertiary times, and learn from the caves of Yorkshire -how elephants, hyenas, rhinoceroses, hippopotami, bears, and wolves, -prowled over the rich valleys; while, from the quarries of the Isle of -Wight, we see how at an earlier time herds of uncouth palæotheres and -slimly-built anoplotheres browsed the plains of Old England. Beyond -the Tertiary ages come those of the Chalk, with its ocean that swarmed -with sea-urchins, terebratulæ, pectens, sponges, and many other forms. -Then arises the era of the Wealden, with its bosky land haunted by the -unwieldy iguanodon; the Oolite, with its land rich in a coniferous -flora, and tenanted by a race of small marsupial animals, and its seas -abounding in corals, encrinites of many a form, cidares, cuttle-fishes, -and ammonites. Further back still, come the times of the Lias, that -strange era in the history of our country, when reptiles huger than -those of the Nile swam the seas, and sped on wings through the air. -Then come the times of the Trias, when a vegetation still further -removed from existing types clothed the land, and frogs large as oxen -waddled along the shores. Then the times of the Permian, with its deep -sea tenanted by a meagre list of corals and shells, and by a type of -fishes that was slowly passing away. We arrive at last at the Coal or -Carboniferous period, to the older ages of which our boulder belongs. - -[Footnote 16: For the names and succession of the rocks of which the -known part of the earth's crust is composed, see the Table at the end -of the volume.] - -These eras may have been some longer, some shorter, but each had a -duration which, when tried by human standards, must be regarded as -immensely protracted. The cycles of astronomy are very vast, yet -I have often thought that the cycles of geology, though probably -of much less duration, impress us more forcibly with the antiquity -of our planet. The astronomer tells us of light that has taken two -millions of years to reach our earth, and of nebulæ that are millions -upon millions of miles distant, but these numbers are so vast that -we cannot bring ourselves to realize them. We _know_ that there is a -great difference between two millions and ten millions, but we cannot -fully _appreciate_ it, and so the periods of the astronomer, beyond a -certain point, cease adequately to impress us. So long as they can be -easily contrasted with our own standards of comparison, they have their -full force; but after that, every additional million, or ten millions, -or ten hundred millions, produces only a confused and bewildered sense -of immensity, and the comparative amount of each addition fails to be -realized. Will my reader forgive a homely illustration:--Some years -ago, I stood at the pier-head of one of our smaller sea-port towns, and -watched the sun as it sullenly sank behind the outline of the opposite -hills. The breadth of the channel, in the direction of sunset, was -several miles, but in the flush of evening one fancied he could almost -have thrown a stone across. The water lay unruffled by a ripple, and -reflected all the thousand varying tints that lighted up the sky. The -harbour, that had been a busy scene all evening, began to grow less -noisy, as one by one the herring-boats pushed out to sea. I found it -not a little interesting to mark, as the boats gained the open firth, -how the opposite coast-line gradually seemed to recede. The farther the -dark sails withdrew, the more remote did the adjacent shores appear, -until, as the last tinge of glory faded from the clouds, and a cold -grey tint settled down over the landscape, the hills lay deep in shade -and stretched away in the twilight as a dark and distant land from -whose valleys there rose troops of stars. The coast-line, as seen in -early evening, reminded me of the periods of the astronomer; as seen in -early night, it reminded me of the periods of the geologist. We fail to -appreciate the real duration of astronomical cycles, because they are -presented to us each as one vast period. They are not subdivided into -intervals, and contain no succession of events, by means of which, as -by milestones, we might estimate their extent; and so their unvaried -continuity tends to diminish the impression of their vastness, just -as the firth, without any islet or vessel on its surface, seemed -greatly narrower than it really was. For it is with time as it is with -space--the eye cannot abstractly estimate distance, nor can the mind -estimate duration. In either case, the process must be conducted by a -comparison with known standards. The geological periods exemplify the -same rule. They may not be greater, perhaps not so great, as those -revealed by astronomy, yet their vastness impresses us more, because we -can trace out their history, and see how step by step they progressed. -Thus, that the interval between the boulder-clay and the coal-measurer -was immense, we learn from the records of many successive ages that -intervened, in the same way that one began to perceive the real breadth -of the firth, by resting his eye on the succession of intervening -herring-boats. In the former case, the mind has ever and anon a sure -footing on which to pause in gauging bygone eternity; in the latter, -the eye had likewise a succession of points on which to rest in -measuring distance. Or, to return to a former illustration: Boadicea -lived eighteen hundred years ago, but who does not feel that the last -nine hundred years look a great deal longer than the first? The one -set has few marked incidents to fix the thoughts; the other is replete -with those of the most momentous kind. In the one, we have M meagre -list of conquerors and kings, from Julius Cæsar down to Athelstan; in -the other, events crowd upon us from the waning of the Saxon power down -through the rising glory of our country to the present plenitude of -its power and greatness. The early centuries, like the cycles of the -astronomer, pass through our mind rather as one continuous period; the -later centuries, like the cycles of the geologist, arrest our thoughts -by a succession of minor periods, and hence the idea of duration is -more vividly suggested by the diversified events of the one series, -than by the comparatively unbroken continuity of the other. - -Let us now open the volume and try to decipher the strange legends -which it contains. On removing some of the upper layers of the boulder, -I found, as I have said, well-preserved remains of several kinds of -plants. One of them was ribbed longitudinally, with transverse notches -every three or four inches, us though a number of slender threads -had been stretched along a rod, and tied tightly to it at regular -intervals. Another, sorely mutilated, was pitted all over somewhat -after the fashion in which the confectioner punctures his biscuits. A -third had a more regular pattern, being prettily fretted with small -lozenge-shaped prominences that wound spirally round the stalk. Other -plants seemed to be present, but in a very bad state of preservation. -They were all jumbled together and converted into a black coaly -substance, in which no structure could be discerned. - -These plants assuredly once grew green upon the land; but where now is -that land on which they flourished? Had it hills and valleys, rivers -and lakes, such as diversify our country? Was it tenanted by sentient -beings, and, if so, what were their forms? Did insects hum their way -through the air, and cattle browse on the plains, and fish gambol in -the rivers? Was the land shaded with forests, dark and rugged like -those of Norway, or fragrant as the orange-groves of Spain? What, in -fine, were its peculiar features, and how far did its scenery resemble -that of any country of the present day? - -That old land has not entirely disappeared. Traces of it are found -pretty extensively in South Wales, in Staffordshire, around Newcastle, -and through central Scotland. Strange as it may seem, its forests -are still standing in many places. The fishes that disported in its -lakes, the insects that fluttered amid its woods, and the lizards that -crawled among its herbage, are still in part preserved to us. Nay, -more; we may sometimes see the sea-beaches of that ancient land pitted -with rain-drops, and roughened with ripple-marks, as freshly as if the -shower had fallen and the tide had flowed only yesterday. The peasants -along the Bay of Naples gathered grapes from the flanks of Vesuvius -for well-nigh seventeen centuries, before it was ascertained that they -daily walked over the site of buried cities, with temples, theatres, -and private houses still erect. It was many more centuries ere the -people of Great Britain discovered that not a few of their villages and -towns stood on the site of buried forests, and lakes, and seas. We have -now, however, become aware of the fact, and are making good use of it. -We dig into the earth and exhume these old forests to supply us with -light and fuel; we quarry into the ripple-marked shores which fringed -that old land, and build our houses with the hardened sand; we calcine -the ferruginous mud that gathered in its swampy hollows, and extract -therefrom our most faithful ally both in peace and war--metallic iron; -we burn the delicate corals and shells and lily-like zoophytes which -lived in the sea of that far-distant era, to enable us to smelt our -iron, to build our houses, and manure our fields; in short, every year -we are discovering some new and valuable material in the productions -of that period, or finding out some new use which can be made of the -substances already known. A more than ordinary interest, therefore, -attaches to the history of the land and sea which have furnished us -with so many aids to comfort as well as power; and we shall find, as we -go on, that that history is a very curious one. - -I shall describe some of the more common plants and animals of the -period, that we may be able, in some measure, to look back through the -ages of the past, and see how these plants would appear when they cast -their broad shadow over river and lake, and how these animals would -have seemed to human eye in the twilight of the forest, in the sluggish -flow of the river, and in the stagnant waters of the lagoon. - -The _Flora_, or vegetation of the Carboniferous era, differed -widely from any that now exists. With the exception of the highest -or exogenous class, it possessed representatives of all the existing -classes of the botanic scale, but in very strange proportions. The -number of species of carboniferous plants already found in Great -Britain amounts to about three hundred, amongst which the ferns are -especially abundant. Some of them seem to have been low-growing -plants, like the bracken of our hillsides, but others must have shot -up to the height of forest trees. We can recognise a few coniferous -and cycadaceous plants, a good many stems resembling the "horse-tail" -of our marshy grounds, and some of large size akin to the creeping -club-moss of our heaths; but there are still many to which there exist -no living analogues. - -When we examine the roof of a coal-pit, or split open plates of shale -in a quarry of the coal-measures, we are struck with the similarity -which the ferns in the stone bear to those among our woods and hills. -One of the most common, and, at the same time, most elegant forms, -is the _Sphenopteris_ or wedge-leaved fern, of which a large list -of species is known. One of them (_S. crenata_) had a strong stem, -from which there sprung straight tapering branches richly dight with -leaflets. The leaflets--somewhat like minute oak-leaves--were ranged -like those of our modern ferns, along two sides of the stalk, in -alternate order, and tapered gently away to its outer extremity. The -effect of the whole is singularly rich, and one can well believe that a -garland of this ancient fern would have wreathed as gracefully around a -victor's brow as the parsley of Nemea or the laurel-leaves of Delphi. - -Another plant of the same genus (_S. affinis_, Fig. 3) has leaflets -like the petals of the meadow-daisy, arranged in clusters along its -slim diverging stalks. From a collection and comparison of many -specimens, the late lamented Hugh Miller was enabled to make a drawing -of this fern as it must have appeared when it waved green along the -old carboniferous hill-sides. I enjoyed the privilege of going over -these specimens with him, and marked how, under a master-hand, piece -by piece fell into its proper place, and yielded up its evidence. His -restoration, which forms the frontispiece to his last work, is a very -beautiful one, and it is as true as it is beautiful. - -[Illustration: Fig. 3. Sphenopteris affinis.] - -[Illustration: Fig. 4. Pecopteris.] - -[Illustration: Fig. 5. Cyclopteris.] - -[Illustration: Fig. 6. Neuropteris.] - -The _Pecopteris_ (Fig. 4, _P. heterophylla_) or comb-fern, is so called -from its stiff thick leaflets being in some species arranged along the -stalk like the teeth along the centre of a comb. Of all the plants -of the coal-measures this is the one that approaches most closely to -living nature. It appears to be almost identical with the _pteris_, -of which one species is well known as the bracken of our hill-sides. -Dr. Hooker figures together a frond of a New Zealand species (_P. -esculenta_) and a fossil frond from the Newcastle pits. They are -so similar as to be easily mistaken at first sight for drawings of -the same plant.[17] The _Neuropteris_ (as _N. gigantea_, Fig. 6) or -nerve-leaved fern, is remarkable for its strongly-defined venation. It -is scarcely, perhaps, so elegant in its outline as the _sphenopteris_, -or some of the other ferns. Its leaflets are large and thick, with an -oblong or rounded form, and arranged either singly along the frond -stem, or along secondary foot-stalks, which diverge from the main stem. -Of the latter kind, some of the species have a good deal of resemblance -to our _Osmunda regalis_ or royal fern. A species of the former class -(_N. cordata_) might readily enough be mistaken for the young leaves -of the _Scolopendrium_ or hart's-tongue, which hangs out its glossy -green amid the gloom of dank and dripping rocks. There are, besides, -several other genera of ferns in the Carboniferous strata, such as the -_Cyclopteris_ (_C. dilatata_, Fig. 5) or round-leaved fern, and the -_Odontopteris_ or tooth-fern. Most of these seem to have been lowly -plants, like the ferns of our own country. But there was another class -to which no analogue can be shown in Europe. They rose high over their -humbler congeners as lofty trees, and must be studied by a reference to -the existing tree-ferns of intertropical countries. - -[Footnote 17: Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 400.] - -[Illustration: Fig. 7. Living Tree-fern.] - -Tree-ferns flourish in warm climates, and are met with in Brazil, the -East and West Indies, New Zealand, &c. They rise sometimes to the -height of fifty or sixty feet, with a long tapering stem surmounted -by a dense crown of graceful fronds, and might easily be mistaken at -a little distance for palms. All the known species belong to the same -division (_Polypodiaceæ_) with the common polypodium of our road-sides. -In some genera, as the _alsophila_ of the East Indies, the trunk is -ribbed by long creeping branches, or rather rootlets, which descend to -the soil, giving the tree somewhat of the appearance so often seen in -old woods, where venerable fir-trees have been firmly encased by the -bearded stems of the ivy. Another genus, the _Cyathea_, has its stem -covered with oblong scars where leaves were attached, and a circle of -rich outspread fronds surmounts its summit. One of the coal-measure -tree-ferns seems to have resembled this recent type. It is named -the _Caulopteris_ or stalk-fern, and had a thick stem picturesquely -roughened by irregular oblong leaf-scars, that wound spirally from its -base to its point. No specimen has hitherto been found showing the -fronds in connexion with the stem, so that we are still ignorant of -the kind of foliage exhibited by this ancient tree. There can be no -doubt, however, that it was crowned with a large tuft of boughs that -cast their shadow over the sward below, and we may, perhaps, believe -that some of the numerous detached ferns found in the shales of the -coal-series, once formed part of this lofty coronal. - -An important section of the carboniferous plants is embraced under -the generic name of _Calamites_. They had smooth jointed stems, like -reeds, and terminated beneath in an obtuse curved point (Fig. 8), -from which there sprang broad leaflets or rather rootlets. After many -years of research our knowledge of these plants is still very scanty. -Some of them have exhibited a highly-organized internal structure, -from which it appears that they consisted--first, of a soft central -cellular pith; second, of a thick layer of woody tissue; and third, an -external cylinder of strong bark, ribbed longitudinally, and furrowed -transversely. They have been ranked with the common horse-tail of our -ponds, but they would rather appear to belong to a higher family. The -breadth of the stem is very various, some specimens being a foot or -more in diameter, others scarcely half an inch. From the discoveries of -Professor Williamson and Mr. Binny of Manchester, it seems not unlikely -that what we call calamites may be really the inner core of a plant not -yet named, just as a set of fossils were long called _sternbergiæ_, -before they were discovered to be really the pith of coniferous trees. -With regard to the branches of the calamites, Brongniart's conjecture -may be true, that they exist among the group of plants called -_asterophyllites_. It is not unlikely that many dissimilar plants have -been grouped together as calamites, and, on the other hand, that plants -allied to the typical species have been thrown into separate genera. -For it requires but a slight acquaintance with the vegetable kingdom to -know how many forms analogous parts of the same plant may assume, and -how impossible it would often be to guess the real relationship of such -varieties if they were not found growing together on one plant. - -[Illustration: Fig. 8. Terminal portion of a calamite stem.] - -[Illustration: Fig. 9.[18]] - -[Footnote 18: The fossil given in Fig. 9 is named by Lindley (_Foss. -Flo._ t. 15, 16), _Calamites nodosus_. He admits, however, that it was -not found in actual contact with a calamite stem. It has exactly the -contour of an asterophyllite, and might, perhaps, be referred to that -genus. It is inserted here that the reader may see the general form of -the asterophyllites, and the close relationship that subsists between -these plants and the calamites.] - -A remarkably graceful class of the coal-plants are known as -_asterophyllites_. They had slim fluted and jointed stalks, apparently -of humble growth. From each of the joints there sprang two thin -opposite branches with stellate clusters of leaflets arranged round -them at equal distances. If the reader will take a young rush-stalk, -and string along it a number of the flowers of the little star-wort, -keeping them a little distance apart, he may form some idea of the -appearance of a single branch of the star-bearing _asterophyllite_. -Some of the plants embraced under this genus are conjectured to have -been aquatic, spreading out their clusters of leaflets in the green -sluggish water of stagnant pools; but many of them are evidently -related to the calamites, and may possibly have formed part of these -plants. - -Whoever has rambled much in a coal-country, scrambling through briars -and brambles in old quarries, or threading his way among the rocks -of river-courses, must often have noticed, on the exposed surface of -sandstone blocks, dark ribbon-like bands fretted over with little -diamond-shaped knobs. They are so common in some districts, that you -can scarcely light upon a piece of sandstone which does not show one -or more. They belong to a carboniferous plant known as _lepidodendron_ -(Fig. 10) or scaly tree, from the peculiar style of ornamentation which -adorned its bark. Its structure and affinities have puzzled botanists -not a little. A well-preserved specimen reminds one of the appearance -presented by a twig of the Scotch fir, when stripped of its green -spiky leaflets. The scars thus left at the base of the leaflets are -of a wedge-like form, and run spirally up the branch in a manner very -like those on the branches of lepidodendron; and it was accordingly -supposed at one time that the latter plant belonged, or at least was -allied, to the conifers. But the branches of lepidodendron possessed a -peculiarity that is shared in by none of our present coniferous trees. -They were what botanists call _dichotomous_,--that is, they subdivided -into two equal branches, these again into other two, and so on. Their -internal texture,[19] too, differed from that of any known conifer. -The only tribe of existing plants with which the lepidodendron seems -to bear comparison, are the _Lycopodiaceæ_, or club-mosses, of which -we have several species in the moor-lands of our own country. They -are low trailing plants, with moss-like scaly branches, bearing at -their ends shaggy little tufts, whence the popular name of the genus. -In warmer climates, they are both more numerous and attain a larger -size, sometimes standing erect to about the height of an ordinary -gooseberry-bush. But though the lepidodendron appears to have been -allied to these plants in structure, it greatly differed from them in -dimensions. The club-mosses of the coal-measures shot up as goodly -trees, measuring fifty feet and upwards in height, and sometimes -nearly five in diameter. Their general effect must have been eminently -picturesque. A shaggy covering of green spiky leaflets bristled over -their multitudinous pendant boughs; and where on the older stems these -leaflets had decayed and dropped off, the outer bark was laid bare, -fretted over with rows of diamond-shaped or oval scars, separated -by waving lines of ridge or furrow, that wound spirally round the -stem. From not a few of the branches there sprang oblong hirsute -cones called _lepidostrobi_ (Fig. 11), which bore the sporangia, or -seed-cases. These cones are of frequent occurrence in the shales of -the coal-measures, and may be readily recognised. They had a central -axis round which the oblong sporangia were built, the whole being -protected externally by a thick covering of pointed scales, imbricated -like the cone of the Scotch fir. The leaflets of lepidodendron, called -_lepidophylla_, were broader than those of the Scotch fir, and had a -stout mid-rib, which must have given them a rigidity like that of the -araucarian pine a plant they may also have resembled in the dark glossy -green of its leaves. - -[Footnote 19: See Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 436.] - -[Illustration: Fig 10. Lepidodendron Sternbergii.] - -[Illustration: Fig. 11. Lepidostrobus.] - -Of all the common coal-measure plants, there is perhaps none so -abundant as that known by the name of _stigmaria_, or punctured-stem. -It is found spreading out its rootlets for several yards in beds of -shale and under-clay, and sometimes even limestone,[20] while, in -many sandstones, fragments of its blackened stems lie as thickly -strewn as twigs among the woods in autumn. I have said that several -of the plants above described have greatly puzzled botanists. None -of them, perhaps, has given rise to so much conjecture and variety -of opinion as the stigmaria. The history of the discussion regarding -its nature and affinities, would be not a little interesting as an -illustration of the slow hindered progress often attendant on the -researches of science, and an instance of how a few simple facts are -sometimes enough to overturn the most plausible theories and probable -conjectures. Many thousands of specimens had been examined ere one was -found that revealed the true nature of the stigmaria. It was by some -imagined to be a soft succulent marshy plant, consisting of a number -of long branches radiating from a sort of soft disk, like spokes from -the centre of a wheel. Analogies were suggested with dicotyledonous -tribes, as the _cacti_ and _euphorbiæ_, though it was at the same time -admitted that the ancient plant presented appearances which seemed very -anomalous. - -[Footnote 20: The fresh-water limestone of Mid-Calder abounds in -long trailing stems and rootlets of stigmaria, mingled with other -terrestrial plants, and shells of _cyprides_.] - -[Illustration: Fig. 12 Stigmaria rootlets springing from Sigillaria -stem.] - -In the course of an extensive survey of the coal-field of South -Wales, Mr. (now Sir William) Logan ascertained the important fact, -that each coal-seam is underlaid by a bed of clay, in which the stems -of stigmaria, branching freely in all directions, may be traced to -the distance of many feet or even yards. They were recognised as -undoubtedly occupying the site on which they grew, and consequently -each coal-seam was held to rest upon an ancient soil. Some years -afterwards, in making a cutting for the Lancaster and Bolton Railway, -several upright massive stems belonging to a plant called _sigillaria_, -were found to pass downwards into true stigmaria stems (Fig. 12). -There could be no doubt that they were different parts of one and the -same plant. This fact has since been abundantly demonstrated from the -Nova Scotia coal-field. Many sigillariæ have been found there passing -down into the fire-clay below, where they branch out horizontally as -true stigmatiæ. It is evident, therefore, that the stigmaria was the -under-ground portion of a plant, which, judging from the nature of the -soil, and the free mode in which the tender rootlets branched off, -appears to have lived in aquatic or marshy stations. - -[Illustration: Fig. 13. Stigmaria.] - -The stigmaria is too well marked to be readily confounded with any -other coal-measure plant. It had a rounded stem, seldom more than -four or five inches across, which was marked by a series of circular -tubercules with a puncture in the centre, arranged in spiral lines -round the stem. Each of these tubercules is surrounded, in ordinary -specimens, by a circular depression,[21] and the whole plant (if -one may use the comparison) looks as if it had been smitten with -small-pox. From the hollow in the centre of each protuberance, there -shot out a long round rootlet, formerly thought to be a leaf, and -since the tubercules are pretty thickly set, the stigmaria must have -had a somewhat hirsute appearance as it crept through the mud. It -would resemble a thick bearded stem of ivy, save that the fibres, -instead of running up two sides, were clustered all round it. Along -the centre of the root, there ran a woody pith of a harder and more -enduring texture than the surrounding part of the plant. The space -between the outer tuberculed rind and the inner pith, seems to have -been of a soft cellular nature, and to have decayed first, for the pith -is sometimes hollow, and may not unfrequently be seen at a distance -from the centre, and almost at the outer bark--a circumstance that -seems only explicable on the supposition, that while the surrounding -portions were decaying, the firmer pith altered its position in the -hollow stem, sinking to the lower side, if the plant lay prostrate, and -that it did not itself begin to decay until the interior of the stem -had been at least partially filled up with sand or mud, or fossilized -by the infiltration of lime. From the root of the sigillaria, which -has a curious cross-shaped mark on its base, the stems of stigmaria -strike out horizontally, first as four great roots which subdivide as -they proceed. Their subdivisions are dichotomous, each root splitting -equally into two, and thus they want that intricate interlacing of -rootlet which is so familiar to us. The whole disposition of these -under-ground stems is singularly straight and regular, leading us to -believe that they shot out freely through a soft muddy soil. - -[Footnote 21: Such is the usual aspect of the plant. But as the stems -have been, for the most part, greatly flattened by the pressure of -the superincumbent rocks, the sharpness of the pattern has been much -effaced. In some specimens described by Dr. Hooker, as having been -found in an upright position, the external ornamentation presents an -appearance somewhat different. What in the common specimens stand out -as tubercules, are there seen to be deep circular cavities, in which -the shrunk flagon-shaped bases of the rootlets are still observable. -(See above, Fig. 13 _b_, which is taken from one of Dr. Hooker's -plates. For a detailed description of the structure of stigmaria, see -the paper above referred to in the _Geological Survey Memoirs_.) A -very ornate species is mentioned by the late Hugh Miller, in which -each tubercule formed the centre of a sculptured star, and the whole -stem seemed covered over with flowers of the composite order. And what -is, perhaps, still more curious, the stem was seen to end off 7 in an -obtuse point, tuberculed like the rest of the plant.--_Testimony of the -Rocks_, p. 461.] - -Some time ago I chanced to visit the remains of what had once been -a royal residence, and still looked majestic even in decay. It gave -a saddened pleasure to thread its winding stairs, and pass dreamily -from chamber to hall, and chapel to closet; to stand in its gloomy -kitchens, with their huge fire-places, whose blackened sides told of -many a roaring fagot that had ruddied merry faces in days long gone -by; to creep stealthily into the sombre dungeons, so dank, earthy, and -cold, and then winding cautiously back, to emerge into the light of the -summer sun. The silent quadrangle had its encircling walls pierced with -many a window, some of which had once been richly carved; but their -mullions were now sorely wasted, while others, with broken lintels and -shattered walls above, seemed only waiting for another storm to hurl -them among the roofless chambers below. In the centre of the court-yard -stood a ruined fountain. It had been grotesquely ornamented with heads -of lions and griffins, and was said to have once run red with wine. But -it was silent enough now; the hand of time, and a still surer enemy, -the hand of man, had done their worst upon it; its groined arches -and foliaged buttresses were broken and gone, and now its shattered -beauty stood in meet harmony with the desolation that reigned around. -I employed myself for a while in looking over the fragments, marking -now the head of some fierce hippogryph, anon the limbs of some mimic -knight clad in armour of proof, and ere long I stumbled on a delicately -sculptured _fleur-de-lis_, that might have surmounted the toilet-window -of some fair one of old. Turning it over, I found its unhewn side -exhibited a still more delicately sculptured stigmaria. The incident -was certainly simple enough, perhaps even trifling. And yet, occurring -in a spot that seemed consecrated to reverie, it awoke a train of -pleasant reflection. How wide the interval of time which was bridged -across in that sculptured stone! Its one side carried the mind back -but a few generations, the other hurried the fancy away over ages and -cycles far into the dim shadows of a past eternity. The one told of a -land of flowers, musical with the hum of the bee and the chantings of -birds, and gladdened by the presence of man; the other told of a land -luxuriant, indeed, in strange forms of vegetation--huge club-mosses, -tall calamites, and waving ferns--yet buried in a silence that was only -broken fitfully by the breeze as it shook the spiky catkins or the -giant fronds of the forest. The _fleur-de-lis_ recalled memories of -France--the sunny land of France--which stood out so brightly in the -dreams of our school-days; the stigmaria conjured up visions of a land -that was never gazed on by human eye, but rolled its rich champaign -during the long ages of the Carboniferous era, and sometimes rises up -dimly in the dreams of our maturer years. Between these two epochs -how many centuries, how many cycles must have slowly rolled away! The -_fleur-de-lis_ was carved but yesterday; the stigmaria flourished when -the earth was young, and had seen scarcely a third part of its known -history. - -I have said that the stem of the stigmaria is called sigillaria. -The name may be translated _signet-stem_,[22] and has reference to -one of the distinguishing peculiarities of the plant. About twenty -British species are enumerated, some of them very dissimilar, yet -they all agree in having long fluted stems with parallel rows of -prominent seal-like tubercules. The sigillaria differed so widely in -its whole contour and ornamentation from every living plant, that it -is impossible to convey an idea of its form by reference to existing -vegetation. Some of the species, as _S. organum_ (Fig. 14), had their -trunks traversed longitudinally by broad ridges separated by narrow -furrows. Along the summit of each ridge there ran a line of tubercules, -set regularly at distances varying from a third or a quarter of an inch -to close contact. One may sometimes see no unfair representation of -the bark of this ancient tree, when looking at a newly ploughed field -in spring-time, having each of its broad ridges dotted with a row of -potato sacks. Other species, while exhibiting the same plan, differed -not a little in the details. In some the tubercules are round, in -others angular, and in a third set double or kidney-shaped. In some -they are far apart, in others they are strung together like a chain of -beads. Sometimes they exist as mere specks, while occasionally they -broaden out so as to equal in width the ridge that supports them. -One species (_S. reniformis_), instead of the broad ridge and narrow -furrow, exhibits an arrangement exactly the reverse. It looks not -unlike a cast of the species first described, save that its broad flat -furrows support rows of much larger tubercules. The breast of a lady's -chemisette, with a thick-set row of buttons down each plait, would -be somewhat like this species of sigillaria, with this difference, -however, that the buttons on the plant were of a form that does not -appear as yet to have come into fashion among the fair sex. Yet they -had no little elegance, and like many other objects in the geological -storehouse, might be a useful model for our students of design. They -were neither round nor quite oval, but rather of a kidney-shape, or -like a double cherry. - -[Footnote 22: The word sigillaria is really plural, and was used by the -Romans to denote the little images which friends were wont to present -to each other at the end of the Saturnalia. They answered pretty nearly -to christmas-boxes and new year's gifts among ourselves. It is not -uninteresting thus to find among the hard dry names of science, one -that two thousand years ago was synonymous with all the kindliness of -friendship.] - -[Illustration: Fig. 14. Sigillaria, with black carbonized bark -partially removed.] - -There can be no doubt that these tubercules must once have supported -leaflets. They are true leaf-scars, like those on the Scotch fir, and -the lozenge-shaped knobs on the bark of lepidodendron. But of the form -of these leaves we are still in ignorance, for no part of the plant, -save the stem and roots, has yet been found. The sigillaria must have -been a tree that could not long withstand maceration, for not only are -its leaves gone, but, in many cases, the outer bark has partially or -wholly decayed, leaving a scarcely distinguishable mass of carbonized -matter.[23] When this outer rind is peeled off, the inner surface of -the stem is seen to be ridged, furrowed, and tuberculed in the same -way, but the markings are much less distinct than on the outside. The -bark sometimes attains the thickness of an inch, and is always found -as a layer of pure coal enveloping the stem where it stands erect, or -lying as a flat cake without any central cylinder where the stem is -prostrate. (See Fig. 14.) - -[Footnote 23: Another proof of the looseness of the texture of this -ancient vegetable may be gathered from the almost invariable truncation -of even the largest erect stems; they are snapped across at the height -of a few feet from their base. The famous "Torbanehill Mineral" -contains many such fragmentary stems, often of considerable thickness. -Their interior consists of the same material as the surrounding bed, -and displays many dissevered plants that may have been washed into -the decaying trunks. For the internal structure of sigillaria see Dr -Hooker's _Memoir_, and the authorities therein cited.] - -Another remarkable feature in this carboniferous plant is that it -appears to have had no branches along its stem. Trunks have been found -four and five feet in diameter, and have been traced to a distance of -fifty, sixty, and even seventy feet, without any marks of branches -being detected. Brongniart examined the portion of one stem, which, -at its thicker end, had been broken across, but still measured a -foot in breadth. It ran for forty feet along the gallery of a mine, -narrowing to a width of not more than six inches, when it divided into -two, each branch measuring about four inches across. The sigillaria -stems, accordingly, must have shot up, slim and straight, to a height -of sometimes seventy feet before they threw out a single branch. We -know nothing of the coronal of these strangely-formed trees. From -Brongniart's observations, it would seem that the upper part of the -stem, like that of the lepidodendron, was dichotomous, that is, it -branched out into two minor stems; but how these were disposed is -unknown. We are wholly ignorant, too, of the foliage of these branches, -though, from the general structure of the plant, as well as from the -number of fern-fronds often found around the base of the stems, it has -been conjectured that the sigillaria was cryptogamous, and, like the -tree-ferns, supported a group of sweeping fronds. If so, it differed in -many respects from every known member of the cryptogamic tribes. - -Putting together, then, all that we know of the exterior of the -sigillaria, we find that it was a tall slender tree, with, palm-like, -a clump of foliaged branches above, its stem bristling thickly, in at -least its upper part, with spiky leaves, and its roots equally hirsute, -shooting out to a distance of sometimes forty feet through the soft -muddy soil. Future researches may bring us better acquainted with this -ancient organism. In the meanwhile, enough of it is known to mark it -out as one of the most ornate forms of vegetation that the world has -ever seen. - -In addition to the above, the coal strata have yielded many other -fragmentary remains, to which names have been given, but of which very -little is known. It is pleasant, amid such a wide sea of doubt and -uncertainty, to alight upon some well-known form of whose affinities -there can be no question, since it still finds its representatives in -living nature. Of such a kind are the coniferous stems occasionally met -with in the sandstones of the coal-measures. . - -It is now many years since the operations of the quarryman in the -carboniferous sandstones of Edinburgh and Newcastle disclosed the -remains of huge gnarled trunks deeply imbedded in the rock. The -neighbourhood of the latter town yielded, in 1829,[24] the stem of -a tree seventy-two feet long, without branches, but roughened with -numerous knobs, indicative of the places whence branches had sprung. -At Craigleith, near Edinburgh, a trunk thirty-six feet long, and three -feet in diameter at the base, was disinterred in the year 1826. Since -then, several others have been found in the same neighbourhood; some -of them sixty and even seventy feet in length, and from two to six in -breadth. They were, for the most part, stripped of roots and branches, -and lay at a greater or less angle among the white sandstone beds, -which they cut across obliquely. It was unknown for some time to what -division of the vegetable kingdom these trunks should be referred. -Their irregular branched surface and undoubted bark indicated a higher -kind of structure than that possessed by any of the other carboniferous -plants; but the conjecture remained unverified until an ingenious -and beautiful method was discovered of investigating their internal -organization. Two Edinburgh geologists, Mr. Nichol and Mr. Witham, -succeeded in obtaining slices of the plants sufficiently transparent to -be viewed under the microscope by transmitted light, and in this way -their true structure was readily perceived. The method of preparing -these objects was simply as follows:--A thin slice of the plant to be -studied was cut by the lapidary, or detached by the hammer. One side -having been ground down smooth, and polished, was cemented by Canada -balsam to a piece of plate-glass, and the upper surface was then ground -down and polished in like manner, so as to leave the slice no thicker -than cartridge-paper.[25] When the preparation was then placed under -a magnifying power, the minute cells and woody fibre of the plant -could be detected as clearly as those of a recent tree. The Craigleith -fossils were in this way recognised as belonging to the great -coniferous family, and to that ancient[26] division of it which is, at -the present day, represented by the pine of Norfolk Island--"a noble -araucarian, which rears its proud head from 160 to 200 feet over the -soil, and exhibits a green and luxuriant breadth of foliage rare among -the coniferæ."[27] Some of these plants have yielded faint traces of -the annual rings shown so markedly in the cross section of our common -forest-trees; whence it would appear, that even as far back as the -times of the coal-measures, there were seasons of alternate heat and -cold, though probably less defined than now. - -[Footnote 24: Witham's _Foss. Veget._ p. 31.] - -[Footnote 25: For a more detailed description of the process, see -Witham's _Foss. Veget._ p. 45.] - -[Footnote 26: The solitary lignite of the Lower Old Red Sandstone, -seems to have been araucarian. Miller's _Footprints of the Creator_, p. -203.] - -[Footnote 27: _Footprints of the Creator_, p. 192.] - -These coniferous trees do not appear to occur among the erect stems of -the coal-beds, at least they are very rare in such a position. Their -more usual appearance is that of drifted, branchless trunks, imbedded -along with other fragmentary plants in deep strata of sandstone. -They probably grew on higher ground than the swamps which supported -the sigillariæ and their allies, and might have been carried down by -streams, freighted out to sea, and so deposited among the sediment that -was gathering at the bottom. - -The remains of cycadaceous plants have been described among the -vegetation of the coal-measures; but only fragments have as yet been -found. The modern _Cycadeæ_ are low shrubs or trees, with thick stems -of nearly uniform breadth, crowned with a dense clump of spreading -fronds which resemble both those of the palms and the ferns. They are -natives of the warmer regions of both hemispheres. - -So long ago as the year 1835, Dr. Lindley figured a flower-like -plant, to which he gave the name of _Antholites_, ranking it among -the _Bromeliaceæ_, or pine-apple group. It was afterwards suspected -by Dr. Hooker to belong rather to the coniferæ; and he supposed that -the so-called flowerets might be really tufts of young unexpanded -leaves. An examination of a more perfect specimen, however, has induced -that distinguished botanist to alter his convictions and return to -the original decision of Lindley, that the antholites are really -flowers.[28] In Fig. 15, therefore, which represents one of these -coal-measure fossils, the reader beholds the oldest flower that has -yet been found; and surely it is of no little interest to know, that -amid the rank, steaming forests of the Carboniferous era, with all -their darkness and gloom, there were at least some flowers--flowers, -too, that were allied to still living forms, and breathed out a rich -aromatic fragrance. - -[Footnote 28: See Dr. Hooker's remarks in the Supplement to the fifth -edition of Lyell's Manual, p. 31.] - -[Illustration: Fig. 15.--Antholites.] - -In fine, from all the genera and species of plants that have been -detected in the strata of the coal-measures, it would appear that the -flora of that ancient period was in a high degree _acrogenous_--that -is to say, consisted in great measure of ferns, club-mosses, and other -members of the great group of plants known as _acrogens_. This word -literally means _top-growers_, and is applied to those plants which -increase in height, but not in width, since they attain at first -nearly their ultimate diameter. Such plants occupy a low position in -the botanical scale. Mingled with the numerous genera of carboniferous -ferns and club-mosses, we find the remains of a much higher grade -of vegetation--that of the _gymnogens_, or plants that bear naked -seeds--such as the firs and pines. There also seem to have been a -few _endogenous_ flowering plants. Viewing, then, this flora on the -whole, it presents us with many striking resemblances to certain -botanical regions of the present day. Many of the tropical islands -abound in ferns, and contain very few flowering plants. But New Zealand -affords perhaps the closest parallel. That island is in certain parts -highly mountainous, its loftiest summits being covered with glaciers. -The hills throughout large districts are bare, or covered with a -scanty herbage, while in other localities they are densely clothed -with forests of pine, beech, and other trees. These forests sweep -on to the lower grounds, where they are replaced by a thick growth -of fern and flax-plant intermingled with dragon-trees and graceful -tree-ferns, while the more swampy regions support a rich profusion of -reeds and rushes. Such a condition of things affords a close parallel -to the probable vegetation of the Carboniferous period--an immense -preponderance of ferns and arborescent acrogens, with an intermixture -of large coniferous trees. From the general scantiness of a flora -where ferns predominate, it has been argued that the swamps of the -coal-measures nourished a luxuriant repetition of comparatively few -species; and this hypothesis also receives confirmation from the -vegetation of New Zealand. Another deduction founded on the resemblance -of the ancient to the modern flora, refers to the conditions of heat -and moisture. It has been inferred that the climate of the coal period -was equable and humid, like that of New Zealand--a supposition much -more natural and simple than that, once so much in vogue, of a heated -atmosphere densely charged with carbonic acid gas. That the air of the -Carboniferous period differed in no material respect from the air of -the present day, seems at last proved by the remains of air-breathing -animals having been found among the coal-beds; and there seems no -reason why the higher mountain-tops of the same epoch may not have been -clothed with glaciers as those of New Zealand are. As yet we have no -evidence of the fact, but it is by no means beyond the possibility of -proof.[29] - -[Footnote 29: See Professor Ramsay's suggestive Memoir on Permian -Breccias in _Quarterly Journal of the Geological Society_, vol. xi. p. -185.] - - - - -CHAPTER V. - - - Scenery of the carboniferous forests--Contrast in the - appearance of coal districts at the present day--Abundance - of animal life in the Carboniferous era--Advantages - of palæontology over fossil-botany--Carboniferous - fauna--Actiniæ--Cup-corals--Architecture of the present - day might be improved by study of the architecture of the - Carboniferous period--Mode of propagation of corals--A - forenoon on the beach--Various stages in the decomposition of - shells--Sea-mat--Bryozoa--Fenestella--Retepora--Stone-lilies-- - Popular superstitions--Structure of the stone-lilies--Aspect - of the sea-bottom on which the stone-lilies - flourished--Sea-urchins--Crustacea, their high - antiquity--Cyprides--Architecture of the Crustacea and - mollusca contrasted--King-crabs. - -The forms of vegetation that flourished during the Carboniferous era -seem to have been in large measure marshy plants, luxuriating on low -muddy delta-lands, like the cypress-swamps of the Mississippi, or the -Sunderbunds of the Ganges. We can picture but faintly the general -scenery of these old forests from the broken and carbonized remains -that have come down to us. But though perhaps somewhat monotonous -on the whole, it must have been eminently beautiful in detail. The -sigillariæ raised their sculptured stems and lofty waving wreaths of -fronds high over the more swampy grounds, while a thick underwood of -ferns and star-leaved asterophyllites clustered amid the shade below. -The lepidodendra shot forth their spiky branches from the margin of -green islets, and dropped their catkins into the sluggish water that -stole on among the dimpled shadows underneath. Tree-ferns spread out -their broad pendant fronds, and wrapt the ground below in an almost -twilight gloom, darker and deeper far than that - - "Hospitable roof - Of branching elms star-proof," - -which rose so often in the visions of Milton; or that "graceful arch" -so exquisitely sung by Cowper, beneath which - - "The chequered earth seems restless as a flood - Brushed by the wind. So sportive is the light - Shot; through the boughs, it dances as they dance, - Shadow and sunshine intermingling quick, - And darkening and enlightening, as the leaves - Play wanton, every moment, every spot." - -Thickets of tall reeds rose out of the water, with stems massive as -those of our forest-trees, encircled at regular distances by wreaths of -pointed leaflets, and bearing on their summits club-like catkins. Far -away, the distant hills lay shaggy with pine-woods, and nursed in their -solitudes the springs and rivulets that worked a devious course through -forest, and glen, and valley, until, united into one broad river, they -crept through the rich foliage of the delta and finally passed away out -to sea, bearing with them a varied burden of drift-wood, pine-trees -from the hills, and stray leaves and cones from the lower grounds. - -How different such a scene from that now presented by the very same -areas of country! These old delta lands are now our coal-fields, and -have exchanged the deep stillness of primeval nature for the din and -turmoil of modern mining districts. In these ancient times, not only -was man uncreated, but the earth as yet lacked all the higher types of -vertebrated being. None of the animals that we see around us existed -then; there were no sheep, nor oxen, horses, deer, nor dogs. Neither -were the quadrupeds of other lands represented; the forests nourished -no lions or tigers, no wolves or bears, no opossums or kangaroos. In -truth, the land must have been a very silent one, for we know as yet of -no animated existence that could break the stillness, save perchance -some chirping grasshopper, or droning beetle, or quivering dragon-fly. -No bee hummed along on errands of industry; it is doubtful, indeed, -whether honey-yielding flowers formed part of the carboniferous flora; -no lark carolled blithely in the sky, nor rook croaked among the -woods. All was still; and one might, perhaps, have stood on some of -those tree-crested islets, and heard no sound but the rippling of the -water along the reedy and sedgy banks, and the rustling of the gloomy -branches overhead. - -To one who muses on these bygone ages it is no unimpressive situation -to stand in the midst of a large coal district and mark its smoking -chimneys, clanking engines, and screaming locomotives, its squalid -villages and still more squalid inhabitants, and its mingled air of -commercial activity, physical wretchedness, and moral degradation. -It is from such a point of view that we receive the most forcible -illustration of those great changes whereof every country has been the -scene, and which are so tersely expressed by one who has gazed on the -revelations of geology with the eye of a true poet-- - - "There rolls the deep where grew the tree. - O earth, what changes hast thou seen! - There where the long street roars, bath been - The stillness of the central sea." - -But the lifelessness of the carboniferous forests was amply compensated -by the activity that reigned in river, lagoon, and sea. Coral groves -gleamed white beneath the waves, fishes of many a shape disported in -stream and lake, and the bulkier forms, armed in massive plates of -bone, ascended the rivers or haunted the deeper recesses of the open -sea. In some beds of rock the remains of these various animals lie -crowded together like drifted tangle on the sea-shore, and the whole -reminds us of a vast cemetery or charnel-house. The bones lie at all -angles, many of them broken and disjointed as though the owner had died -at a distance, and his remains, sadly mutilated on the way, had been -borne to their last resting-place by the shifting currents; others -lie all in place, covered with their armature of scales, as though -the creature, conscious of approaching dissolution, had sought out a -sheltered nook and there lain down and died. It is not uninteresting -or uninstructive to tract; out in an old quarry stratum above stratum, -each with its groups of once living things. I know of few employments -more pleasant than to sit there, amid the calm stillness of a summer -evening, when the shadows are beginning to steal along the valleys and -creep up the hill-sides, and in that dim fading light to try in fancy -to clothe these dry bones with life, to picture the time when they -lived and moved in the glassy depths of lakes and seas, or amid the -solitudes of jungles and forests, and so to spend a pleasant hour in -reverie, till roused at last by the vesper song of the lark, or the low -meanings of the night wind as it sighs mournfully through the woods. - -The study of fossil animals embraces a much greater range of subject -than that of fossil plants. The _fauna_ of any particular geological -formation, that is to say, its embedded animal remains, for the most -part vastly exceeds in number its _flora_, or vegetable remains, and is -likewise usually better preserved. About the nature and affinities of -several tribes of fossil plants there hangs an amount of uncertainty -which renders them a dubious guide to the climatal and other conditions -of the period and locality in which they lived. Generic distinctions -among living plants often rest on the character of those parts which -are the most perishable, such as flowers and seed-vessels. These -delicate structures we, of course, can hardly look to find preserved -in the rocks, and we have in place of them only detached leaflets, -twigs, branches, and stems, often sorely mutilated in outward form, and -presenting no trace of internal organization. But the tribes of the -animal kingdom have, for the most part, harder frameworks. The minute -infusoria, which by their accumulated remains help to choke up the -delta of the Nile, and swarm by millions in every ocean of the globe, -have their silicious or calcareous shells so minute that Ehrenberg -has estimated a cubic inch of tripoli to contain forty-one thousand -millions of them. The polypi have their internal calcareous skeletons, -which abound in all the older limestones, and form the coral reefs of -the present day. The mollusca, too, though, as their name imports, they -have perishable bodies, are yet, in most cases, furnished with hard -calcareous shells, that indicate by their various modifications of form -and structure, the character of the animal that lived within them. They -are found in all the formations from the earliest upwards, and as they -vastly exceed in numbers all the other classes with which the geologist -has to deal, they form the larger part of that basis of evidence from -which he interprets the past history of organized existence. Hugh -Miller loved to talk of them as the "shell alphabet," out of which the -language of palæontological history should be compiled. The vertebrata, -too, all have their hard skeletons, easily capable of preservation, -whether it be in the form of the massive exo-skeleton of bone that -characterized the older ganoidal fishes, or the compact endo-skeleton -of the reptiles and mammals. A greater amount of attention is, -therefore, due to the study of fossil animals, since they thus not -only far exceed fossil plants in number, but possess a higher value as -evidence of ancient physical conditions. - -The _fauna_ of the Carboniferous system is a very numerous one, -exhibiting specimens of almost every class of animal life, from the -tiny _foraminifer_ up to the massive bone-covered sauroidal fish, -and even to occasional traces of true reptilian remains. By far the -larger number are peculiar to the sea, such as the molluscan tribes -and corals; others are undoubtedly terrestrial organisms, such as the -wings and wing-sheaths of several kinds of insects; while some appear -to be peculiar to fresh or brackish water, such as shells allied to -our _unio_ or river-mussel, and minute crustaceous animals known as -_cyprides_, of which we have still representatives in our ponds and -ditches. It is plain, then, that if we rightly ascertain the class or -family to which one of these fossils belonged, we shall obtain a clue -to the history of the physical geography, during Carboniferous times, -of the district in which the fossil occurs. A bed of unios will tell -us of old rivers and lakes that spread out their blue waters where -now, perchance, there lie waving fields of corn. A bed of corals and -stone-lilies will lay before us the bottom of an ancient ocean that -rolled its restless waves where to-day, perhaps, the quarryman plies -his task amid the gloom of dark pine-woods. In short, these organic -remains are to the history of the earth what ancient monuments are to -the history of man. They enable us to trace out the varied changes -of our planet and its inhabitants down to the human era, just as the -wooden canoe, the flint arrow-head, the stone coffin, the bronze sword, -the iron cuirass, the ruined abbey, and the feudal castle, teach us the -successive stages of progress in the history of our own country. - -Whoever has spent a few days on some rocky coast, must have noticed -adhering to half-tide stones numerous solitary _actiniæ_. Arrayed in -all the colours of the rainbow--purple, green, and gold--these little -creatures hang out their tentacles like so many flowers, and have hence -received the popular name of sea-anemones. Their internal structure is -no less beautiful. They resemble so many large plump gooseberries, and -consist of a little sack suspended within a larger one. The outer sack -is fringed along its upper edges with one or more rows of slim hollow -tentacles, which diverge outwards like the petals of the daisy, and can -be contracted at pleasure so as somewhat to resemble the daisy when -folded up at sunset. The inner sack, which forms the stomach of the -animal, has a short opening or gullet, at the upper part of which is -the mouth lying in the centre of the cavity surrounded by the fringes -of tentacles. The inner sack is connected with the outer by means of -thin membranes, like so many partition-walls, which radiate inwards -like spokes towards the axle of a wheel. The space between each of -these membranes, or lamellæ, forms an independent chamber, but it has -a communication with those on either side by a window in each wall, -and further opens upwards into the hollow tentacles, which, with minute -orifices at their outer points, may be compared to chimneys. These -chambers form the breathing apparatus of the little creature. Sea-water -passes down through the tentacle into the hollow chamber below, -whence, by the constant action of minute hairlike cilia that line the -walls like tapestry, it is driven through the window into the next -chamber, thence into the next, and so on, passing gradually through the -tentacles back to the sea. - -The actiniæ are of a soft perishable substance, but many of the other -_Anthozoa_, or flower-like animals, have hard calcareous skeletons. Of -such a kind are the polypi that in the Pacific Ocean have raised those -stupendous reefs and islands of coral. It does not appear that, during -the Carboniferous period, there existed any reef-building zoophytes, -but some of the most abundant forms of life belonged to a kindred -tribe, and are known by the name of _Cyathophyllidæ_, or cup-corals. - -As the name imports, the typical genus has a general cup-shaped form, -but this is liable to many aberrations in the cognate genera. The -younger specimens of one species (_Cyathopsis fungites_) have a curved -outline somewhat like the bowl of a tobacco-pipe, whence the quarrymen -know them as pipe-heads. The older individuals are generally more or -less wrinkled and twisted, sometimes reaching a length of eight or nine -inches, and have been named by the workmen _rams'-horns_. - -[Illustration: Fig. 16.--Cyathopsis (clisiophyllum ?) fungites.] - -The annexed figure (Fig. 16) shows their general appearance and -structure. The lower end was fixed to the rock like the flat -sucker-like disc of the actinia. Around the outer margin there diverged -one or more rows of slim tentacles, hollow, soft, and retractile, like -those of the actinia. From the margin to the centre there radiated -more than a hundred lamellæ, but these differed from the corresponding -membranes of the modern animal, inasmuch as they were strengthened -internally by a skeleton of hard carbonate of lime; and to this -difference we owe their preservation. They stand out in high relief -upon weathered specimens, showing the long, narrow chambers that ran -between them. Their walls were once doubtless hung with countless -vibratile cilia, and perhaps pierced each with its window, through -which the currents of water passed in their ceaseless progress to and -from the sea. At the centre lay the mouth, communicating by a short -gullet with the stomach, which occupied the central portion of the -animal, and from the outer walls of which the lamellæ diverged like so -many buttresses. In its youngest stages, the animal occupied the whole -length of the cup, but, us it increased in size, it gradually retreated -from the narrow end, which was then divided off by a thin calcareous -membrane. At each successive stage of its growth, a new membrane was -added, each further and further from the lower end, so that eventually -the creature left below it a series of empty chambers all firmly -built up. Thus, in a specimen six or eight inches long, there would -in reality only be a small part tenanted--in fact merely the upper -floor--all the lower storeys remaining silent and uninhabited. The -house of this old-world architect differed widely in one respect from -human dwellings. Man begins his basement story of the same dimensions -as those that are to succeed it, or, if any difference is made at all, -the upper floors are built each less than the one below it, so that -the whole structure tapers upward to a point, as in the Pyramids. But -the cyathopsis reversed this latter process; it inverted the cone, -commencing the smallest chamber at the bottom, and placing the widest -at the top. Indeed, one is sometimes puzzled to conjecture how so bulky -a building could be securely poised on so narrow a basis, and it is -certainly difficult to see how the creature could move about with such -a ponderous load to drag along. The snail carries his house on his -back, yet it is a slim structure at the best; but the cup-coral must -not merely have carried his house, but some dozen or two of old ones -strung one after another to his tail. Perhaps, though free to move -about and try change of residence in its youthful days, the creature -gradually settled down in life, and took up its permanent abode in some -favourite retreat, the more especially as in process of time it became -what we should call a very respectable householder. - -Allied to the cyathopsis is another and still more beautiful coral, -described so long ago as the latter part of the seventeenth century -by the Welsh antiquary and naturalist, Lhwyd, under the name of -_Lithostrotion_. Although many perfect specimens of it have been -found, and it is usually as well preserved as any of its congeners, -men of science have been sadly at a loss what to call it. Four or -five synonyms may be found applied to it in different works on -palæontology. There seems now, however, a tendency to return to the -name that old Lhwyd gave it two centuries ago; the family to which -it belongs, and of which it is the type, has accordingly been termed -the _Lithostrotionidæ_, and the species in question _Lithostrotion -striatum_ (Fig. 17). It differed from the cyathopsis in several -respects, but chiefly in this, that it lived in little congregated -groups or colonies, whereas the cyathopsis, like our own actinia, dwelt -alone. - -[Illustration: Fig. 17.--Lithostrotion striatum.] - -Each of these colonies was formed of a cluster of hexagonal, or rather -polygonal pillars, fitting closely into each other, like the basaltic -columns of Fingal's Cave, and springing from a common base at the sea -bottom.[30] Each pillar constituted the abode of a single animal, -and resembled generally the stalk of the cyathopsis. It had the same -minute diverging partitions running from the outer walls towards the -centre, and the same thin diaphragms, which, stretching horizontally -across the interior of the column at short intervals, marked the -successive stages of the animal's growth. Within these partitions, -which vary from forty to eighty in number, there runs an inner circular -tube with thin lamellæ and diaphragms. The exterior of the columns is -ribbed longitudinally by a set of long fine striæ, which give somewhat -the appearance of the fluting on a Corinthian pillar. The columns, -moreover, are not straight, but have an irregular, wrinkled outline, -so that, by a slant light, they look like some old pillar formed of -many layers of stone, the joints of which have wasted away, producing -an undulating profile in place of the original even one. But in these -ancient coral columns there is no blunted outline, no worn hollow; -the sculpturing stands out as sharp and fresh, and the wavy curves as -clearly defined, as though the creature had died but yesterday. They -resemble no order of human architecture, save faintly, perhaps, some of -the wavy outlines of the Arabesque. - -[Footnote 30: Sir Roderick Murchison figures in his _Siluria_, p. 282, -a gigantic specimen, which measured two feet four inches in width.] - -Despite all the improvements and inventions of modern times, classic -architecture has made no progress since the days of Pericles. All that -we do now is but to reproduce what the Greeks created 2000 years ago, -and he is reckoned the best architect who furnishes the best imitation. -Our architects might find some useful hints, however, by studying the -lowlier orders of nature. They would see there patterns of beauty far -more delicate than the Grecian capital, and more light and airy than -the Gothic shaft. And whether or not they could found a new order of -architecture, they could not fail to discover many modifications and -improvements upon some of the old. They could not readily light upon -a more graceful form than that of the lithostrotion, would they but -picture it as it grew at the bottom of the old carboniferous sea. A -group of hexagonal pillars, firmly compacted together like those of -the Giant's Causeway, or Fingal's Cave, rose from a white calcareous -pediment, as columns from the marble steps of an Athenian temple. Each -side of the pillar had a wavy undulating surface, delicately fluted by -long slender striæ, the whole being so arranged that the convexities -of one surface fitted into the sinuosities of the adhering one. Each -pillar was crowned above by a capital, consisting of the soft vibratile -tentacles of the animal, that hung over like so many acanthus leaves. -Of the form of these tentacles, their design and grouping, we know -nothing save what may be gathered from the analogy of living corals. -There can be little doubt, however, that, like the flower-shaped buds -of the existing reef-building polyps, they must have been eminently -beautiful, and in strict keeping with the graceful column which they -crowned. - -Another kindred form was that known as the _lithodendron_. It, too, -grew in colonies, and seems to have closely resembled the last, save -that the pillars, in place of being six-sided, were round. I have seen -a bed of these corals several yards in extent, and seven or eight -inches deep, where the individuals were closely crowded together, so -as to resemble a series of tobacco-pipe stems, or slim pencils set on -end. The tubes, however, were not all quite straight; many being more -or less curved, and sometimes crossing their neighbours obliquely. -The internal arrangement was on the same plan as in the two previous -corals. The same numerous partitions ran from the exterior wall -towards the central tube, the same thick-set diaphragms crossed the -entire breadth of the column, imparting the same minute honey-combed -appearance to a cross section. The exterior of the column (in _L. -fasciculatum_) was likewise traversed by the same longitudinal striæ. - -Both these corals seem to have been _fissiparous_, that is to say, they -propagated by splitting into two parts, each of which formed the base -of a new column with a new animal. The evidence for this statement -rests on the fact, that many of the tubes are seen to bifurcate in -their course, so that two new tubes are produced equal in size and -completeness to the old one from which they proceed. Another mode of -generation which, in at least its earlier stages, would produce a -somewhat similar appearance is called _gemmation_, and consists in -the protrusion of a bud or gemmule from the side of the animal, which -shortly develops into a new and perfect individual. It is probable, -however, that the ordinary mode of propagation among these old corals -was the usual one by impregnated ova. These ova, like those of our -sea-anemones, were probably generated within the partitions, between -the central stomach and the outer wall, whence they passed down into -the stomach, and were ejected by the mouth of the parent as little -gemmules, furnished with the power of locomotion by means of vibratile -cilia. Some of the _Medusa_ family possess this three-fold mode of -propagation; but, in all, the last-mentioned is the most usual. - -Has the reader ever stretched himself along the shore, while, perhaps, -a July sun blazed overhead, and a fitful breeze came over the sea, just -strong enough to chase ashore an endless series of rippling wavelets, -and breathe over his temples a delicious and refreshing coolness? -Thus placed, and gazing dreamily now, perchance, at the distant sails -like white specks along the boundary line of sea and sky; now at the -gulls wheeling in broad circles through the air, and shooting swift -as arrows down into the blue water, he must often have turned to look -for a little at the sand which, heaped up in little mounds around him, -formed a couch well-nigh as soft as the finest down. Many a varied -fragment entered into the composition of that sand. Mingled among the -minuter quartzy particles lay scores of shells, some with the colour -not yet faded, and the valves still together--the delicate tellina, -with its polished surface, and its flush of pink; the cardium with its -strong white plaited sides, and the turritella with its circling spire; -some were worn down and sorely effaced, others broken into fragments -by the ceaseless grinding of the waves. It was pleasant labour in such -a sultry noon to pick out the shells of one species in all stages of -decay. The _Trochus lineatus_, or Silver Willie, as young ramblers by -the sea-shore love to call it, showed well the process of destruction. -The perfect shell, cast ashore, perhaps, by the last storm, and still -uninjured by the tides, displayed its russet epidermis, or outer skin, -covered with fine brown zig-zag lines, running across the whorls from -the creature's wide pearl-lined mouth to the apex. A second shell -exhibited a surface that had begun to suffer; the point had been -divested of its thin outer skin, and laid bare the silvery coating of -pearl below. A third had undergone a still longer period of abrasion, -for the whole of the epidermis was gone, and the surface gleamed with -a pearly iridescence. In yet a fourth, this bright exterior had been -in large measure worn away, and the blunted, rounded shell displayed -the dull white calcareous substance of which it was mainly built up. -But there were other objects of interest in the sand: bits of tangle, -crusted over with a fine net-work of gauze, and fragments of thin -leaf-like membrane, consisting of a similar slender network known -popularly as the _sea-mat_, occasionally turned up among the pebbles -and shells. No one who met with these organisms for the first time -could fail to be struck with the extreme delicacy of finish, if one -may so speak, that characterizes them. And yet he might be puzzled to -know what to make of them. The leaf-like membrane, at a first glance, -looks not unlike some of the flat-leaved algæ, and such the observer -might readily take them to be. Such, too, they were long regarded by -naturalists; but a more careful examination of them showed that the -so-called plants really belonged to the animal kingdom, and that the -supposed leaves were, in truth, the organic dwelling-places of minute -zoophytes, of which many hundreds lay grouped together on every square -inch. For many years these little creatures were called "celliferous -corallines," and classed among the polypi, that great tribe which has -its representatives in every ocean, from the coral reefs of the Pacific -to the little bell-shaped _hydra_ amid the tangle of our own seas. -But the microscope--that lamp which lights us into the inner recesses -of nature--revealed at last their true character. Fixed to one spot, -living in communities, and exceedingly minute, in short, with many of -the outward features of the true corallines, they were yet found to -possess a structure so complex and highly organized, as to entitle them -to rank among the higher tribes of the invertebrate animals, and they -are now accordingly pretty generally subjoined to the mollusca, under -the name of _Bryozoa_. - -Each bryozoon consists externally of a single horny or calcareous cell, -sometimes furnished with a valve-like lid that folds down when the -animal withdraws itself. When danger is past, and the creature begins -again to emerge, the upper parts, which were drawn in like the inverted -finger of a glove, are pushed out until a series of tentacles, covered -with minute hair-like bodies, called cilia, are expanded. The vibratile -motion of these cilia causes a constant current in the direction of -the mouth, which lies in the centre of the hollow whence the tentacles -spring; animalcules are in this way brought in rapid succession within -reach of the mouth, and form a never-failing source of nourishment. -The interior is greatly more complex than that of the _polypi_. The -stomach is connected above with a cavity like the gizzard of a bird, -furnished with pointed sides, which serve to triturate the food before -it passes into the stomach. There is also a distinct intestine. The -muscular action for the expansion and retraction of the animal is -highly developed, and the generative system is a greatly more complex -one than that of the polyps already referred to. In short, however -closely they might be thought to resemble the corals in outward form, -their internal structure undoubtedly links them with a much higher type -of organization, and justifies the naturalist in subjoining them as a -sub-order to the mollusca. - -The cells are grouped at short intervals along a horny or calcareous -substance, that sometimes encrusts sea-weed, or spreads out as a flat -leaf-like membrane, or rises into cup-shaped or dendritic forms. A -series of cells constituting a separate and independent colony, is -termed a polypidom. The cells are further connected together by an -external jelly-like integument, in which they are sunk, and which -serves to secrete the calcareous particles from the sea. - -It is interesting to know that creatures so minute and yet so complexly -organized, existed abundantly in the seas of the Carboniferous period. -No less than fifty-four species are enumerated as having been obtained -from the carboniferous strata of the British Islands, and scarcely a -year passes without one or two new species being added to the list. -The most frequent belong to the genus _Fenestella_, or little window, -a name indicative of the reticulated grouping of the branches like the -wooden framework of a window. Each of these branches, or interstices, -as they are called, was more or less straight, being connected with -that on either side by a row of transverse bars, just as the central -mullion of an abbey window is connected with the flanking ones by means -of cross-bars of stone. Not unfrequently some of the branches subdivide -into two, as we saw to be the case among the cup-corals. - -[Illustration: Fig. 18.--_a_, Fenestella oculata (M'Coy), nat. size; -_b_, magnified portion of the same.] - -Fig. 18 illustrates the relative disposition of these branches. In _a_, -the natural size of the fossil is given; _b_ is a portion of the same -magnified, to show the form and arrangement of the ribs and cross-bars. -Each rib is seen to have two sides separated by a rounded ridge. Along -each side there runs a row of circular hollows or cells, every one -of which once formed the abode of a distinct bryozoon. The back or -inner surface of the branch, was ribbed and granulated irregularly, -without any cells. The connecting bars or dissepiments have no cells, -and served merely to bind the interstices together into one firm -organically-united polypidom. Such fragments as that here figured -are the most usual traces to be found of these animals among the -carboniferous rocks. But perfect specimens are sometimes met with which -show how delicate and graceful a structure the polypidom of some of the -fenestellæ must have been. All these bars sprung from a common point -as their basis, and rose up in the form of a cup. It was, in short, a -cup of network, hung with waving tentacles and quivering cilia. I have -seen some dissections of flowers in which all the softer tissue had -been removed, so as to present only the harder veinings of the leaves -with their thousand ramifications bleached to a delicate whiteness. -Out of these skeleton-leaves there were formed groups of lilies, -crocuses, geraniums, and roses, like patterns of the finest gauze. Some -of the larger-stemmed leaves that had been artistically moulded into a -tulip form, seemed not inaptly to represent the general contour of the -skeleton of the old carboniferous fenestella. - -An allied form is called the _Retepora_. It differed from the previous -organism in having the ribs not straight, but irregularly anastomosing, -that is, running into and coalescing with each other, so as to form -a close network with oval interspaces, like a piece of very minute -wire-fence. Each of these wavy libs was completely covered over on one -side with oval pores or cells, which, as in the fenestella, formed the -abode of the living animals. The differences in organization between -the animal of fenestella and that of retepora can, of course, only be -matter of speculation. The general structure in both must, however, -have been pretty much alike. The former genus is now no longer extant, -but the latter, which was ushered into the world during the era of the -Old Red Sandstone, still lives in the deeper recesses of the ocean, and -manifests in its structure and habits the leading characteristics of -bryozoan life. - -What rambler among old lime-quarries is not familiar with the -stone-lily, so abundant an organism in most of the Palæozoic and -many of the Secondary limestones? In some beds of the carboniferous -limestone its abundance is almost incredible. I have seen a weathered -cliff in which its remains stood out in bold relief, crowded together, -to use an expression of Dr. Buckland's, "as thickly as straws in a -corn-rick." The joints of this animal, known now as _entrochi_ or -wheel-stones, forced themselves on the notice of men during even the -middle ages, and an explanation was soon found for their existence. -From their occurring largely about the coast at Holy Island, they were -set down as the workmanship of Saint Cuthbert. - - "On a rock by Lindisfarne, - St. Cuthbert sits and toils to frame - The sea-born beads which bear his name." - -The aged saint was represented as employing his nights in this highly -intellectual task, sitting on a lone rock out in the sea, and using an -adjacent one as his anvil. - - "Such tales had Whitby's fishers told, - And said they might his shape behold, - And hear his anvil sound, - A deaden'd clang,--a huge dim form - Seen but, and heard, when gathering storm - And night were closing round." - -But these wheel-stones were not the only geological curiosities -to which this simple mode of explanation was applied. In the same -storied neighbourhood there occur in considerable numbers the round -whorled shells of the genus _Ammonites_. These were gravely set down -as petrified snakes wanting the head, and their petrifaction and -decapitation were alike reverently ascribed to the power of the sainted -abbess of Whitby. - - "They told - How of a thousand snakes each one - Was changed into a coil of stone - When holy Hilda prayed." - -The stone-lily belonged to that large class of animals ranked together -as _Echinodermata_, a name taken from one of the leading subdivisions -of the group--the _Echini_ or sea-urchins. It seems to have been one -of the earliest forms of life upon our planet, its disjointed stalks -occurring largely in some of the oldest Silurian limestones. In the -Secondary ages it began gradually to wane, until at the present day its -numerous genera appear to be represented by but the _comatula_ and the -_pentacrinite_, two tiny forms that float their jointed arms in the -profounder depths of the sea. - -[Illustration: Fig. 19.--_a_, Cyathocrinites planus. _b_, Encrinal -stem, with uniform joints. _c_, Single joint, or wheelstone.] - -As its name imports, the stone-lily or encrinite had a plant-like -form. It consisted of a long stalk fixed by the lower end to the -sea-bottom, and supporting above a lily-shaped cup, in which were -placed the mouth and stomach (Fig. 19 _a_). The stalk consisted of -circular plates (some of them not so thick as a sixpence), having -their flat sides covered with a set of minute ribs radiating from the -centre, and so arranged that the prominent lines of one joint fitted -into corresponding depressed lines of the adhering ones. The centre -of each joint was pierced by a small aperture, like the axle of a -wheel, which, when the stem was entire, formed part of the long tube -or canal that traversed the centre of the stem, and served to convey -aliment to the remotest part of the animal. Detached joints have thus -a wheel-like appearance (Fig 19 _c_), and hence their common name of -wheel-stones. In many species they were not all of the same diameter, -but alternately larger and smaller, as if the stem had been made up of -a tall pile of sixpences and threepenny pieces in alternate succession. -This variation gives a remarkably elegant contour to the stalk. The -flower-shaped cup consisted of a cavity formed of geometric calcareous -plates, and fringed along its upper margin with thick calcareous -arms, five or ten in number, that subdivided into still more slender -branches, which were fringed along their inner side with minute _cirri_ -or feelers. All these subdivisions, however fine, were made up of -calcareous joints like the stalk, so that every stone-lily consisted -of many thousand pieces, each perfect in its organization and delicate -in its sculpturing. One species peculiar to the Liassic formation -(_Extracrinus Briareus_) has been calculated to contain one hundred and -fifty thousand joints! - -The effect of this minute subdivision was to impart the most perfect -flexibility to even the smallest pinnule. The flower could instantly -collapse, and thus the animals on which the encrinite preyed were -seized and hurried to the central mouth. The lower part of the cup, or -_pelvis_, as it is called, contained the stomach and other viscera, and -communicated with the most distant part of the body by the central -alimentary canal. - -But while this continued the general type on which the encrinites -were constructed, it received many minor modifications. These were -effected chiefly on the form and arrangement of the cup-shaped body -and its appendages, and form now the basis of our classification -into genera and species. Thus, in the genus known as _Platycrinus_, -the lower part of the cup consists of two rows of large hexagonal or -polygonal plates fitting closely into each other, while the upper part -rises into a dome-like elevation formed of smaller polygonal plates, -which have often a mammillated exterior. The arms sprang from the -widest part of the body where the large pieces of the lower cup were -succeeded by the small pieces of the upper. In an Irish species (_P. -triacontadactylus_), the arms subdivided into thirty branches, each -fringed with minuter pinnules and folding round the central elevated -spire, as the petals of a crocus close round its central pistil. In -another encrinite (_Poteriocrinites conicus_), the cup was shaped like -an inverted cone, the point being affixed to the summit of the stalk, -and the broad part throwing out from its edges the lateral arms. The -_Woodocrinus macrodactylus_ had such gigantic arms as well-nigh to -conceal the position of the cup, which relatively was very small in -size. They sprang from near the base of the cup, five in number, but -soon subdivided each into two, the ten arms thus produced being closely -fringed with the usual jointed calcareous pinnules. - -The size and arrangement of the joints of the stalk also differed in -different genera. The Woodocrinus and many others had them alternately -broad and narrow, like a string of buttons of unequal sizes; others had -all the joints of the same relative diameter (Fig. 19 _b_), so that the -stalk tapered by a uniform line from base to point. I may add, that -on some specimens of both these kinds of stems, we can notice small, -solitary _areolæ_, or scars, which may mark the points of attachment -of cirri, or little tentacles, like those on the stem of the existing -Pentacrinite. But though each of these varieties of stem is peculiar to -a certain number of genera, there is often so little distinction among -the detached fragments, that it becomes difficult, indeed impossible, -to assign each to its appropriate individual. We may say, that certain -encrinal stalks could not have belonged to a poteriocrinus, and others -could never have fitted on to the cup of an actinocrinus; but we cannot -often say positively to what species they actually would have fitted. -There can, however, be no doubt about their being encrinites, and so -we have in them a safe and evident test for the origin of the rock in -which their remains occur. But to this I shall afterwards revert. - -In the meantime, I would have the reader to fix the stone-lily in -his memory as peculiarly and emphatically a marine animal, dwelling -probably in the deeper and stiller recesses of the ocean, like the -Pentacrinite of existing times. Let him try to remember it, not in the -broken and sorely mutilated state in which we find it among the blocks -of our lime-quarries, but as it must have lived at the bottom of the -carboniferous seas. The oozy floor of these old waters lay thickly -covered with many a graceful production of the deep, submarine gardens -of - - "Violet, asphodel, ivy, and vine-leaves, roses and lilies, - Coral and sea-fan, and tangle, the blooms and the palms of the ocean." - -Amid this rich assemblage of animated forms, the stone-lilies must have -occupied a conspicuous place. Grouped in thick-set though diminutive -forests, these little creatures raised their waving stems, and spread -out their tremulous arms, like beds of tulips swaying in the evening -air. Their flower like cups, so delicately fringed, must have presented -a scene of ceaseless activity as they opened and closed, coiling up -while the animal seized its prey, or on the approach of danger, and -relaxing again when the food had been secured, or when the symptoms -of a coming enemy had passed away. Only from this animated action -would one have been apt to conjecture these organisms to be other -than vegetable. They lived, too, not in detached patches, like the -tulip-beds of the florist, but, to judge from the abundance of their -remains, must have covered acre after acre, and square mile 'after -square mile, with a dense growth of living, quivering flowers. As one -individual died out, another took its place, the decaying steins and -flowers meanwhile falling to pieces among the limy sediment that lay -thickly athwart the sea-bottom, and contributing, by their decay and -entombment, to build up those enormous masses of rock, known as the -mountain-limestone, which stretch through Yorkshire and the central -counties into Wales. - -In addition to the stone-lilies, the carboniferous rocks contain the -remains of several other kinds of _Echinodermata_. Some of them find -their nearest modern analogues among the sea-urchins so common on -our shores; but I pass on to notice another very interesting class -of fossils known by the name of _Crustacea_, and still abundantly -represented, the crab and lobster being familiar examples. - -The Crustacea, so called from the hard crust or shell which envelops -them, form, with all their orders and genera, a very numerous family. -They are of interest to us as containing among their number some of -the oldest forms of life. Away down in the lower Silurian rocks, among -the most ancient fossiliferous strata, we find the crustacean with -its armour of plates and its prominent sessile eyes set round with -lenses, still visible on the stone. Thus, on the first page of the -stony records of our planet's history are these primeval organisms -engraved. In some localities, where oxide of iron is largely present, -they are coated with a bright yellow efflorescence, and stand out -from the dull grey stone like figures embossed in gold.[31] On all -the subsequent leaves of this ancient chronicle, we can detect the -remains of crustacean life, and many tribes still swarm in our seas -and lakes. It is interesting, however, as marking the onward progress -of creation, to notice that, though this great family has continued -to live during all the successive geological ages, its members have -ever been changing, the older types waning and dying out, while newer -genera rose to supply for a time their place, and then passed away -before the advance of other and yet later forms. The trilobites that -meet us on the very verge of creation, swarmed by millions in the seas -of the Silurian ages, diminished gradually during the era of the Old -Red Sandstone, and seem to have died out altogether in the times of -the Coal. In no ocean of the present day is a trace of any of their -many genera to be seen. The _decapods_, of which our common crab is a -typical form, began to be after the trilobites had died out. In all the -subsequent eras they gradually increased in numbers, and at the present -day they form the most abundant order of crustacean life. The history -of these two divisions, to adopt Agassiz's mode of representation, may -be illustrated by two long tapering bands like two attenuated pyramids. -The one has its broad base resting upon the existing now, and thinning -away into the past, till at last it comes to a point. At a little -interval the apex of the other begins, and gradually swells outward as -it recedes, till the wide base terminates at the first beginnings of -life. - -[Footnote 31: Such is the aspect of the organisms in some of the -Silurian sandstones near Girvan. I have seen the same bright tint -on a set of fossils from the Llandeilo flags of Wales, and from the -slates of Desertcreat, Ireland, and have disinterred similarly gilded -shells from the vertical greywackè slates of the Pentland Hills and -Peeblesshire. Nothing can be more beautiful than the aspect of these -fossils when first laid open, but the bright gleam eventually passes -away on exposure.] - -But there are also some orders that would be best illustrated by a -long line of nearly uniform breadth, extending from the first geologic -periods to the present day. In other words, they seem to have retained -during all time pretty much the same amount of development. I shall -confine my notice of the carboniferous Crustacea to the description of -a single genus belonging to a family that seems to have begun during -the period of the Lower Silurian, and still flourishes abundantly in -existing waters. - -[Illustration: Fig. 20. - - 1. Carboniferous cypris, nat. size, and magnified. - 2. Recent cypris, highly magnified. - 3. Carboniferous king-crab (_Limulus trilobitoides_). -] - -The genus to which I refer is a well-known fossil in some parts of -the Coal-measure series, and has been named _Cypris_. The shells of -_cyprides_ are very minute, considerably less than the heads of small -pins (Fig. 20-1). They can be seen quite well, however, without the -use of a magnifying power. In shape they resemble beans, and when seen -scattered over a slab of shale, look much liker seeds than the relics -of animal life. Yet, under this simple exterior, they concealed a -somewhat complex organization. The little bean-shaped shells, which -are all that now remains to us of their structure, formed the crust -or outer shell in which their viscera were contained, and answered to -the massive carapace and segments of the crab. They consisted of two -valve-like cases fitting to each other, so as to resemble the united -valves of a bivalve shell. From the upper end there were protruded -through the opening between the valves a pair of slim jointed antennæ, -each furnished at its point with a bundle of minute hair-like cilia -(Fig. 20-2). These, when set in rapid motion, served to impel the -creature through the water. The legs, four in number, were encrusted -with the same hard membrane, and had the same jointed structure as -those of our common shrimps and crabs. The foremost pair were pointed -like the antennæ with fine hairs, the incessant action of which -assisted the animal in swimming. Of the little, confluent, sessile -eyes, the delicate branchia or gills, and all the complex internal -structure of the nervous, circulating, and other systems, no trace -has survived on the stone; but enough of the general external form is -left to show us the true affinities of these organisms in the animated -world of the present time. By studying the forms and habits of the -cyprides that swarm in some of our ponds and marshes, a just conception -is obtained of the structure and habitat of the animals that once -occupied the minute bean-shaped shells, which lie by millions among the -shales of the Carboniferous system. From such a comparison we infer, -that just as the cyprides of to-day are fresh-water animals abounding -among the green slime of stagnant pools, so, in past ages, they must -have preserved with the same organization the same habits. And thus -we arrive at the important conclusion that the strata in which the -remains of cyprides abound must have been deposited in lakes or rivers. -This gives us a key by which to interpret some of the changes of a -geological system, and the ancient physical revolutions of large tracts -of country. - -The shales of the coal-measures sometimes contain the cypris cases in -such abundance as to derive therefrom a sort of fissile structure. -It should be borne in mind, however, that each animal may during its -lifetime have possessed in succession several of these cases. Among the -shell-bearing molluscous animals, the little shell which contains the -creature in its youngest stages remains ever after as an integral part -of the outer calcareous case. As the inhabitant grows, it continues -to add band after band to the outer edge of the shell, each of which, -whilst preserving the general symmetry and proportions of the whole -structure, increases its dimensions in every way. Among the univalves, -such, for example, as the turritella, so common on our shores, the -layers of growth succeed each other like the steps of one of those -long spiral stairs that our feudal forefathers loved to build from -the court-yard to the watch-tower of their castles. Each new layer -exceeding in bulk its predecessors, adds a new step to the ascending -pile, and thus the ever-widening mouth winds spirally upwards around -the central pillar. The bivalves exemplify the same principle. The -successive additions are made in a crescent form to the outer edges, -and form those prominent concentric ridges so conspicuous on many of -our commoner shells, such, for instance, as some of those in the genera -_Astarte_ and _Venus_. - -But the architecture of the Crustacea (and, of course, that of the -cyprides) is conducted on a very different principle. Their houses -admit of no additions or enlargements, and so, when the animals find -themselves getting somewhat straitened, they retire to a sheltered -spot, and there, separating the walls that hem them in, crawl out like -soft lumps of dough. The outer membrane of the moulted animal quickly -acquires strength and hardness, and in a day or two the renovated -creature is as healthy and vigorous as ever. In this process it is -not merely an external shell, like that of a mollusc, which is thrown -off, but a veritable skin, so that when the old shell is abandoned -it frequently could not be detected on a first glance to be empty, -the outer crust of every leg and joint, and sometimes even of thin -bristles, remaining just as in the living animal. - -It is not unlikely that this process of moulting takes place annually -in most of the Crustacea, so that if we suppose a fossil member of the -group to have lived six years, it would have left six crusts to be -entombed in any deposits that might be forming at the time. Of course -there would be many chances against all the six being preserved, but -the possibility of at least several of them becoming fossilized should -be borne in mind when we speculate on the abundance of such organisms -in any geological formation. - -I might refer to another very interesting group of crustacean animals -known as the _Limuli_, or king-crabs, of which there were at least -three representatives during the times of the English Carboniferous -system (Fig. 20-3). They are remarkable chiefly for their large -crescent-shaped shield, their long sword-like tail, and their double -pair of eyes, of which the outer ones are large, sessile, and compound, -like those of the trilobites, while the middle pair are small, simple, -and set close together on the forehead, like those of the single-eyed -Cyclops in the old mythology. Altogether, with their shields, swords, -watchful waking eyes, strong massive armour, and great size (for some -of them measure two feet in length), they form a most warlike genus. - - - - -CHAPTER VI. - - Carboniferous fauna continued--George Herbert's ode on - "Man"--His idea of creation--What nature teaches on - this subject--Molluscous animals--Range of species in - time proportionate to their distribution in space--Two - principles of renovation and decay exhibited alike in the - physical world and the world of life--Their effects--The - mollusca--Abundantly represented in the carboniferous - rocks--Pteropods--Brachiopods--Productus--Its alliance with - Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods-- - Land-snail of Nova Scotia--Cephalopods--Structure of orthoceras-- - Habits of living nautilus. - -Holy George Herbert, in one of the most remarkable odes of the -seventeenth century, sang quaintly, yet nobly, of the dignity of man. -He looked into the design and nature of the human heart, and saw there -a palace that had been built for the abode of the Eternal. Deserted -though it might be, broken down and in ruins, yet there still lingered -a trace of its ancient glory, and the whole material world still -testified to its inherent greatness. He looked abroad on the face of -nature, and saw, in all its objects and all its movements, a continued -ministration to man. - - "For us the windes do blow; - The earth doth rest, heav'n move, and fountains flow. - Nothing we see, but means our good, - As our delight, or as our treasure; - The whole is, either our cupboard of food, - Or cabinet of pleasure. - - "The starres have us to bed; - Night draws the curtain, which the sunne withdraws; - Musick and light attend our head. - All things unto our flesh are kinde - In their descent and being; to our minde - In their ascent and cause." - -The idea is a very natural one, and is consequently as old as man -himself. Human vanity is soothed by the reflection that all this varied -world, with its countless beauties, has been designed and arrayed -solely for the use of man. And yet, if we but think of it, such a view -of creation, however natural and pleasing, is at the best but a narrow -and selfish one. It assuredly finds no response in nature, and grows -more and more out of fashion the further our investigations proceed. -Nature teaches us that long ere man appeared upon the earth there were -successive generations of living things just as now; that the sun -shone, and the waves rolled, and the wind blew, as they do to-day; and -that, on as lovely a planet as that whereon we dwell, there lay forests -and prairies nursing in abundance animals of long-extinct forms; lakes -and rivers, haunted by creatures that find no representatives now; -and seas teeming with life, from the minute infusory up to the most -unwieldy icthyosaur, or the most gigantic cetacean. And all this, too, -ere a reasoning, intelligent being had been numbered among terrestrial -creatures, and when, perhaps, each successive creation was witnessed -by none save those "morning stars who sang together, and those sons -of God who shouted for joy." The delight and comfort of the human -race formed, doubtless, one of the many reasons why this globe was so -bountifully garnished.[32] But the workmanship of a Being infinitely -wise, and good, and powerful, could hardly have been other than -complex and beautiful. That symmetry and grace which we see running -as a silver thread through every part of creation, forms one of the -characteristics of the Almighty's mode of working. From the Fountain -of all Beauty nothing unseemly or deformed can proceed. And so we -find, away back among the ages of the past, that, though the material -world might be less complete, it was not less beautiful than now. Nay, -those bygone millenniums stood higher in one respect, for the eye of -God rested upon their unsullied glory, and he pronounced them very -good; but these last ages of creation are dimmed and darkened, and -that Eye now watches a world trodden down by the powers of evil. There -is profound truth in the sublime allegory of Milton that represents -Sin girt round with clamorous hell-hounds, and the two grisly forms -sitting at the farthest verge of purity and light, to keep the gates of -darkness and chaos. With the introduction of moral evil into our planet -came the elements of deformity and confusion. The geologist can go back -to a time ere yet the harmony of nature had been broken. The Christian -looks forward to a day when that harmony shall be again restored, and -when guilt with all its hideous train shall be for ever chased away -from the abodes of the redeemed. - -[Footnote 32: In connexion with this subject I have been often struck -with a passage in St. Paul's Epistle to the Colossians, i. 10, "All -things were created by him [Christ] and for [εις--with a view to, -on account of] him." It is probable that these words, in their full -meaning, cannot be understood by us. Yet they seem to point to Christ -as at once the Creator, and himself the acme and design of creation; -and perhaps they may contain what hereafter shall prove the key to -the mystery of creation. On this impressive and difficult subject the -reader should refer to the closing chapter of Hugh Miller's _Footprints -of the Creator_. See also M'Cosh on _Typical Forms_, 2d edit. p. 531.] - -Such thoughts as these sometimes arise in the mind of one who labours -much among organic remains. By no class of fossils are they more -vividly suggested than by those which we come next to examine--the -various tribes of molluscous animals. This results from the high -antiquity of these organisms, and the similarity of type which they -have manifested in all ages. In the very earliest geological periods -they exhibited the same symmetry of external form as now, the same -beauty of structure, and apparently the same delicacy of colour. Nay, -so closely did they resemble their existing congeners that we are -seldom at a loss as to their affinities, and can refer them to their -places in the scale of creation, and sometimes even to genera still -living.[33] - -[Footnote 33: It must be admitted, however, that not a few of the -identifications already made are somewhat suspicious The natural -tendency is to perceive resemblances--a tendency which even the most -rigid science sometimes fails to control.] - -The geological ages saw many strange types of creation. One era, in -especial, furnished reptiles which united in their structure the snout -of the porpoise, the head of the lizard, the teeth of the crocodile, -the paddles of the whale, and the backbone of the fish. Some displayed -the long pliant neck of the swan, and others careered through the -air on wings like those of the bat. But the molluscous tribes have -never exhibited such aberrant forms. The existing classes and orders -of the naturalist are still the same as those which nourished during -the successive geological periods. Hence their value as evidence of -physical changes in the ancient world. Hence, too, the conviction, -forced upon the mind of the observer, that the conditions for the -support of life never deviated much from those now in operation; that -in place of all the varied beauty of the world having arisen for the -use of man, it existed millions of years ere the breath of life had -been breathed into his nostrils; that in fine, man is but a new-comer, -a creation of yesterday. - -There is another point suggested by the occurrence of mollusca in the -Carboniferous system, to which it may be well to refer, namely, the -curious, and as yet not wholly understood fact, that the range of -animals in time is in some way proportionate to their range in space. -In other words, it often happens (so often, indeed, as apparently to -indicate a law) that the more widely diffused a genus is found to be -at the present day, the farther back can we trace its remains into the -geological ages. This fact probably depends upon causes, many of which -are still unknown to us; but the following remarks may help the reader -to a notion of the general bearings of the subject.[34] - -[Footnote 34: The law is more especially exemplified by the mollusca, -but it may eventually be found to characterize other classes. We, -perhaps, see traces of it in the present distribution of the two most -ancient orders of icthyic life--the placoids and ganoids.] - -In the profounder recesses of the ocean, the temperature remains more -or less uniform all over the globe.[35] In these undisturbed regions -there occur, along with corals and other humble animals, many kinds of -mollusca, such as terebratulæ, craniæ, scissurellæ, &c. These are very -generally found not to be confined to one province or limited district, -but to flourish in every sea from Hudson's Bay to Hindustan. One of -the causes of this wide distribution is the uniformity of temperature -that characterizes the depths in which they live. They can migrate -from one ocean to another, from the torrid zone to the polar circle, -without experiencing any destructive change in the thermal conditions -of their element. And provided only they meet with no barrier in the -form of a lofty submarine mountain chain or profound abyss, and can -secure the requisite food in their journey, we know no reason why some -of these shells may not thus extend themselves over wide areas. Of the -two species of _rhynconella_ now living, one inhabits the depths of -the icy sea, the other enjoys the warmer waters that lave New Zealand. -The species, in this case, seem (for the fact cannot yet be accepted -as fully proved) to occupy a more limited area, while the genus has a -larger range. - -[Footnote 35: The stratum of constant temperature runs in a wave-like -form from pole to pole. In the arctic and antarctic oceans it is found -at a depth of 4500 feet, whence it slopes upwards so as to reach the -surf ice at the temperate zone on both sides of the equator. It then -gradually sinks down in the warmer regions, till at the equator it is -7200 feet below the sea-level. There are thus one tropical and two -polar basins separated by two wave-like circles, or, as a geologist -would say, three synclinal troughs separated by two anticlinal ridges.] - -Now, a genus widely diffused, and capable of enduring great differences -in the temperature and other conditions of the ocean, would probably -suffer least from any great physical changes. If all the sea at one -locality were converted into land, the genus would be driven into other -districts, and thrive as abundantly as ever; or, even supposing that it -should become locally extinct, it would still be abundantly represented -in other oceans of the globe. In the course of many ages, after many -such slow revolutions in the configuration of land and sea, the genus -might perhaps become greatly reduced in numbers, until at length some -final elevation of the sea-bed, or other change, might cause its total -extinction. In the _rhynconella_, we perhaps see one of these genera in -its last stage. Any great change in northern latitudes would probably -destroy the arctic species, and a similar change around New Zealand -might gradually extinguish the southern one. - -Looking, then, from this point of view into the past history of life -upon our planet, we see that such extinctions have often taken place. -At first, many of these widely-diffused genera were created. They -were represented by a large number of species as well as individuals, -and ranged over all the oceans of the globe; but in tracing out their -history, we mark one species after another passing away. Some of them -lived for but a comparatively short period; others came in with the -beginning and saw out the end of an entire geological system; but of -all these early species there is not now a single one extant, though -some of the genera still inhabit our seas. It is plain, therefore, -that but for the operation of another principle, all the genera, too, -would ere this have become extinct, for the whole can contain no more -than the sum of its parts; and if these parts are destroyed the whole -must perish simultaneously. As the species of certain genera died out, -however, their places were from time to time filled up with new ones, -yet the rate of increase became ever less and less than the rate of -decrease, so that the numbers of such genera grew fewer with every -successive period, and have reached their minimum in existing seas. -There are instances, however, in which this ratio was reversed, the -list of added species continually outnumbering that of the extinct, -till the genus reached its maximum, when it either continued at that -stage till the present day, or began slowly to decline. - -In the physical world around us, we behold a perpetual strife between -the two great principles of renovation and decay. Hills are insensibly -crumbling into valleys; valleys are gradually cut down, and their -debris transported to the sea. Our shores bear witness to the slow -but ever onward march of the ocean, whether as shattered cliff's worn -by the incessant lashing of the surge, or as sand-banks and submerged -forests that represent the wolds and holms of our forefathers. We mark, -too, how the sediment thus borne into the main is sowing - - "The dust of continents to be;" - -while the slow elevation of large tracts of country, or the sudden -upheaval of others, shows us by how powerful an agency the balance of -land and sea is preserved, and how sometimes the paroxysm of an hour -may effect a mightier change than the wasting and decay of a thousand -years. We choose to call these two principles antagonistic, because in -their effects they are entirely opposite; yet there is no discordance, -no caprice in their operation. Each works out its end, and the result -is the harmony and stability of the face of nature. - -In the world of life, too, there seems to have been a double principle -of decline and renewal. The natural tendency of species and genera, -like that of individuals, has been towards extinction. Why it should be -so we know not, further than that they are for the most part influenced -by every change in physical geography. But they probably obey a still -higher law which governs their duration, as the laws of vitality govern -the life of an individual If we are but slightly acquainted with the -agency by which the degradation of land is counter-balanced, we are -still more ignorant of the laws that preserve the balance of life. -Creation is a mystery, and such it must for ever remain. So, too, are -the principles on which it has been conducted. We can but mark their -results. We see new species appear from time to time in the upward -series of the geological formations, but they tell not whence they -came. Of two genera created together at the beginning, one ere long -died out, but the other still lives; yet here there is assuredly -nought like discordance or caprice. Nay, these two principles--death -and creation--have been in active operation all through the ages, and -the result is that varied and exquisitely beautiful world wherein we -dwell. - -The Mollusca are so named from the soft nature of their bodies, and are -familiar to us as exemplified in the garden-snail and the shells of the -sea-shore. The general type upon which they are constructed is that of -an external muscular bag, either entire or divided into two, called -the mantle, in which the viscera are contained. In most of the orders, -they have likewise an outer hard calcareous shell, consisting of one or -more parts. It is of course this shell alone that can be detected in -the rocks, but by attending to the relations between the living animals -and their shells, we ascertain the nature and affinities of the fossil -species. - -Few who ramble by the sea-shore, gathering limpets, whelks, and -cockles, are aware how complex an anatomy is concealed within one -of those brown discoloured shells. There are elaborate nervous and -muscular systems--sometimes several hearts with accompanying arteries -and veins--often dozens of rudimentary eyes--capsules which perform -the function of ears--jaws, teeth, a strongly armed tongue--gullet, -gizzard, stomach, liver, intestine, and complete breathing apparatus. -The structure and grouping of these parts vary in the different genera -and orders, and upon such variations is founded the classification of -the naturalist. Thus, the mollusca of the highest class are called -the _Cephalopoda_, or _head-footed_, because their feet, or rather -arms, are slung in a belt round the head. They contain, among their -number, the cuttle-fish, with its curious internal bone that shadows -forth, as it were, the coming of the vertebrate type; and the nautilus, -with its many-decked vessel of pearl. The second class is termed the -_Gastropoda_, or _belly-footed_, as the genera embraced under it -creep on the under side of the body, which is expanded into a broad -retractile foot. The common snail and whelk are familiar examples. The -third class is formed by the _Pteropoda_, or _wing-footed_--delicate -animals, found only in the open sea, and remarkable for a pair -of wing-like expansions or fins on the sides of the mouth. The -_Lamellibranchiata_ form the fourth class, and receive their name -from the laminated form of their branchia, or gills. They contain the -two-valved shells, such as the oyster and scallop, and are one of the -most abundant groups of animals on our coasts. The fifth class consists -of the _Brachiopoda_, or _arm-footed_ molluscs a name given to them -from their long spiral arms, once thought to be the instruments of -motion, but now ascertained only to assist in bringing the food to the -mouth. The sixth, and humblest class, has received the designation of -_Tunicata_, from the thick bladder-like tunic, or sac, which supplies -the place of an outer shell. - -The geologist finds the remains of all these classes in the different -rock-formations of the crust of the earth. They flourished so -abundantly in the earliest seas, that the first geological period -has sometimes been called the Age of Molluscs; and, during all the -subsequent eras, they held a prominent place among the inhabitants of -the deep. Let us look for a little at their development in the times of -the Carboniferous system. - -As the Carboniferous group of rocks exhibits the remains of ocean-bed, -lake-bottom, and land-surface, so we find in it shells of marine, -fresh-water and (though rarely) terrestrial mollusca. The marine -genera greatly predominate, just as the shells of the sea at the -present day vastly outnumber those either of lakes or of the land. In -England they occur chiefly in the lower part of the formation, giving a -characteristic stamp to the deep series of beds known as the mountain -limestone. There they are associated with the corals and stone-lilies -already described--all productions of the sea. In Northumberland, -however, and generally throughout Scotland, they occupy a somewhat -different position. The great mountain limestone of central England -gets split up into subdivisions as it proceeds northward, and beds of -coal, full of land plants, become mingled with the ordinary marine -strata. Sometimes we may find a group of brachiopods scattered over the -macerated stem of a stigmaria; and the writer has himself collected a -sigillaria in a limestone crowded with stone-lilies and _producti_. -But this intermingling is still further carried on in the upper part -of the series. The coal-beds, with their underclays and stigmaria -rootlets, evidently representing ancient vegetation with the soils -on which it grew, are succeeded by beds of limestone, full of marine -mollusca; and these, again, are erelong replaced by sandstones, -shales, and ironstones, charged with land-plants and fresh-water -shells. To this curious blending of very different organic remains, I -shall have occasion to refer more at large in a subsequent chapter. I -mention it now as a sort of apology for the dryness of details which -it is necessary to give, in order to complete our picture of the -carboniferous fauna, and to understand the principles upon which the -ancient history of the earth is deciphered. - -[Illustration: Fig. 21.] - -Of the _Pteropoda_, we have, as yet, but one carboniferous genus, -the _conularia_ (Fig. 21). It was a slim delicate shell, in shape an -oblong cone, having four sides, finely striated with a sort of zig-zag -moulding like that of the Norman arch. Each of the four angles was -traversed along its whole extent by a narrow gutter-like depression, -and this style of fluting, combined with the markings on the sides, -imparted no little elegance to the shell. The conularia is not a -common fossil. It has been found among the coal-bearing strata of -Coalbrook-Dale, and was noticed long ago by Dr. Ure in his _History of -Rutherglen_. - -The _Brachiopoda_ are bivalve molluscs, but unlike most other molluscs -they are rooted to one spot, and destitute of any power of locomotion. -Their shells are unequal, the dorsal, or upper valve, being smaller -and usually more bulged out than the under or ventral valve, which -in most species is prolonged at its narrow end into a kind of beak. -In the terebratula this beak has a little circular hole, from which -there emerges a short peduncle or stalk, that fixes itself firmly to a -rock or other substance at the sea-bottom, and serves the purpose of -an anchor and cable to keep the little vessel safely moored. When the -shells are detached, these perforated ventral valves have so exactly -the form of the old Roman lamps, "that they were called _Lampades_, -or lamp-shells, by the old naturalists."[36] Other species, as the -_lingulæ_, have no beak, and the long peduncle passes out between the -valves, which are of nearly equal size, and have been compared to the -shape of a duck's bill. In yet another genus, the _crania_, there is -no peduncle, but the animal adheres by its lower valve, much like -the oyster, and may often be seen clustered in groups on decayed sea -urchins or other organisms, particularly in the chalk formation. - -[Footnote 36: See the excellent _Manual of Mollusca_, by Woodward, p. -209.] - -The internal structure of these animals is singularly beautiful. The -inner surface of each valve is lined with a soft membranous substance, -called the pallial lobe, the margin of which is set round with stiff -hair-like bristles, that prevent the ingress of any foreign body likely -to interfere with the play of the delicate filaments of the arms. These -two soft lobes are furnished with veins, and supply the place of a -breathing apparatus. The body of the animal occupies not quite a third -part of the interior of its valves, and is situated at the narrow end. -There are thus two distinct regions within the shell, separated from -each other by a strong membrane, through the centre of which is the -opening of the mouth. The smaller cavity next the hinge contains the -viscera, and the outer larger one, the folded and ciliated arms. These -arms form one of the most characteristic features of the brachiopods. -They are two in number, and proceeding from the margin of the mouth, -advance into the outer empty chamber of the shell, and return upon -themselves in spiral curves and folds. They are fringed with slim, -flat, narrow filaments, set along the arm like teeth along the back of -a fine comb. Though called arms, these long ciliated appendages are -rather enormously protruded lips. The vibratory action of the fringes -causes currents to set inwards towards the mouth, which is placed at -the inner end or base of the arms. To support these long convoluted -arms, many of the genera are furnished with slender hoops of hard -calcareous matter, which are hung from the dorsal valve, and are still -found within the shells of some of the most ancient fossil brachiopods. - -The little visceral cavity contains the complex groups of muscles for -opening and closing the valves, a simple stomach, a large granular -liver, a short intestine, two hearts, and the centre of the nervous -system. Without going into the details of these various structures, -the reader will see that the brachiopoda are really a highly organized -tribe; and I am thus particular in the enumeration, partly that he may -the better understand the mechanism of the carboniferous shells of -that type, and partly that he may mark how the oldest forms of life, -those that meet us on the very threshold of animated existence, were -not low in organization, but possessed an anatomy as complex as it was -beautiful. - -Who that has ever wielded an enthusiastic hammer among the richly -fossiliferous beds of the mountain limestone, does not remember with -delight the hosts of delicately fluted shells that the labour of an -hour could pile up before him? There was the striated productus, -with its slim spines scattered over the stone. There, too, lay the -spirifer with its broader plications, its toothed margin, and its -deeply indented valve. Less common, and so more highly prized, was the -slimly-ribbed rhynconella, with its sharp, prominent beak, or perhaps -the smooth, thin terebratula, with its colour-bands not yet effaced. -These were pleasant hours, and their memory must dwell gratefully -among the recollections of one whose avocations immure him throughout -well-nigh the livelong year amid the din and dust of town--the _fumum -et opes strepitumque Romæ_. - -[Illustration: Fig. 22.--Productus giganteus.] - -In the _productus_ the dorsal valve is sometimes quite flat, while -the ventral is prominently arched, and the shell resembles a little -cup with a flat plate of the same diameter placed over it. Usually, -however, both the valves are concavo-convex, or arched in the same -direction like two saucers placed within each other. The exterior -surface of each valve is differently ornamented in the various species. -A very common style of sculpturing is by a set of fine hair-like -longitudinal ribs, diverging more or less regularly from the hinge line -to the outer margin. In some species these ribs are wider, and are -furnished with little prominent scars. In others (as _P. punctatus_) -a set of semicircular ridges runs round the shell, narrowing as they -converge from the outer lips to the centre of the hinge line, and -bearing each an irregular row of small scars or tubercules. Some of -the species are very irregularly ornamented into a sort of wrinkled -surface, in which the striæ seem, as it were, thrown over the valves in -bundles at random. - -The productus was furnished with slender hollow spines, which rose up -from the surface of either valve, chiefly, however, about the hinge. In -_P. spinosus_ they were long and stout, like thin rush stalks, while in -the smaller species they rather resembled stiff bristles. The use of -these spines is not very well made out. As most of the producti appear -to have been free, that is, without any peduncle fixing them to the -sea-bottom, it has been conjectured that the spines, by sinking deep -into the mud, may have served the place of a peduncle to moor the shell. - -As regards size, the productus is very variable. You may gather some -species in the young form, not larger than peas, while others may -reward your search, having a breadth of six or eight inches (_P. -giganteus_). But however much they may vary in dimensions, they usually -remain pretty constant in their abundance, being among the most common -fossils of the mountain limestone, and even of some limestones in the -true Coal-measures;[37] and that must be a poor stratum indeed which -cannot yield you a bagful of producti. - -[Footnote 37: See the table given below in Chap. X.] - -The productus no longer ranks among living forms. It began during the -times of the Upper Silurian system, lived all through the Old Red -Sandstone, and attained its maximum of development in the seas of the -Lower Carboniferous group. As the coal forests began to flourish, the -productus seems to have waned; but it is still sometimes found in -considerable numbers in the ironstones and limestones intercalated -among the coal seams of northern England and central Scotland. In the -period which succeeded the coal, that, namely, of the Permian, it seems -to have died out altogether, at least no trace of its remains have as -yet been detected in strata of a later age. But whilst it lived, the -productus must have enjoyed a wide range of climate, for its valves -have been found by thousands both in the old world and in the new. I -have seen several that were brought from the hills of China, and they -occur likewise in Thibet. Specimens have been brought, too, from the -warm plains of Australia, and from the snows of Spitzbergen. - -In looking over the fossils that lie grouped along beds of the -mountain limestone, there are two forms that we find almost invariably -side by side--the productus and the spirifer. They seem to have -begun life together, or rather, perhaps, the spirifer is somewhat -the older brother. They voyaged through the same seas, and anchored -themselves to the same ocean-bed, sometimes among mud and ooze, and -often among bowers of corals and stone-lilies. They visited together -the most distant parts of the world, from China to Chili, and from -Hudson's Bay to New Zealand. I have sometimes laid open fragments of -limestone where they lay thickly clustered as though they had ended -a life of friendship by dying very lovingly together. But after all -the varieties of the productus had died out, some species of the -spirifer still lived on, and it was not until the period of the lias -that they finally disappeared. I remember meeting with one of these -latest spirifers in the course of a ramble in early morning along the -shores of Pabba, one of the lone sea-girt islands of the Hebrides, -where the Scottish secondary rocks are represented. The beach was -formed of low shelving reefs of a dark-brown micaceous shale, richly -charged with the characteristic fossils of the Lias--ammonites, -belemnites, gryphææ, pectines, &c. In the course of the walk I came to -a lighter coloured band, with many reddish-brown nodules of ironstone, -but with no observable fossils. A search, however, of a few minutes -disclosed a weathered specimen, near which a limpet had made good its -resting-place; and this solitary specimen proved to be one of the last -lingering spirifers (_S. Walcottii_). The form struck me at once as a -familiar one, and recalled the fossils of the mountain limestone. It -may seem a puerile fancy, but to one who had lately been working among -palæozoic rocks, and remembered the history of the spirifer, there -was something suggestive in the loneliness of the specimen. With the -exception of one or two other organisms (as _rhynconella_), it was by -far the most ancient form of the deposit. Its family had come into the -world thousands of years before that of the large pinnæ that lay among -the neighbouring shales, and perhaps millions of years before that of -the gracefully curved ammonites. But the family was nearly extinct -when these shales were being thrown down as sandy mud, and this wasted -specimen, worn by the dash of the waves, seemed in its solitariness no -inapt representative of an ancient genus that was passing away. - -The spirifer received its name from the two highly developed spiral -processes in the interior of the shell attached to the dorsal valve. -They were hard, like the substance of the shell, and sprang from -near the hinge, each diverging outwards to near the border of the -valve. They resembled two cork-screws, but the loops were much closer -together. These coiled calcareous wires almost filled the hollow of the -shell (Fig. 23), and ample support was thus afforded to the filamentous -arms. In recent brachiopods, these arms do not always strictly follow -the course of the calcareous loops. Among palæozoic genera the case may -have been similar, so that the complex calcareous coil of the spirifer -may not perhaps indicate a corresponding complexity of the arms. But -none of the few recent forms exhibit anything like the coiled processes -of the spirifer. - -The Carboniferous system of Great Britain and Ireland is stated to -have yielded between fifty and sixty species of spirifers. Of course, -in such a long list the gradations are sometimes very nice, and to an -ordinary eye imperceptible, but there exist many marked differences -notwithstanding. The general type of the spirifers is tolerably well -defined. They had both valves arched outwards, not concavo-convex as -in the productus. Their hinge-line, like that of the latter shell, -ran in a straight line, and their dorsal valve was raised along its -centre from hinge to outer margin, into a prominent ridge, while in the -ventral valve there was a furrow exactly to correspond. Most of the -species were traversed by sharp ribs radiating from the centre of the -hinge-line like those on the surface of the common cockle. But some -were quite smooth, retaining only the high lobe in the centre, such as -_S. glaber_. In a noble specimen figured by M'Coy[38] under the name -of _S. princeps_, the valves are covered with broad plaits that sweep -gracefully outward from the centre of the hinge-line. - -[Footnote 38: _Carb. Limest. Foss. of Ireland_, pl. 21, fig. 1.] - -[Illustration: Fig. 23.--Spirifer hystericus. _b_, Interior of the -same, showing the arrangement of the spiral arms.] - -The spirifers vary more in form than in external ornament. Some are -triangular, others nearly semicircular, others long and attenuated. In -some species (as the _S. glaber_), the central ridge is very prominent, -taking up about a third of the entire area of the shell, and thus -giving it a trilobed appearance. In others (as _S. symmetricus_) it is -less marked, and bears a minor furrow down its centre; while in yet a -third class (as in some specimens of _S. trigonalis_) the median fold -scarcely rises above the ribs that are ranged on each side. - -These old shells probably anchored themselves to the sea-bottom by -means of a thin peduncle, and lived by the vigorous action of those -complex fringed arms, whose screw-like skeleton still occasionally -remains, and which conveyed to the mouth the animal substances that -served as food. - -[Illustration: Fig. 24.--Terebratula hastata.] - -I shall refer to but one other brachiopod of the carboniferous rocks, -interesting both as one of the forms of life still living in our -seas, and as exhibiting, after the lapse of such a vast interval, the -form of the coloured bands which adorned it when alive. It is called -_Terebratula hastata_; a slim delicate shell like its representatives -of the present day, narrow at the beak, and bulging out towards the -outer margin, which is slightly curved. The surface is smooth, and -in the older specimens has numerous concentric layers of growth, -especially marked near the margin. The stripes of colour radiate from -the beak, outwards, and though the tint which once brightened them is -no longer visible, it may be that the vessel of the little terebratula, -which lay anchored perhaps fifty fathoms down, was well-nigh as -gaily decked as a felucca of the Levant. But the existence of these -colour-bands is not merely interesting; the geologist can turn it -to account in investigating the physical conditions of an ancient -ocean. The late Professor Edward Forbes, after a careful series of -investigations in the Mediterranean, brought to light the fact, that -below a depth of fifty fathoms shells are but dimly coloured, and hence -he inferred, from the numerous coloured shells of the carboniferous -limestone, that the ocean in which they lived was not much more than -fifty fathoms deep.[39] - -[Footnote 39: Similar coloured bands are found even in the Lower -Silurian, e.g., on turbo rupestris (Murchison's _Siluria_, p. 194), while -on many of the carboniferous gastropods and lamellibranchiate bivalves, -they are of frequent occurrence.] - -The _lamellibranchiate_ bivalve shells of the British Carboniferous -system, so far as yet discovered, number about 300 species, belonging -to genera some of which are still familiar to us. There were the -_pectens_ or _scallops_, the _pinnas_ with their beards of byssus, -the _cardiums_ or cockles, the _mytili_ and _modiolæ_ or mussels, -all sea-shells. Then among the fresh-water bivalves we can detect -several species of the unio or river mussel, that perhaps displayed -valves as silvery in their lining as those of our own pearl-mussels. -But with these well-known forms there co-existed some that no longer -survive. Such was the _conocardium_, a curious form that looks like a -_cardium_ cut through the middle, with a long slender tube added to -the dismembered side (Fig. 25). The _aviculopecten_, a shell allied to -our common scallop, and sometimes showing still its colour-bands (Fig. -25), and the _cardinia_ or _anthracosia_, a small bivalve that abounds -in the shales and ironstones of our coal-fields, along with nautili, -producti, and conulariæ at Coalbrook Dale, and with a thin leaf-like -lingula at Borrowstounness. - -[Illustration: Fig. 25.--Carboniferous Lamellibranchs. - -1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium -aliformis.] - -The _Gastropods_ of the carboniferous rocks in the British Islands -embrace from twenty-five to thirty genera, with upwards of 200 -species. Here, too, we can detect some forms that have not yet passed -away. The _trochus_, so universally diffused over the globe at the -present day, also lived in the palæozoic seas. Its companions, the -_natica_, the _turritella_, and the _turbo_, likewise flourished in -these ancient waters. Among the genera now extinct we may notice the -_euomphalus_, with its whorls coiled in a flat discoidal form; and the -_bellerophon_, with its simple coiled shell, resembling in general form -the nautilus. The gastropods are numerously represented in our gardens -and woods, by the various species of the snails, animals that have a -most extensive distribution over the world, and number probably not -much under two thousand species. - -[Illustration: Fig. 26.--Carboniferous Gastropods. - -1. Euomphalus peatangulatus. 2. Pleurotomaria carinata (showing -colour-bands).] - -For a long time it was matter of surprise that no such land shells -had ever been detected in the carboniferous rocks. Trees and forests -had been turned up by the hundred, but never a trace was found of -any air-breathing creature. From this fact, and from the enormous -amount of vegetable matter preserved, it was once hastily inferred -that the atmosphere of that ancient period must have been uncongenial -to air-breathers; that, in short, it was a dense heated medium -of noxious carbonic-acid gas, wrapt round the earth like a vast -mephitic exhalation, favourable in the highest degree to the growth -of vegetation, yet deadly as the air of Avernus to all terrestrial -animals. But this notion, like most other bold deductions from merely -negative evidence, has had to be abandoned, for traces of air-breathers -have at last been found. Among these, not the least interesting is -the shell of a _pupa_, a sort of land-snail, which Sir Charles Lyell -detected, along with the bones of a small reptile, embedded in the -heart of an upright sigillaria stem in the carboniferous rocks of Nova -Scotia. Small as was the organism, the evidence furnished by it proved -scarcely less valuable than if it had been a large mammal that might -have afforded material for weeks of study. The similarity of the shell -to existing forms, showed that the ancient carboniferous forests had -at least one race of air-breathing creatures among their foliage, and -that the atmosphere of the period could have differed in no material -point from that of the present day, for as the snails breathe by lungs, -and require, consequently, a continual supply of oxygen to support -respiration, they could not have existed in an atmosphere charged with -carbonic acid. - -[Illustration: Fig 27.--Carboniferous Cephalopods. - -1. Nautilus Koninckii. 2. Goniatites crenistria. 3. Orthoceras laterale -(fragment).] - -The _Cephalopods_, or highest class of mollusca, are represented among -the British carboniferous strata by seven genera. Of these the most -characteristic is the _orthoceras_, so named from its shell being -like a long straight horn. When the animal was young it inhabited a -single-chambered shell like that of many of the gastropods, but as -it increased in size and prolonged its shell in a straight line, it -withdrew from the first occupied chamber. This was partitioned off by a -thin wall called a _septum_, through the centre of which a tube ran to -the narrow end of the shell (Fig. 27). As the creature grew, chamber -after chamber was in this way formed, each of them quite air-tight, and -traversed by the central tube. Suppose a graduated series of diminutive -watch-glasses to be pierced by a long tapering glass-tube in such a -way that they should have their convex faces towards the narrow end of -the tube, and be arranged at short intervals, the smallest one placed -near the point of the tube, and the largest a little below the wider -end. Suppose, further, that this piece of mechanism were placed within -another tube tapering to an obtuse point, and that the edges of the -watch-glasses fitted tightly to the inner surface of this larger tube. -Such would be a rough model of the structure of the orthoceras. - -The inner tube that traverses the centre of the chambers from end -to end of the shell is called the _syphon_, but its uses are very -problematical. At one time naturalists inclined to regard it as -intended to be filled with fluid, which, by expanding the membrane -of the tube, would compress the air in the chambers, and thus, -increasing the specific gravity of the animal, enable it to sink to -the bottom. In this way, by emptying or filling the syphonal tube, -the orthoceras might have risen rapidly to the surface of the deep, -or sunk as swiftly to the bottom. But this view, so pretty that one -wishes it were confirmed, must be regarded as at least doubtful. The -orthoceras more probably owed its power of progression to the action of -a funnel connected with the breathing apparatus, whereby jets of water -were squirted out that drove the shell rapidly along. The use of the -air-tight chambers was, perhaps, to give buoyancy to the shell so as to -make it nearly of the same specific gravity as water. Such a provision -must have been amply needed, for Professor Owen mentions an orthoceras -from Dumfries-shire that measured six feet in length, and similar -gigantic specimens have been found in America. Unless the chambers in -these shells had been air-tight, the animals that inhabited them would -have been held down about as firmly to one spot as if they had been -tied to a sheet-anchor. No mollusc could have possessed much locomotion -with so ponderous a tail, six feet or more in length, to drag after it. -But this inconvenience was obviated by the simple plan of having the -chambers close, and filled with nitrogen or other gas evolved by the -chemistry of the inmate. The shell, in this way, acquired no little -buoyancy, and probably stood up like a church spire, the animal keeping -close to the bottom to lie in wait for any hapless mollusc or trilobite -that might chance to come in its way. - -The _nautilus_ (Fig. 27), which still lives in our seas, occurred -likewise in those of the Carboniferous period. It was a coiled shell; -in truth, just an orthoceras rolled up in one plane like a coil of -watch-spring. An allied form, called the _goniatite_ (Fig. 27), had -the margins of its septa of a zig-zag form, like the angles of the -wall round a fortified town. When the thin outer coating of the -shell is removed, the ends of these partition-walls are seen to form -strongly-marked angulated sutures or joints, where they come in contact -with the shell. Hence the name of the genus--_angled_ shell. - -All these animals were predaceous. They did not confine themselves to -the lower forms of life, polyps and medusæ, nor even to the humbler -tribes of their own sub-kingdom, but hesitated not to wage war with -creatures greatly higher in the scale of creation than themselves, such -as the smaller fishes. They swarmed in the palæozoic seas, and well -merited the title of scavengers of the deep, that has been bestowed -on the sharks of our own day. They seem to have performed a function -now divided partly among the fishes and partly among the higher -gastropodous molluscs. And accordingly we find that as these latter -tribes increased, the orthoceratites, and goniatites, and ammonites -waned. At the present day, of all the palæozoic cephalopods there -remains but one--the nautilus[40]; a and so rare is it, that up to -the year 1832, all sorts of fanciful notions existed as to its nature -and functions. In fact, the nautilus was a sort of myth which any -naturalist could dress up as he chose, much as the old poets used to -picture the ship Argo. A specimen was at length procured and intrusted -to the examination of Professor Owen, by whom its anatomy was studied, -and afterwards philosophically described in an elaborate monograph. -Then, for the first time, did geologists obtain a true notion of the -nature of those siphonated shells, which lie grouped by hundreds in the -palæozoic and secondary formations. Yet we still want an account of the -habits of the nautilus. The older naturalists alleged that it could -at pleasure rise to the surface or sink into the depths of the ocean; -that it could spread out its fleshy arms and float across the waves or -draw them in, capsize the little vessel, and so return to a creeping -posture among the sea-weed at the bottom. These statements may to some -extent be true, for the chambers of the nautilus shell must impart -great buoyancy to it. But in the meantime the story of the sailing -propensities of the animal is derived from a sort of mythic age, and -must be viewed with some little suspicion. Until further observations -are made, we shall neither fully understand the economy of the nautilus -nor the habits of the cephalopods of the palæozoic seas. But the day -is probably not far distant when such doubts will be set at rest, and -we shall know whether the nautili and orthoceratites swam in argosies -over the surface of the ocean, or, keeping ever at the bottom, left the -waves to roll far above them, unvaried save perchance by some floating -sea-weed or drifted tree. - -[Footnote 40: And perhaps even that is doubtful, for it is not unlikely -that after all, the palæozoic nautili may belong in reality to another -genus. Twenty years hence will probably see no little change on our -present identifications.] - - - - -CHAPTER VII. - - - Classification of the naturalist not always correspondent with - the order of nature--Incongruous grouping of animals - in the invertebrate division--Rudimentary skeleton - of the cephalopods--Introduction of the vertebrate - type into creation--Ichthyolites of the carboniferous - rocks--Their state of keeping--Classification of fossil - fishes--Placoids--Ichthyodorulites--Ganoids--Their structure - exemplified in the megalichthys and holoptychius--Cranium of - megalichthys--Its armature of scales--Microscopic structure of - a scale--Skeleton of megalichthys--History of the discovery - of the holoptychius--Confounded with megalichthys--External - ornament of holoptychius--Its jaws and teeth--Microscopic - structure of the teeth--Paucity of terrestrial fauna in coal - measures--Insect remains--Relics of reptiles--Concluding - summary of the characters of the carboniferous fauna--Results. - -The organic remains hitherto described belong to that large division -of the animal kingdom instituted by Lamarck, to comprehend all those -whose internal structure is supported by no vertebral column, and -which are hence termed invertebrate. They are for the most part -protected by a hard outer covering, or exo-skeleton, which assumes -many different modifications. We have seen it in the calcareous -cells of the little net-like fenestella, in the geometric cup of -the stone-lily, in the double case of the cypris, and in the shells -of the mollusca. But the order of nature does not always exactly -correspond with the classification of the naturalist. His system -must necessarily be precise, formal, and defined. One tribe ends off -abruptly, and is immediately succeeded by another, with different -functions and structure, and dignified with a separate name. But in -the order of creation, such abrupt demarcations are few, for if they -exist in the present economy, they can not unfrequently be filled up -from the existences of the past. There is usually a shading off of -one class into another, like the blending of the tints of sunset, -and it often baffles all the skill of the profoundest anatomist, by -drawing a distinct line, to pronounce where the one division actually -ends and the other begins. Any name, therefore, which is intended to -embrace a large section of the animal kingdom, must ever be more or -less arbitrary. It will extend too far in one direction, and embrace -organisms which might be classed in a different section. It will -probably not extend far enough in another, and thus leave beyond its -pale animals possessing strong affinities to the majority of those -included under it. More especially is this true of every system of -classification that proceeds upon the modifications of a single -feature, or upon mere negative resemblances. Suppose, for instance, -that it were proposed by some highly systematic individual to divide -the inhabitants of our country into two great classes--the bearded -and the beardless. In the latter category he would arrange all the -more quiet and orderly portion of the community, with perchance a -tolerable intermixture of rogues. The bearded group would present a -most motley array--from the fierce-visaged heroes of the Crimea to the -peaceable stone-mason or begrimed pitman--all brought into one list, -and yet agreeing in no single feature save that of being like Bully -Bottom the weaver, "marvellous hairy about the face." But Lamarck's -invertebrate division of the animal kingdom presents a grouping of yet -more diverse characteristics, as cannot fail to be confessed when we -recollect that it embraces among its members the microscopic monad, -the coral polyp, the lobster, the butterfly, the limpet, the nautilus, -and the cuttle-fish. Cuvier's three-fold grouping of the division -into _mollusca_, _articulata_, and _radiata_, has now supplanted the -old name, though the latter is still retained as a sort of convenient -designation for all the animals below the vertebrate type. - -The most highly developed of the recent cephalopods exhibit a true -internal skeleton, in the form of a strong oblong bone, on which the -body is hung. In this respect they occupy a sort of intermediate place -between the lower molluscs on the one hand, and the lower fishes on -the other. Theirs is not a vertebral column, but rather, as it were, -a foreshadowing of it; not, however, as a link in some process of -self-development from mollusc to fish, for these higher cephalopods do -not appear to have been created until fishes and reptiles had lived -for ages. The vertebrate type has been traced well-nigh as far back -into the past as we have yet been able to penetrate. Once introduced, -it has never ceased to exist, but in the successive geological ages -has been ever receiving newer and higher modifications, reaching its -perfection at length in man. The vertebrate form of structure fulfils -the highest adaptations of which terrestrial beings seem capable. We -can hardly conceive of corporeal existence reaching a more elevated -stage of development, save in thereby becoming less material, and -receiving an impartation of some higher element. The vertebrate -animals display not merely the most complexly organized structures, -but manifest in their habits the workings of the higher instincts -and affections. Among the invertebrate tribes the propagation of the -species is, in the vast majority of cases, a mere mechanical function, -like that of feeding or respiration, and the eggs once deposited, the -parent has no further care of her young. But among the vertebrated -animals, on the other hand, the perpetuation of the race forms the -central pillar round which the natural affections are entwined. It -parcels out every species into pairs, in each of which the mates are -bound together by the strongest ties of attachment. It gives birth, -too, to that noble instinct which leads the mother to expose her own -life rather than suffer harm to come to her offspring. It produces, at -least in man, that reciprocal attachment of offspring to parent, from -which springs no small part of all that is holiest and best in this -world. These attributes, to a greater or less extent, belong to all -the vertebrate animals, from the fish up to man. In looking over the -relics of animal life in the earlier geological formations, we are apt, -as we gaze on the massive jaws and teeth, the strong bony armour, and -the sharp, barbed spines, to think only of a time of war and carnage, -when the larger forms preyed upon the smaller, or ruthlessly sought -to exterminate each other. Yet should we not remember, that with all -these weapons and instincts of self-preservation there were linked -attributes of a nobler kind; that the earliest vertebrate remains point -to the introduction--though perhaps in but a rudimentary form--of -self-sacrificing love into our planet? The march of creation from -the first dawn of life has ever been an onward one, as regards the -development not only of organic structure but of the social relations; -and if it be true that physical organization finds its archetype in -man, it is assuredly no less so that in him too we meet with the -highest manifestation of those instincts which, by linking individual -to individual, have ever marked out the vertebrate tribes of animals -from the more machine-like characteristics of the invertebrate. - -We pass now to the vertebrate animals, and shall look for a little into -the general grade and organization of the fishes that characterized the -carboniferous rivers and seas. - -A collection of the ichthyolites of the carboniferous rocks presents -almost every variety in the mode of preservation. The smaller species -are frequently found entire, and show their shining scales still -regularly imbricated as when the creatures were alive. The larger forms -seldom occur in other than a very fragmentary condition. The limestones -yield dark-brown or black, oblong, leech-like teeth, which are found -on examination to be those of an ancient family of sharks. The shales -are often sprinkled over with glittering scales and enamelled bones. -Some of the coals and ironstones yield in abundance long sculptured -spines, huge jaws bristling with sharp conical teeth, and detached -tusks, sometimes five or six inches long. In short, the naturalist who -would decipher the ichthyology of the Coal formation, finds before -him, in the rocks, not a suite of correctly arranged, and carefully -preserved skeletons, but a set of disjointed, unconnected bones; here -a tooth, there a scale, now a jaw, now a dermal plate, all mingled at -random. And yet, though the evidence lie in this fragmentary state, our -knowledge of these ancient fishes is far from being correspondingly -meagre. To such precision has the science of comparative anatomy -arrived, that a mere scale or tooth is often enough to indicate the -nature and functions of the individual to which it belonged, and to -establish the existence in former times of a particular class or order -of animals. Thus the smooth rounded teeth of the mountain limestone are -found to present both externally and internally a close resemblance to -the hinder flat teeth of the sole living cestraciont (_C. Philippi_); -and we hence learn that a family of sharks, now all but extinct, -abounded in the palæozoic seas. The occurrence of a set of dark, -rounded little objects, which by the unpractised eye would be apt to be -mistaken for pebbles, is in this way sufficient at once to augment our -knowledge of the various animals of the Carboniferous period, and to -establish an important fact in the history of creation. - -Of the four great Orders into which Agassiz[41] subdivided the class -_Pisces_, the Placoids and Ganoids, agreeing on the whole with the -cartilaginous fishes of Cuvier, occur abundantly in the palæozoic -rocks, while the Cycloids and Ctenoids, answering to Cuvier's -osseous fishes, began in the Secondary formations, and are found in -all subsequent deposits. The two former reached their maximum in the -earlier geological ages, and have been gradually dwindling down ever -since, till now they are represented by comparatively few genera; the -two latter are emphatically modern orders; they have been constantly -increasing in numbers since their creation, and swarm in every sea at -the present day. The carboniferous ichthyolites belong, of course, only -to the two first-mentioned orders the placoids and ganoids. - -[Footnote 41: The classification of Agassiz, which is certainly not -a little arbitrary and artificial, has been altered by Müller, a -distinguished German anatomist, whose arrangement has been modified -again by Professor Owen. See Owen's _Lectures on Comparative Anatomy_, -vol. ii. p. 47. There is far from anything like unanimity on the -subject. Every naturalist thinks himself at liberty to modify and -restrict the groupings of his predecessors or contemporaries, sometimes -without condescending to give synonyms or any clue by which one may -compare the rival classifications. The geological student cannot engage -in a more sickening task than that of ranging through these various -arrangements, and he must possess some self-command who can refrain -from throwing up the search in disgust. The best way of progressing is -to select some standard work and keep to it, until the characteristics -of the genera and families have been mastered, and as far as possible, -verified from actual observation. After such preliminary training, the -student will be more able to grope his way through the "chaos and dark -night" of synonyms and systems.] - -The Placoid, or _Plagiostome_ fishes, are familiar to us all as -exemplified in the common thornback and skate of our markets. They are -covered with a tough skin, which either supports a set of tuberculed -plates as in the thornback, or a thick crop of small rounded bony -points or plates, as in the shagreen of the sharks. The head consists -of a single cartilaginous box. The spinal column is likewise formed -of cartilage, built up in the higher genera of partially ossified -vertebræ. The tail is heterocercal or unequally lobed, inasmuch as -the spinal column, instead of ending off abruptly as it does in the -herring, trout, and all our commoner fishes, passes on to the extreme -point of the upper half of the tail. This is a noticeable feature, for -it has been found to characterize all the fishes that lived in the -earlier geological periods. The fins are often strengthened by strong -spines of bone, which stand up in front of them and serve the double -purpose of organs of progression and weapons of defence. The teeth vary -a good deal in form. In the larger number of existing placoids they -are of a sharp cutting shape, often with saw-like edges. Among the -sharks they run along the jaws in numerous rows, of which, however, -only the outer one is used, those behind lying in reserve to fill up -the successive gaps in the front rank. The teeth do not sink into the -jaw, as in the ganoids, but are merely bound together by the tough -integument which forms the lips. Another form of tooth, abundant among -the ancient placoids, and visible on some of those at the present -day, shows a smooth rounded surface, the teeth being closely grouped -together into a sort of tessellated pavement which, in the recent -species, runs round the inner part of the jaws, while a row of conical -teeth guards the entrance of the mouth. - -[Illustration: Fig. 28.--Ctenacanthus hybodoides. (Edgerton.)] - -The animals which possess these characteristics include the various -tribes of the sharks and rays, and form the highest group of fishes. -They are all active and predaceous, frequenting every part of the ocean -where their prey is to be found. The formidable spines and hideous -"chasm of teeth" belonging to the bulkier forms, render them more than -a match for any other denizens of the deep, and thus they reign in -undisputed supremacy--the scourge of their congeners, and a terror to -man. - -The seas of the Carboniferous era abounded with similar predaceous -fishes, some of which must have been of enormous size. An entire -specimen has never been obtained; nor, from the destructible nature -of the animal framework, can we expect to meet with one. But the hard -bony parts of the animals, those capable in short of preservation -in mineral accumulations, are of common occurrence in the mountain -limestone beds and even among the coal seams. The dorsal spines or -_ichthyodorulites_, are especially conspicuous (Fig. 28). They stood -up along the creature's back like masts, the fin which was attached to -the hinder margin of each, representing the sail. The spine could be -raised or depressed at pleasure, its movements regulating those of the -fin, much as the raising or lowering of the mast in a boat influences -the lug-sail that is attached to it. The general form of these spines -was long, tapering, and more or less rounded. But they assumed many -varieties of surface ornament. Some species were ribbed longitudinally, -and had along their posterior concave side a set of little hooks -somewhat like the thorns of a rose. Others seem to have been quite -smooth, and of a flattened shape, with a thick-set row of sharp hooks -down both of the edges, like the spine on the tail of the sting-ray -of the Mediterranean. Such weapons have considerable resemblance to -the barbed spear-heads of savage tribes, and it is certain they were -intended to act in a similar way, as at once offensive and defensive -arms. The toothed spines of the sting-rays are still used in some -parts of the world to point the warrior's spear and arrow. Is there -not something suggestive in the fact that these stings, after having -accomplished their appointed purpose as weapons of war in the great -deep, should come to be employed over again in a like capacity on the -land; and that an instrument, which was designed by the Creator as a -means of protecting its possessor, should be turned by man into an -implement for gratifying his cupidity and satiating his revenge? Other -ichthyodorulites are elegantly ornamented by long rows of tuberculed -lines arranged in a zig-zag fashion, or in straight rows tapering from -base to point. In all there was a blunt unornamented base, which sank -into the back and served as a point of attachment for the muscles -employed in raising or depressing the spine. In some specimens the -outer point appears rounded and worn, the characteristic ornament being -effaced for some distance--a circumstance which probably indicates that -these fishes frequented the more rocky parts of the sea.[42] - -[Footnote 42: See Egerton, _Quart. Jour. Geol. Soc._, vol. ix. p. 281.] - -The placoid teeth of the carboniferous rocks show the usual forms -of the order. Some of them are sharp and pointed, as those of the -hybodonts; others have a smooth, rounded, or plate-like form, as in the -cestracionts. The latter often show a dark brilliant surface, and might -be readily enough mistaken for well-worn pebbles. In the oblong rounded -teeth of _psammodus_ the surface is densely covered with minute points -like grains of sand, whence the name of the genus. These teeth, when -sliced and viewed under the microscope by transmitted light, exhibit a -complex reticulated internal structure. - -Agassiz' second great Order of fishes is named Ganoid, from a Greek -word signifying brightness, in allusion to the brightly enamelled -surface of their dermal covering. They differ from the placoids in -having their outer surface cased in a strong armature of bone, which -is disposed either in the form of large overlapping plates, as among -the strange tortoise-like fishes of the Old Red Sandstone, or as thick -scales, which are either placed at intervals, as along the back and -sides of the sturgeon, or closely imbricated, as in the stony-gar -(_lepidosteus_) of the American rivers. This strong, massive skeleton -constitutes in many genera the sole support of the animal framework, -the inner skeleton being of a gristly cartilaginous kind, like that -of the skate. On this account traces of the vertebral column are by -no means abundant among the older formations. But as the ganoids form -a sort of intermediate link between the placoid or gristly fishes -on the one hand, and the bony fishes on the other, they are found -to present in their different genera examples of both these kinds -of structure. Thus, the skeleton of the sturgeon consists of a firm -cartilage, out of which the vertebræ are moulded, so that this fish -was at one time ranked with the sharks in the cartilaginous tribe of -Cuvier. The skeletons of some of the older ganoids (as _holoptychius_), -on the other hand, manifest such a decidedly osseous structure, with -sometimes so much of a reptilian cast, that the bones were at first -referred to some huge extinct saurians. The head of the ganoid fishes -is encased in a set of large massive plates of bone, and the jaws -are furnished with several rows of small sharp teeth, intermingled -with a less numerous but larger-sized and more formidable kind. The -interior of the mouth likewise displayed in many ancient genera groups -of palatal teeth, so that the dental apparatus of these animals -must have been very complex and complete. The tail in all the older -ganoids was heterocercal, like that of the sharks, the lobes being -not unfrequently densely covered with minute overlapping scales of -bone--a peculiarity which also extended to the fins. But the fins -were sometimes strengthened in another way by having the foremost ray -greatly thickened and enlarged, so as to form a stiff spine like the -ichthyodorulites of the placoids. The whole of the external surface -of these ganoidal fishes glittered with enamel, and was usually -sculptured in the most graceful patterns or ornamented with fine -lines and punctures so minute as to be almost invisible to the naked -eye. Every plate, scale, fin-ray, nay, the very lips exhibited the -characteristic enamel mottled over with the style of ornament peculiar -to the species. And when we think we have exhausted the contemplation -of these beauties, it needs but a glance through an ordinary microscope -to assure us that the unassisted eye catches only a superficial glimpse -of them. The more highly we magnify any portion of these old-world -mummies, the more exquisite does its structure appear. - -In the carboniferous rocks of Great Britain, upwards of forty species -of ganoids have been detected. They have a wide range in size, the -smallest measuring scarce two or three inches, while the largest, to -judge at least from the bones which they have left behind, must have -reached a length of twenty, or perhaps even thirty feet The lesser -genera (Fig. 29) were characterized by small, angular, glossy scales, -usually ornamented either with a very minute punctulation, or with -fine hair-like lines which sometimes exhibited the most complicated -patterns. The scales were likewise occasionally serrated along the -exposed edges--a style of ornament which gives no little richness to -the aspect of the dermal covering. The fins, closely imbricated with -small angular scales of bone, sometimes displayed a striated ray in -front, but this neither possessed the strength nor the formidable -aspect of the corresponding spine among the placoids. The head was -encased in a set of bony plates fitting tightly into each other, and -ornamented with various patterns according to the species. The teeth -were very small and fine, resembling the bristles of a brush, but in -at least some species intermingled with teeth of a larger size. The -minute style of dentition in these smaller fishes has been thought to -indicate their habit of keeping to the bottom of the water and feeding -on the soft decaying substances lying there. Nowhere have I seen the -small rhomboidal scales of the _palæoniscus_ so abundant as among dark -shales charged with cypris cases and fragments of terrestrial plants, -and on such occasions the idea has often occurred that these graceful -little fishes, like the _amia_ of the American rivers, may have fed on -the cyprides that swarmed along the bottom of the estuary. - -[Illustration: Fig. 29.--Amblypterus macropterus (a Carboniferous -ganoid).] - -Scattered over the fresh-water limestones, ironstones, and shales, or -crowded together along the upper surface of some of the coal-seams, -there occur the remains of two very remarkable ganoidal fishes. They -deserve our attention for their great size, their complex organization, -and the important place in the scale of animal life which they occupied -during a former period. One of them has been called _megalichthys_ -or _great fish_--an unhappy name, since the animal did not reach the -dimensions attained by not a few of the other ganoids, and was even -surpassed by at least one of its contemporary congeners. The other -is known as the _holoptychius_ or _wrinkled scale_. A more detailed -examination of these two animals will perhaps best enable us to -understand the character of the ganoid fishes that lived in the waters -of the Carboniferous period. - -The megalichthys had an average length of about three feet. Like -the other members of the ganoid order it had a glittering exterior, -every scale and plate being formed of strong bone, and coated with a -bright layer of enamel. Wherever this polished surface extends, it is -found to be ornamented with a minute punctulation, the pores of which -lie thickly together like the finer dots of a stippled engraving. -The cranial plates are further varied by a scattered and irregular -series of larger punctures that look as if they had been formed by the -insertion of a pin-point into a soft yielding surface. The examination -of the head of the megalichthys as depicted in Fig. 30, will convey an -adequate conception of the structure of a ganoidal cranium. - -[Illustration: Fig. 30.--Head of Megalichthys Hibberti, one-sixth of -natural size (Agass. _Poiss. Foss._ Tab. 63). - -A Upper side. B. Under side. C Profile.] - -The snout is formed of an elegantly curved bone (_c_) fringed along -its under edge with minute thick-set teeth. On either side it is -flanked by two triangular plates, which occupy the space between the -intermaxillary bone (_c_) and the upper jaws (_q q_). The eye orbits -seem to have been at the corners of the intermaxillary, circumscribed -by the sub-orbitals (_f g h_) and the ethmoids (_b_). The massive -intermaxillary bone had its posterior margin of an angular form, and -into the notch thus formed there was wedged the anterior end of a long -strip of plates, which expanded as they approached the occipital part -of the cranium, and terminated in three irregular plates that may -represent the place of the parietal and occipital bones. The space -between this belt and the upper jaws was occupied by three large plates -(_i k l_) which in other ganoids, as the _osteolepis_ of the Old Red -Sandstone, were united into a single pre-opercular bone of considerable -size. The operculum or gill cover (_m_) was relatively large, and -had an elegantly curved anterior margin. The upper jaws (_q_) were -comparatively small, and had a fringe of small conical teeth. The under -jaws (_r r_) reached to nearly double the length of the upper, and were -similarly set round with teeth. The teeth of the megalichthys, like -those of the living lepidosteus, consisted of two kinds, of which the -one bristled thickly along the outer edge of the jaw as sharp minute -points, averaging about a line in length, while behind this outer row -lay a scattered series of much larger teeth that sometimes rose nearly -an inch above the jaw. The external surface of these more formidable -tusks is smooth, glittering, and minutely striated with fine lines from -base to point, while the root of each is farther marked by a circle of -short, deep, longitudinal furrows. The internal structure displays a -close ivory, which when viewed under a microscope is seen to be made up -of fine tubes radiating from the outer surface to the hollow central -cavity. Some of the bones in the interior of the mouth seem to have -been also furnished with an apparatus of teeth. The under surface of -the cranium between the arch of the under jaws consists of two oblong -central plates (_t_) surrounded by a row of sixteen irregular ones, -eight on each side, and terminated in front by a large lozenge-shaped -scale (_u_) which fits into their angle of junction on the one side, -and into the symphysis of the jaws on the other. In the osteolepis -there were likewise two large plates terminating in a similar -lozenge-shaped one, but without the flanking rows. In the famous Old -Red holoptychius of Clashbennie, the under surface of the head had -but two plates, and in the still older and more gigantic asterolepis, -there was but one. It is the delightful task of the paleontologist -to compare and contrast these various pieces of mechanism, to mark -how what seems lacking in one comes to be supplied in another, and to -trace out the various modes in which, during the ages of the past, -Nature has wrought out the same leading plan, sounding, as it were, an -ever-changing series of modulations upon one key-note. In comparing -together the ganoids of the Old Red Sandstone and the Carboniferous -rocks, he finds that in the asterolepis--a fish belonging to the lower -part of the former formation--the pointed arch formed by the sweep of -the lower jaws is filled up by a single plate like some abbey-window -with its mullions knocked away, and built up with rude stone and lime. -Higher in the same group of rocks he meets with the cranium of the -holoptychius, where there is one straight central mullion running -in an unbroken line from the angle of the arch to its base. In the -osteolepis[43] he sees this mullion branching into two at its upper -end, so that the window consists of three divisions, as in the simplest -style of Gothic. Passing upwards into the Carboniferous system, he -encounters a still more ornate arrangement in the cranium of the -megalichthys. The central mullion with its two upper branches still -remains, but it is flanked by an additional one on each side, from -which there spring six cross bars that diverge obliquely with a slight -curve, so as to join the outer arch and subdivide the window into -nineteen compartments. So varied are the plans of the Divine Architect -in what to man may seem such a little matter as the piecing together of -a fish's skull. - -[Footnote 43: Hugh Miller's _Footprints of the Creator_, p. 91.] - -The body of the megalichthys was cased in an armature of as solid and -glittering bone as that which defended its head. Where the plates of -the cranium ended off they were succeeded by large rhomboidal scales -that crossed the body obliquely, and overlapped each other like the -metal plates in the antique scale-armour. Each scale consisted of -two parts, of which one had a rhomboid form and was covered over with -enamel, while the other ran round the two inner sides of the rhomb -as a broad unenamelled selvage deeply indented along its centre. It -was the enamelled portion alone that formed the outer surface, the -rough unpolished border being covered by the overlapping edges of -the adjoining scales. The scales had not a uniform thickness, but -were strongest at the covered part from which each thinned off to -the outer edges. In this way the thin edge of one scale pressed down -on the thick part of the subjacent one, and a covering of uniform -strength and smoothness was produced. Looking at a set of these scales -as they still occupy their original position on the creature's body, -it is scarcely more than a half of each which meets our eye; for the -unenamelled border occupied about a third of the entire surface, and -a fourth of the remainder was covered by the overlapping scales. The -effect of this arrangement must have been to combine great strength -with the most perfect flexibility. Notwithstanding the bulk of his -helmet and the weight of his scale-armour, we cannot conceive the -megalichthys to have been other than a lithe, active, predaceous fish, -dealing death and destruction among the herring-like shoals of little -palæonisci and amblypteri, though able to maintain perhaps but a -doubtful warfare with his more bulky contemporary, the holoptychius. -The internal structure of the scales of the megalichthys exhibits the -same provision for combining strength with the least possible amount -of material. Viewed in a transverse section under a magnifying power -of about eight diameters, they are seen to consist of three layers of -bone; each possessing a peculiar structure. The outermost is formed -of a tessellated pavement of minute round ocelli, having a fine brown -colour, and placed close together with considerable regularity. They -somewhat resemble little wheels, the axle being either a dark solid -nucleus or a small circular aperture, whence there radiates to the -outer rim a set of exceedingly minute fibres which were originally -hollow, and served as canals to carry on the growth of the scale. The -vacant space left where four wheels impinge on each other, forms one -of the pores that cover the enamelled surface of the scale. The whole -structure of this outer layer very closely resembles that presented by -the internal part of the base of the teeth, save that the confluent -lobes shown in the teeth become in the scale detached into separate and -independent circles. The central stratum of each scale is composed of -a loose open network of cancellated bone that passes into the layer on -either side, and resembles in its general texture the osseous vertebræ -of the same fish. The under layer, one end of which rested immediately -on the skin, approaches more to the firmness and solidity of the outer -one, but, in place of a tessellated, ivory-like pavement, it had a -close fibrous texture, with here and there a scattered cavity, and the -fibres were matted together so as to resemble the more solid structure -of the cranial bones. The effect of this triple arrangement must have -been to impart great strength and lightness to the external armature -of the fish; the middle spongy layer serving, by its porosity, at once -to deaden the effect of any blow aimed at the outside, and to give -buoyancy and lightness to what would otherwise have been a coat of -mail well-nigh, as ponderous as that of a feudal chief. One can hardly -conceive any implement of warfare in use among the lower animals of -strength enough to pierce this massive covering. But we shall find -as we go on that if the megalichthys had a strong defensive armour, -a bulkier neighbour had a still stronger offensive one, and that the -enamelled plates of the one fish were scarcely a match for the huge -pointed tusks of the other. - -The megalichthys had an osseous skeleton, with vertebræ of a discoidal -form. These internal bones when viewed under the microscope are found -to display an open cancellated structure, resembling that of the -central layer in the scales. It thus appears that this ancient fish -was not merely defended by a hard external armour, but possessed an -equally solid framework of bone within. - -Mingled with the scales and bones of the megalichthys, there are found -the remains of a still larger fish, to which the name of Holoptychius -has been given. Its external ornament differed entirely from that of -the animal last described. It possessed teeth sometimes six or seven -times larger, and jaws, plates, and bones of a form and dimensions -totally distinct. Strange as it may seem, however, these two fishes -have been constantly and systematically confounded from the time when -they were first discovered. Two or three years ago, there might be -seen in the British Museum several specimens of the holoptychius, -of which some bore the correct name, while the rest were labelled -"Megalichthys;" and a similar error prevailed in several of the other -museums.[44] The confusion can be traced very distinctly in the memoir -of Dr. Hibbert, who for the first time described the remains of these -fishes, and wrote according to information received from Agassiz. - -[Footnote 44: The mistake was noticed in 1845 by Hugh Miller, who, in a -foot-note to his _First Impressions of England and its People_, p. 71, -well defines the distinctions between the two ichthyolites.] - -In the autumn of 1832, the attention of the scientific public of -Edinburgh was directed to the extraordinary character of some fossil -remains obtained from the lime-quarries of Burdiehouse, a village about -four miles to the south of the town. Dr. Hibbert visited the locality, -and soon saw enough to excite his lively interest in its thorough -investigation. The Royal Society of Edinburgh warmly supported his -exertions, and by their means a large suite of specimens was eventually -obtained, which the Doctor from time to time described as they were -successively received. At the meeting[45] of the British Association in -Edinburgh, in 1834, the specimens were exhibited before the Geological -Section, and a memoir upon them read by their successful discoverer. -On the conclusion of the paper, a lively discussion ensued upon the -nature of the animal to which the scales and teeth had belonged. -Dr. Hibbert argued, from the deeply-furrowed teeth, and the strong, -massive cranial plates, that the animal must have been a reptile, and -supported his assertion by no small amount of anatomical skill. In the -midst of the discussion, a message was sent to the great ichthyologist -of Neufchatel, who happened to be at that time busily engaged in the -Zoological Section. Passing over the fossils as they lay grouped -upon the table, with that quick perception for which he is so justly -celebrated, Agassiz at once decided that the bones must have been -those of some large and hitherto undescribed _fish_. Such a decision -from such an authority produced of course no little sensation, and the -naturalist was told with some surprise that the remains had just been -elaborately described as those of extinct reptiles. "Reptiles!" thought -Agassiz, and again his quick eye darted over the table; but the fossils -would yield no other answer than what they had already given. Despite -their seeming reptilian character, they were undoubtedly ichthyic, -though belonging to an animal up to that time unknown. In the completed -memoir which Dr. Hibbert subsequently submitted to the Royal Society, -his mistake was freely acknowledged, and the remains there flourish as -those of a true fish. But with this amendment a grave error of another -kind was committed, though in this the Doctor seems to have been -supported by the authority of Agassiz himself. The large bones, scales, -and teeth of the Burdiehouse limestone, were all indiscriminately -thrown into one genus, to which Agassiz gave the name of Megalichthys; -and in the memoir we find the different kinds of scales and teeth -described and figured without the slightest intimation or suspicion -that they might possibly have belonged to different animals. The -novelty of the discoveries soon attracted general attention to -Dr. Hibbert's paper. It was quoted or referred to in almost every -scientific work treating of general geology, while in some instances -(as in Dr. Buckland's _Bridgewater Treatise_) the erroneously-named -bones were re-engraved. A tooth from the Fife coal-field, drawn -for one of the woodcuts in a popular elementary manual, was also -named megalichthys; an error perpetuated through every edition till -the last, where the tooth has been restored to its true owner--the -holoptychius. In truth, no two organisms have ever been so maltreated; -and if the reader will kindly bear with me a little further, it will -not be difficult to show him that the holoptychius had peculiarities -of its own quite as distinct as those that have come before us in the -megalichthys, and that each animal has a full and legitimate claim to a -separate and independent niche in the gallery of fossil fishes. - -[Footnote 45: See Agassiz, _Poiss. Foss._, tom. ii. Part 2, p. 89 _et -seq._] - -The word _holoptychius_ means, as I have said, "wrinkled or folded -all over,"--a name truly expressive of the peculiar style of ornament -displayed by every part of the exterior of the animal's body. The -head-plates, which are of great size, exhibit a fine corrugated -shagreen-like surface, roughened into knobs, and wavy lines of -confluent tubercules, that remind one disposed to be fanciful, of a -frosty December moon with its isolated peaks, and confluent mountain -chains. The scales are of a rounded or oval form, and vary from less -than half an inch to fully four or even five inches in diameter. -Their upper side consists of two parts, one of which with a crescent -shape lay beneath, the over-lapping scales, while the other passed -outwards to form a portion of the outer visible surface. The part -that was hidden by the overlapping scale was smooth, with a finely -striated surface. The exposed portion displayed the usual corrugated -sculpturing, many of the little tubercules having striated sides, and -showing, in consequence, no little resemblance to the star-like knobs -on the dermal covering of the Old Red Sandstone asterolepis. The inner -surface of the scales was concave, with a central prominent oblong -point surrounded by encircling scaly ridges, and forming what is called -the centre of ossification.[46] - -[Footnote 46: The above descriptions of the scales and teeth of these -two fishes, are taken from specimens in my own collection. None of my -holoptychian scales show incontestable the proportion of the covered to -the exposed part. Judging from the aspect of one of them, the wrinkled -portion occupied perhaps about three-fifths of the entire scale, -the remaining part being covered by the overlapping edges of those -adjacent; for the characteristic corrugated surface was essentially -an external ornament, and ceased at the point where the external -bone passed into the interior. I may remark, that the upper side of -the scales is not very frequently seen in the Burdiehouse limestone, -the rough surface usually adhering to the rock, and leaving only the -smooth inner side exposed. Out of seven specimens from that locality, -only one shows the upper side, and that by no means in a perfect state -of keeping. The structure alike of scales and bones can be seen to -much greater advantage in the shales, ironstones, and coals of the -coal-fields, where, owing to the soft nature of their matrix, the -fossils can be readily cleared and exposed.] - -But perhaps the most remarkable and characteristic parts of the -carboniferous holoptychius were its jaws and teeth. As we might -readily conjecture from the great size and strength of the scales and -cranial plates of this fish, its dentition was of a correspondingly -massive type. The under jaw, with the usual corrugated ornament, -frequently exceeded a foot in length, and displayed along its upper -edge a thick-set group of teeth. Of these there were two kinds one -of a smaller size and more blunted form, with short indented furrows -at their base; the other of a greatly more formidable size, grouped -at intervals among the smaller ones. The front end of each under jaw -bore one of these long conical tusks, serving as it were to guard the -entrance of the mouth. Each of the larger teeth had a base strongly -marked with longitudinal furrows, and sank deep into the jaw, with -the bone of which it sometimes anchylosed.[47] The part of the tooth -above this socket had an oval form, so flattened as to present two -cutting edges, one facing the front, the other the back of the mouth, -and meeting at the upper end of the tooth which was sharp and pointed. -Such large conical tusks may frequently be obtained, having a length -of two or three inches, while occasionally they range as high as six -or seven, the smaller teeth seldom reaching so much as an inch. It -is difficult to see how, with such a formidable dentition, the jaws -could readily close. In some specimens I have seen deep hollows beside -the bases of the teeth, which may possibly have received those of the -opposite jaw, but the gigantic tusks at the entrance of the mouth seem -to have stood high over the jaw, passing outside like those of the -wild-boar. If this be correct, the jaw of the holoptychius would unite -the mechanism of both the alligator and the crocodile--its recipient -hollows being analogous to the tooth-pits in the former tribe, and its -protruded teeth to the similarly exposed teeth of the latter. - -[Footnote 47: I have seen detached teeth, wherein the length of -the root, or part imbedded in the jaw, tripled that of the exposed -part, sinking four or five inches into the bone without any trace of -anchylosis. Whether these huge tusks belonged to the upper or under -maxillary, I do not pretend to say, though no specimen of the under -jaw, which has ever come under my notice, would accommodate half of -such a deep-sunk base.] - -[Illustration: Fig. 31.--Jaw of Holoptychius (_Rhizodus_.--Owen) from -Gilmerton, one-fourth nat. size; the large teeth along the middle part -of the jaw are here wanting.] - -When we bring the microscope to bear upon the elucidation of the -structure of these ancient teeth, it seems as if our labour had but -just begun; and that so far from having by an external scrutiny -exhausted all that they have to show us, our knowledge of them can be -but scanty and superficial until we have studied them carefully under -a magnifying power. Microscopic sections of such organic remains are -prepared in the same way as those of the fossil woods already noticed; -and a more interesting or beautiful series of objects cannot be -conceived than a set of slices of these fossil-teeth. - -Viewed, then, in longitudinal section from base to point, the part -above the fluted root of one of the large teeth of the holoptychius is -seen to consist of minute hair-like fibres of extreme tenuity, which -proceed in straight lines from the outer surface to the interior. At -right angles to these, and parallel with the outer edges, there is -a set of dark widely-placed lines conforming to the outline of the -tooth, like so many long sugar-loaf shaped caps, placed within each -other. When this part is cut across, and viewed in transverse section, -the tooth is observed to be of a flattened oval form, with the same -fine fibres or tubes radiating from the centre, and traversed by the -same dark bands which now assume the form of concentric rings. The -appearance thus presented reminds one at once of a cross section -of some dicotyledonous tree, the dark bands resembling the annual -layers of growth, and like these resulting from a similar thickening -of the internal tissue. The upper part of the tooth is solid and the -concentric rings few; the middle exhibits an increase of the rings, and -possesses, moreover, a hollow centre or pulp-cavity,[48] with the usual -diverging fibres. Here the oval form is well shown, and the encircling -rings are considerably flattened at the ends of the long axis. - -[Footnote 48: This hollow centre may be seen occasionally filled up by -a sharp conical tooth like the _phragmocone_ of a belemnite.] - -The lower portion of the tooth exhibits a much more complicated -texture. Externally it is marked by deep longitudinal furrows, that -run down the enamelled sides and sink into the jaw. When cut across -at this ribbed part, the tooth is found to present the most complex -and graceful internal structure. The prominent ridges between the -furrows are seen to be produced by crumpled folds of the substance of -the tooth, which roll inwards towards the centre, coalescing with each -other, and forming intricate groups of circling knots and folds. In -some places they seem all but separated from each other into little -circles, pierced with a central aperture, and recall the aspect of -the upper layer in the scale of megalichthys. Each of these loops and -folds presents a texture exactly similar to that of the upper part of -the tooth. The same minute hair-like tubes, darkened and thickened in -the long axis, radiate towards the centre; the same concentric bands -run from centre to circumference; so that the lower part of the tooth -seems, as it were, made up of a bundle of smaller teeth partially -melted into each other. Between these loops and folds circular meshes -frequently occur, and add to the complexity as well as the beauty of -the whole structure. One of these sections, with all its twisting -crumples, and folds, and knots, and coloured meshes, and encircled -rings, bears no small resemblance to an antique polished table that -has been cut out of the gnarled roots of a venerable oak. This complex -structure arose from the mode of growth of the tooth; each prominent -external ridge continually turning inwards down the furrow on either -side, and mingling in freakish knots with the folds that had gone -before.[49] - -[Footnote 49: For an acquaintance with the remarkable teeth of this -ancient fish, more minute than it had been my good fortune to possess -before, I am indebted to a most interesting series of microscopical -preparations kindly lent me from his extensive collection by my friend -Mr. Alexander Bryson of Edinburgh.] - -The internal bones of the holoptychius were of great size and strength, -as befitted such a bulky ganoid. Some of them had a singular style of -surface ornament, that somewhat resembled a frosted widow on a December -morning. Their internal structure was loose and cancellated; the endo- -being usually of a less compact texture than the exo-skeleton. Judging -from the size of such bones, the carboniferous holoptychius must have -been one of the bulkiest and most formidable denizens of the deep, -reaching sometimes to a length of twenty feet or even more. Such an -animal would have been, perhaps, quite a match for our hugest crocodile -or alligator, for it must have swum about with a litheness and agility -possessed by none of the saurian reptiles. Like that leviathan chosen -by the Almighty, in an age long subsequent, as an illustration of His -power and greatness, the holoptychius must have been king over all -the inhabitants of the sea, and the magnificent language of Job, -descriptive of the living animal, applies not less graphically to -the extinct one: "Who can open the doors of his face? his teeth are -terrible round about. His scales are his pride, shut up together as -with a close seal. One is so near to another, that no air can come -between them. He maketh the deep to boil like a pot: he maketh the sea -like a pot of ointment. He maketh a path to shine after him; one would -think the deep to be hoary." - -Our survey has hitherto been directed to the denizens of carboniferous -lake, river, and sea, and we have found them to be alike important in -numbers and interesting in organization. It is otherwise, however, -when we turn in search of the denizens of the carboniferous lands. -The crowded trees and shrubs of the coal strata recalling as they do -old forest-covered swamps, might seem to indicate the probability of -a pretty numerous terrestrial fauna. Where are we to look for the -fossilized relics of land animals, if not in the remains of a submerged -land-surface? And yet, strange as it may seem, of the inhabitants of -the land during the Coal-measure period we know almost nothing. "We -have ransacked hundreds of soils replete with the fossil roots of -trees,--have dug out hundreds of erect trunks and stumps, which stood -in the position in which they grew,--have broken up myriads of cubic -feet of fuel, still retaining its vegetable structure,--and, after -all, we continue almost as much in the dark regarding the invertebrate -air-breathers of this epoch, as if the coal had been thrown down in -mid-ocean."[50] - -[Footnote 50: Sir Charles Lyell's _Elements_, fifth edition, p. 406.] - -The little land-shell already noticed as having been detected by Sir -Charles Lyell in the carboniferous rocks of Nova Scotia, seems to be -as yet the only air-breathing mollusc obtained from rocks of such -high antiquity. Insect remains have been detected in the English -coal-fields belonging to two or three species of beetles; while on the -Continent, wing-sheaths and other fragments of cockroaches, scorpions, -grasshoppers, locusts, crickets, &c., have been detected. But the most -remarkable traces of air-breathers consist in various indications of -the existence of reptiles during the Carboniferous era. Fragmentary -skeletons, with detached bones and plates, have been found in Bavaria -and America, together with long tracks of footprints, from which it -appears that during the time our coal-seams were forming, there swam -through the sluggish deltas, or crept amid the dank luxuriant foliage, -strange lizard-like forms, large enough to leave behind them on the -soft yielding mud or sand the impress of their double pair of toed -feet. But of these animals we have much to learn. Some of them have -bequeathed to us merely their dismembered broken bones; others have -left but the imprints of their toes. Yet even these remains, trifling -as they may seem, become of importance when we remember that they -demonstrate fishes not to have been the highest types of being during -the epoch of the Coal, and show that while the bulky holoptychius held -the supremacy of the waters, lizard-like forms of a less formidable -type seem, so far as we know, to have ruled it over the land. - -In fine, then, no one can glance at a list of the carboniferous fauna -without perceiving either that the animated world of that ancient -epoch must have had a very different proportioning from what now -obtains, or that we have only a meagre and fragmentary record of it. -That the latter conclusion is the more philosophical will appear if we -reflect upon the many chances that exist against the entombment and -preservation of animal remains, especially of those peculiar to the -land. How very small a proportion of the remains of animals living in -our own country could be gathered from the surface-soil of any given -locality, and how very inadequate would be the meagre list of species -thus obtained, as representing the varied and extensive fauna of Great -Britain! In contrasting, then, the rich abundance of marine organisms -with the extreme paucity of terrestrial animals among the carboniferous -rocks, it would be too hasty to infer a corresponding disproportion -originally. It must be admitted that the rarity of air-breathers, -after such long-continued and extensive explorations among terrestrial -and lacustrine beds, presents a difficult problem, only (if at all) -to be cleared away by patient and persevering investigation. With -this preliminary caution, we may regard the carboniferous fauna as -peculiarly rich in marine species. The sea-bottoms swarmed with -stone-lilies, cup-corals, and net-like bryozoa, mingled with the -various tribes of molluscan life--the brachiopods with their long -ciliated arms; the bivalves and gastropods with their coloured shells -that recall some of the most familiar objects of our shores; and the -cephalopods with their groups of siphonated chambers, straight as in -the orthoceras, or gracefully coiled as in the goniatite. The seas -swarmed, too, with fishes belonging to the two great orders of ganoids -and placoids, the latter represented now by our sharks and rays, -though the exact type of the ancient genera is retained only by the -cestracion or Port-Jackson shark; the ganoids, with their strong armour -of bone, represented by but two genera, the lepidosteus of the American -rivers, and the polypterus of the Nile,--two fishes that seem but as -dwarfs when placed side by side with the gigantic holoptychius of the -coal-measures. The rivers and estuaries of the same period seem to have -been frequented by immense shoals of the smaller ganoidal fishes that -fed on decaying matter brought down from the land, and perhaps, too, -on the minute Crustacea that lay strewed by myriads along the bottom. -Into these busy scenes the bulkier monsters from the sea made frequent -migrations, perhaps in some cases ascending the rivers for leagues to -spawn, and returning again to their places at the mouth of the estuary -or in open sea. The rivers and lakes swarmed with small crustaceous -animals, and nourished, too, shells like those of our pearl-mussels. -The land--so luxuriantly clothed with vegetable forms--was hummed over -by beetles, chirupped over by grasshoppers and crickets, and crawled -over by four-footed reptiles, that united in their structure the lizard -and the frog. But of the general grade and proportions of its denizens -we still remain in ignorance. From all that yet appears, the scenery -of these forests must have been dark, silent, and gloomy, buried in a -solitude that was startled by no tiger's roar, no cattle's low, and -neither cheered with the melody of birds nor gladdened by the presence -of man. - -We have lingered, perhaps, too long over the remains of these old -carboniferous animals. But the delay may be not without its use if, by -thus bringing before us some of the more marked points in the structure -of creatures that for ages peopled our planet, it broaden our view of -creation; and by lifting the curtain from off a dim, distant period of -our world's long history, it show, amid all diversities of arrangement, -and all varieties of form, still the same grand principles of design, -and the same modes of working as those which we can see and compare -among the living forms around us. It is something to be assured that -the race of man has been preceded by many other races, lower indeed -in the scale of being, but manifesting, throughout the long centuries -of their existence, ideas of mechanism and contrivance still familiar -to us, and serving in this way to link the human era with those that -have gone before, as parts of one grand scheme carried on by one great -Creator. - - - - -CHAPTER VIII. - - - Sand and gravel of the boulder--What they suggested--Their - consideration leads us among the more mechanical operations - of Nature--An endless succession of mutations in the economy - of the universe--Exhibited in plants--In animals--In - the action of winds and oceanic currents--Beautifully - shown by the ceaseless passage of water from land to - sea, and sea to land--This interchange not an isolated - phenomenon--How aided in its effects by a universal process - of decay going on wherever a land surface is exposed to - the air--Complex mode of Nature's operations--Interlacing - of different causes in the production of an apparently - single and simple effect--Decay of rocks--Chemical - changes--Underground and surface decomposition--Carbonated - springs--The Spar Cave--Action of rain-water--Decay of - granite--Scene in Skye--Trap-dykes--Weathered cliffs of - sandstone--Of conglomerate--Of shale--Of limestone--Caverns - of Raasay--Incident--Causes of this waste of calcareous - rocks--Tombstones. - -From the blackened plants that darkened the upper layers of the -boulder, the transition was natural to the matrix in which they lay. -The whole rock consisted of a fine quartzy sand more or less distinctly -laminated, and showing in its lower parts well-rounded pebbles of -quartz, green grit, and felspathic trap. The contemplation of these -features suggested the existence of some old land with elevated ranges -of hills, and wide verdant valleys traversed by rivulets and rivers -which bore a ceaseless burden of mud, sand, and gravel, onwards to -the sea. The pebbles afforded some indication of the kind of rocks -that formed the hill-sides. Perhaps the higher grounds exhibited that -grey wrinkled appearance peculiar to the quartz districts of the -north-western Highlands, with here and there a bluff crag of felspathic -trap shooting up from among the fern-brakes of the valley, or cutting -across the channel of some mountain stream that tumbled over the pale -rock in a sheet of foam. And there may have been among these uplands -smooth undulating districts, dotted over with dark araucarian pines, -and densely clothed with a brushwood of rolling fern, but which showed -in all their ravines the green grit that formed the framework of the -country,--its beds twisted and contorted, jointed and cleaved, like -the grits and slates along the banks of many a stream, beloved by -the angler, in the classic ground of the Ettrick and the Yarrow. But -whatever may have been the special features of its scenery, there can -be no doubt of the land's existence. The carbonized plants stand up to -tell us of its strange and luxuriant vegetation. We have listened to -their story, and suffered them to lead us away into forest, and lake, -and sea, to look on the various forms of life, vegetable and animal, -which abounded in that far-distant age. We return again to the boulder, -and shall now seek to learn the lessons which the sand and pebbles have -to teach us. Their subject belongs to what is called physical geology, -and will bring before us some of the more mechanical operations of -nature, such as the slow but constant action of air, rain, and rivers, -upon hard rock, the grinding action of the waves, and the consequent -accumulation of new masses of sedimentary rock. - -In all the departments of nature that come under the cognizance of man, -there is seen to be an endless succession of mutations. According to -the Samian philosopher-- - - "Turn wheresoe'er we may by land or sea, - There's nought around us that doth cease to be. - Each object varies but in form and hue, - Its parts exchange; hence combinations new. - And thus is Nature through her mighty frame - For ever varying, and yet still the same." - -In the world of life we see how animals are sustained by a constant -series of chemical changes in their blood, every respiration of air -adding, as it were, fresh fuel to the flame of life within. In plants, -too, there is an analogous process. The atmospheric air is by them -decomposed, part of it being given off again, and part retained to -build up the organic structure. Plants withdraw mineral matter from the -soil, animals feed upon plants, and thus the earthy substances, after -having formed a part, first of rock masses, then of vegetable, and -subsequently of animal organizations, are returned again to the soil, -whence to be once more withdrawn and undergo new cycles of mutation. -But this perpetual interchange is not confined to the vital world. We -see it in the action of winds, when heated air rises and moves in one -direction, and the colder parts sink and travel the opposite way. The -same principle is exhibited by the oceanic currents, the removal of a -body of water, from whatever cause, always necessitating the ingress of -a corresponding quantity to supply its place. But perhaps one of the -most beautiful instances of these interchanges in the whole inorganic -world is the ceaseless passage of water from the land to the sea, and -from the sea to the land. The countless thousands of rivulets, and -streams, and gigantic rivers, that are ever pouring their waters into -the great deep, do not in the least raise its level or diminish its -saltness. And why? Simply because the sea gives off by evaporation -as much water as it receives from rain and rivers. The vapour thus -exhaled ascends to the upper regions of the atmosphere, where it forms -clouds, and whence it eventually descends as rain. The larger part of -the rain probably falls upon the ocean, but a considerable amount is -nevertheless driven by winds across the land. This finds its way into -the streams, and so back again to the sea, only, however, to be anew -evaporated and sent as drizzling rain across the face of land and sea. -This interchange is constantly in progress, and seems to have been as -unvarying during past ages. - -But the ceaseless passage of water between land and sea is not a mere -isolated and independent phenomenon. Like all the rest of Nature's -processes, even the simplest, it produces important and complicated -effects. And the reader may, perhaps, think it worth looking at for a -little, when he reflects that to this seemingly feeble cause we owe no -small part of our solid lands, whether as islands wasted by the sea, -or as part of vast and variegated continents, wide rolling prairies -covered with verdure and roamed over by herds of cattle, or wintry -Alpine hills lifeless and bare. - -The truth of this will appear when we reflect that the moisture which -rises from the sea and falls on the land as rain, is free from any -admixture of impurities; but by the time it again reaches the sea, -after a circuit of perhaps many miles down valley and plain, it has -grown turbid and discoloured, carrying with it a quantity of mud, sand, -and drift-wood. The sediment thus transported soon sinks to the bottom, -where it eventually hardens into rock, and in course of time is raised -above the waves as part of a new land. Such I conceive to have been the -origin of the sand, gravel, and imbedded plants of our boulder. It may -be well, however, in going into the details of the subject, to take a -wider view of this interesting branch of geology, and look for a little -at the forms and modes of the decomposition of rocks, and the varied -manner in which new sedimentary accumulations are formed. - -All over the world, wherever a land surface spreads out beneath the -sky, there goes on a process of degradation and decay. Hills are -insensibly crumbling into the valleys, valleys are silently eroded, -and crags that ever since the birth of man have been the landmarks of -the race, are yet slowly but surely melting away. It matters not where -the hill or plain may lie, the highest mountains of the tropics and -the frozen soil of the poles, yield each in its measure and degree -to the influence of the general law. It might seem that so universal -a process should be the result of some equally prevalent and simple -cause. But when we set ourselves to examine the matter, we find it far -otherwise. The waste of the solid lands, in place of arising from some -single general action, is found to result from a multiform chain of -causes, often local in their operation and variable in their effects. -Such an investigation affords a good illustration of the general mode -and fashion in which Nature delights to work. It shows us that what -may seem a very simple process may be in reality a very complicated -one; that in truth there exist in the world around us few if any -simple, single processes, which stand out by themselves unconnected -with any other; that, on the contrary, all become intimately linked -together, the effects of one often forming part of the chain of causes -in another, and producing by their combined action that complex yet -strikingly harmonious order that pervades all the operations of Nature. -To an extent of which Cicero never dreamed, there runs through all -the world "such an admirable succession of things that each seems -entwined with the other, and all are thus intimately linked and bound -together."[51] Man separates out these various processes, classifies -and arranges them, because from the imperfection of his mental powers -he cannot otherwise understand their effects; all would seem but -chaos and confusion. But the formal precision and the sharp lines of -demarcation exist only in his mind. They have no place in the outer -world. There we see process dove-tailing with process, and spreading -out over the material world in an endless network of cause and effect. -We feebly try to trace out these interlacing threads, but we can follow -them far in no direction. Proteus-like, they seem to change their -aspect, blending now into one form, now into another, and so eluding -our keenest pursuit. - -[Footnote 51: Cicero, _De Nat. Deor._ lib. i. 4. So, in Bacon's _Wisdom -of the Ancients_, under the fable "Pan or Nature:"--"The chain of -natural causes links together the rise, duration, and corruption; the -exaltation, degeneration, and workings; the processes, the effects, -and changes, of all that can any way happen to things." Such is the -philosopher's explanation of the Destinies as sisters of Pan. In no -part of his writings can the thorough practical character of Bacon's -philosophy be more conspicuously seen than in his treatment of these -ancient fables. Glancing over the titles of the different papers, you -are tempted to wonder what an intellect which could only appreciate -poetry as a mode of narrating history or as a vehicle for the teaching -of truth, will make of such fairy tales as those of Pan, Orpheus, -Proteus, Cupid, and many others. They seem like so many airy Naiads -crushed within the iron grasp of a hundred-banded Briareus. But a -perusal of those delightful pages will show that the giant has really -no malevolent intentions towards his fair prisoners; nay, that he only -wishes, by stripping them of their paint and finery, to show that, -with all their lightness and grace, they are nevertheless strong buxom -dames, of the same doughty race with himself.] - -As an instance, therefore, of this remarkable interlacing of different -causes in what we call a single process, the disintegration of rocks -deserves our attention. In ordinary language, we say a stone rots -away, and its debris is washed down by the rains and streamlets, and -the process does not at first sight seem at all more complex than the -expression used to describe it; yet if we examine the subject, we shall -ere long find that there are in nature many simpler things than the -rotting away of a stone. To effect such a result, there come into play -a whole category of agencies, chemical and mechanical, so combined in -their operation, and so intimately blended in their effects, that it -becomes no easy task to tell where one set ends and another begins. - -A rock is said to undergo a chemical change, when one or more of its -component parts passes from one state of combination to another--as, -for instance, when a mineral absorbs oxygen, and, from the condition -of a protoxide, changes into that of a peroxide; or when, parting with -its silicic acid, it takes an equivalent amount of carbonic acid, and -in place of a silicate becomes a carbonate. Now these, and similar -metamorphisms, are chiefly produced by water permeating through the -rocky mass, and thus no sooner does the old combination cease, than the -new one which replaces it is dissolved by the slowly filtering water, -and carried away either to greater depths, or to the surface. Every -drop of water, therefore, that finds its way through the rock, carries -away an infinitesimal portion of the mineral matter, and the stone is -consequently undergoing a continual decay. This condition of things may -go on either at some depth in the earth's crust, or on the surface. -In the former case, springs and percolating water are the agents in -effecting the change; in the latter, it is produced chiefly by rain and -streams. But wherever the process goes on, the results, unless where -counteracted by some opposite agency, are ultimately the same. It may -be of use to look at some examples of these changes, and, by dividing -them in a rough way, into underground and surface actions, we shall be -enabled to mark more clearly their effects. - -A common source of the decay of rocks arises from the percolation -through them of water charged with carbonic acid. Decomposing -vegetation gives off a large amount of this gas, which is readily -absorbed by rain-water. The water sinks into the ground filtering -through cracks and fissures in the rocks, whence it afterwards -re-emerges in the form of springs. Now wherever, in its passage through -these subterranean rocks, the water meets with any carbonate, the -carbonic acid contained in the liquid immediately begins to dissolve -out the mineral matter, and carries it eventually to the surface. There -the amount of evaporation is often sufficient to cause a re-deposit of -the mineral in solution. If it be lime, a white crust gathers along -the sides of the stream, delicately enveloping grass-stalks, leaves, -twigs, snail-shells, and other objects, which it may meet with in its -progress. Such "petrifying" springs, as they are popularly termed, -occur abundantly in our limestone districts. It should be borne in -mind, however, that they only produce an incrustation round the organic -nucleus, and do not petrify it. That alone is a true petrifaction -where the substance is literally fossilized, or turned into stone. -A familiar instance of a similar chemical process may be seen under -many a bridge, and along the vaulted roofs of many an old castle. -Numerous tapering stalactites hang down from between the joints of the -masonry, resembling, so to speak, icicles of stone, often of a dazzling -whiteness. They are formed by the percolation of carbonated water -through the mortar of the joints, the carbonate of lime thus withdrawn -being re-deposited where the water reaches the air and evaporates. A -little pellicle of lime first gathers on the roof, and every succeeding -drop adds to the length of the column. In some cases, where the supply -of water is too great for the amount of evaporation, part falls on the -floor, and, being there dissipated, leaves behind a slowly-gathering -pile of lime called stalagmite. In some of the Eastern grottos, the -pillars from the roof have become united to those on the floor, forming -the most exquisite and fairy-like combinations of arch and pillar. -An example of a calcareous grotto has now become pretty familiar to -our summer tourists, under the name of the Spar Cave. It lies on an -exposed cliff-line along the western shores of Skye, against which -the surge of the Atlantic is ever breaking. You approach it from the -sea, and enter a narrow recess between two precipitous walls of rock, -open above to the sky, and washed below by the gurgling tide. Crossing -the narrow, shingly beach, you find the ground thickly covered with -herbage, while, grouped along the dark walls, are large bunches of -spleenwort, hart's-tongue fern, and other plants that love the shade. -Soon after entering the cave, all becomes sombre and cold; and the few -candles, with which the party have furnished themselves, only serve -to heighten the gloom. After scrambling on for a time across dank, -dripping rocks, and over a high bank of smooth marble, on which it is -difficult to creep, almost impossible to stand, you arrive at a deep -pool of clear, limpid water, which extends across the cave from side to -side, barring all farther passage. The scenery at this point will not -readily be forgotten. The roof towers so high that the lights are too -feeble to show it, while the walls, roughened into every form of cusp -and pinnacle, pillar and cornice, all glittering in the light, resemble -the grotto of some fairy dream. On returning again to the light of day, -if you ask the cause that has given rise to all this beauty, it will -be found a very simple one. The cleft occupied by the cave has been -once filled by a wall of igneous rock called a trap-dyke. Atmospheric -influences, aided probably by the waves, have caused the decomposition -and removal of this intruded rock, and the calcareous sandstone on -either side 'now stands up in a wall-like form. The upper part of -the dyke remains as a roof to the cave, but it has become completely -covered over with the calcareous deposits left by the carbonated water -that filters through the adjacent limy sandstone. The amount of water -is considerable, and consequently every part of the cavern--roof, -walls, and floor--has been incrusted with a white crystalline carbonate -of lime. In volcanic countries, where the springs often come to the -Surface in a highly heated state, charged, too, with various chemical -ingredients, they produce no slight amount of physical change on the -surrounding districts, and must be regarded as important geological -agents. - -But perhaps the most common and widely-diffused form of decomposition, -is that produced on the surface of the earth by the action of -rain-water, in slowly dissolving out the soluble parts of rocks, and -washing away the loose, incoherent grains that remain behind. It is -hard to say whether this process is more chemical or mechanical. -The solution of the mineral matter belongs to the former class of -changes, while the removal of debris must be ranked among the latter. -The results of these combined forces form one of the most important -branches of investigation which can occupy the attention of the -physical geologist, and in contemplating them, we are at a loss whether -most to admire their magnitude, or the immense lapse of time which -they must have occupied. It may be worth while to look at the progress -of this kind of disintegration, that we may see how wide-spread and -constant is the waste that goes on over the world, and how materially -the effects of running water are by this means increased. A volume -might be written about the decay of rocks, and a most interesting one -it would be, but its authorship would devolve rather on the chemist -than the geologist.[52] We can do no more here than merely glance at -one or two illustrative examples. - -[Footnote 52: A German chemist, Bischoff by name, has written two -learned volumes in which this subject is discussed (translated into -English, and published by the Cavendish Society), valuable for their -facts, but not always very safe in their deductions.] - -Among the mineral substances that most readily yield to the action of -the weather, are the silicates and the carbonates. The rocks containing -the former belong in large measure to what we call the igneous class, -such as the granites and traps; while those containing the latter -form the bulk of our useful stones, such as limestone and sandstone. -The removal of alkaline silicates is due to their conversion into -carbonates, which are readily soluble in water. Rain falling on a rock -in which they are largely present, dissolves a small portion, and -carries it into the soil or into streams, and thence to the ocean. -Every shower in this way withdraws a minute amount of mineral matter, -and tends to leave the harder insoluble grains of the rock standing out -on the surface in the form of a loose pulverulent crust, easily washed -away. The debris thus formed, where allowed to accumulate, makes an -excellent kind of soil known to the Scottish farmers as "rotten rock." - -The tourist who has visited any of our granitic districts, such as the -south-western parts of Cornwall, the rugged scenery of Arran, or the -hills of the Aberdeenshire Highlands, must be familiar with some of the -forms of waste which the rocks of these regions display. Mouldering -blocks, poised sometimes on but a slender base, and eaten away into the -most fantastic shapes, abound in some localities, while in other parts, -as for instance at the summit of Goatfell in Arran, the rock weathers -into a sort of rude masonry, and stands out in its nakedness and ruin -like some crumbling relic of Cyclopean art. In other districts, as in -Skye and in the adjoining island of Raasay, the granitic hills are -of a still more mouldering material. Their summits, white and bald, -sometimes rise to a height of fully two thousand feet above the sea, -while down their sides are spread long reddish-yellow tracks of debris -intermingled with patches of stunted herbage. Every winter adds to the -waste, and lengthens the lines of rubbish. Some of these hills form a -good field wherein to study the disintegration of granitic rocks, such, -for instance, as Beinn na Cailleaich, that rises from the shores of -Broadford Bay. Around the eastern base of that mountain there stretches -a flat moory district, with a few protruding blocks that have rolled -down into the plain. The earlier part of the ascent lies over a region -of metamorphic limestone, where the grey weathered masses of the -calcareous rock, often like groups of mouldering tombstones, are seen -protruding in considerable numbers through the rich soft grass and the -scanty brushwood of hazel and fern. Leaving this more verdant zone, we -enter a district of brown heath that slowly grows in desolation as we -ascend. Huge blocks of syenite--a granitic rock of which the upper part -of the mountain entirely consists--cumber the soil in every direction, -and gradually increase in numbers till the furze can scarcely find a -nestling-place, and is at last choked altogether. Then comes a scene -of utter desolation. Grey masses of rock of every form and size are -piled upon each other in endless confusion. Some of them lie buried in -debris, others tower above each other in a rude sort of masonry, while -not a few perched on the merest point seem but to await the storms of -another winter to hurl them down into the plain. The ascent of such a -region is no easy task, and must not unfrequently be performed on hands -and knees. But once at the top, the view is enough to compensate a -tenfold greater exertion. Far away to the west, half sunk in the ocean, -lie the isles of Eigg, Coll, and Tiree, with the nearer mountains of -Rum. North-west, are the black serrated peaks of the Coolins, that -stand out by themselves in strange contrast with every other feature -of the landscape. Northward, stretches the great range of syenitic -hills, with the sea and the northern Hebrides beyond. Away to the east, -across the intervening strait, lie the hills of the mainland, with -all their variety of form and outline, and all their changing tints, -as the chequered light and shade glide athwart the scene. Southward, -the eye rests on the grey wrinkled hills of Sleat, and far over along -the line where earth and sky commingle, are the mountains of Morven, -stretching westwards till they end in the bold weather-beaten headland -of Ardnamurchan, beyond which lies the blue boundless ocean. The top of -Beinn na Cailleaich is flat and smooth, surmounted in the centre by a -cairn. Tradition tells that beneath these stones there rest the bones -of the nurse of a Norwegian princess. She had accompanied her mistress -to "the misty hills of Skye," and eventually died there. But the love -of home continued strong with her to the end, for it was her last -request that she might be buried on the top of Beinn na Cailleaich, -that the clear northern breezes, coming fresh from the land of her -childhood, might blow over her grave. - -I have already alluded to the wasting away of a trap-dyke. This -decomposition arises from the same cause as among the granites--the -solution, and removal of the silicates. All these trap-rocks are -igneous, and seem to have risen from below through open fissures and -rents. As they contain a large percentage of felspar--the same mineral -that gives to many granites their mouldering character--they may be -seen exhibiting every form and stage of decay. Often they stand out in -prominent relief from some cliff of soft shale, with a brown surface, -picturesquely roughened into spherical masses of all sizes, that give -to the rock somewhat the appearance of a hardened pile of ammunition -in which ponderous shells lie intermingled with round shot, grape, and -canister. Each of the concretionary balls when examined is found to -exfoliate in concentric pellicles like the coats of an onion, and you -may sometimes peal off a considerable number before arriving at the -central core, which consists of the hard rock still undecomposed. In -this case the process of degradation is aided by the decay of another -mineral called augite, which contains a variable percentage of iron, -and imparts the peculiar yellowish-brown tint to the weathered rock. -Trap-dykes may also be seen in a still more wasted condition, where, -in place of protruding from a cliff-line, they recede to some depth -and give rise to deep clefts and fissures. An instance of this kind -has been referred to in the case of the Spar Cave, and many others may -be seen along the same coast-line. The shore there for miles is formed -of a low cliff of white calcareous sandstone, fissured by innumerable -perpendicular clefts of greater or less width, and sometimes only a -yard or two apart. Each of these has once been filled by a dyke of -trap, which originally rose up in a melted state, and after having -solidified into a compact stony mass, began to yield to the process of -decay. In all these and similar cases, the primary cause of the waste -lies in the decomposition of the felspar. Rain-water acts in removing -the soluble portions, and the harder grains that remain, deprived of -the cementing matrix, ere long crumble down and are washed off by the -rains. In this way the rock insensibly moulders away, every frost -loosening its structure, and every shower carrying away part of its -substance. - -Among the many objects of interest along a rocky coast some of the more -striking are certainly to be found in the curious and often grotesque -forms assumed by the weathered cliffs. Above high-water mark and thus -away from the dash of the waves, we can often trace the progress of -decay among such sedimentary rocks as sandstones and conglomerates. -Worn into holes and scars, projecting cusps and tapering pinnacles, or -eaten away into the rude semblance of a human form, headless perchance, -or into the shape of a huge table poised on a narrow pedestal, the -rock affords an endless variety of aspects and a continual source of -pleasure. If we chance to light upon any building constructed out of -the sandstone of such cliffs, it is worth noting that the removal of -the stone has not deprived it of its mouldering qualities; nay, that -houses erected within the memory of people still living already begin -to wear an aspect of venerable antiquity. I remember meeting with an -interesting example in the case of an old castle built on a similar -rocky coast-line. It stood on a little ness or promontory of dull red -stone, washed on all sides save one by the wild sea. The walls, of -which but a fragment remained, were built of a dark red sandstone; but -the lapse of centuries had told sadly on their masonry. The stones -rose over each other tier upon tier, corroded sometimes into holes -and hollows, sometimes into a close honey-combed surface, but the -mortar that had been used to cement them together still stood firm and -protruded from between the tiers to show, by no doubtful or ambiguous -sign, how silently yet how surely the wasting forces had been at -work. The scutcheon over the only remaining gateway had been carved -out of another kind of stone of a lighter colour and harder texture, -and so its grim lions looked nearly as fresh and formidable as when -first raised to the place of honour which they still occupied. In -this case, as before, the decomposition was owing to the presence of -a considerable proportion of soluble matter, which the rains of four -centuries had carried away along with the loosened incoherent sandy -grains. - -Conglomerate or pudding-stone has often a picturesque outline in its -decay, more especially if its included fragments have a considerable -range of size. Large tracts Of this rock exist in various parts of -Britain, particularly in Scotland, where the basement beds of the Old -Red Sandstone consist of a coarse conglomerate, sometimes several -thousand feet in thickness. Such enormous masses form the scenery of a -large part of East Lothian, and are found in detached patches across -into Peeblesshire and Lanark. In the north, too, the neighbourhood of -Inverness and other parts of the same district display conspicuous -conglomerate hills. Unless where laid bare by streams or by the action -of breakers, the contour of these hills is rounded and tame, with a -scanty covering of short scrubby grass and very few protruding bosses -of rock. But where a mountain torrent has cut its way down the hill -side, the ravine thus formed exhibits broken walls and pillars of -rock made up of rounded balls of every shade and size, cemented by a -dark-red or green paste. The cementing material is sometimes clay, -sometimes lime, and its variable nature gives rise to a corresponding -inequality in the amount and form of decomposition. Where the rounded -pebbles are bound together by clay, rains act with rapidity in washing -away the cement, and the component balls fall out by degrees, leaving -a cliff strangely roughened by protruding knobs, and eaten away into -clefts and hollows. When the pebbles are held in a crystallized matrix -of lime, they usually remain longer together, and may sometimes be -seen standing up in the form of detached rugged pillars that defy -all regularity of size or outline, and remind one of a sort of rude -grotto-work. Such irregularities become still more marked where to the -action of the rains there has been added the spray of the ocean. A -coast-line of conglomerate, where the rock rises into cliffs, is always -a romantic one; caves, pillars, and ruined walls, all in the same rough -grotto style, meet us at every step. Here, too, we can mark the varying -effect of the waves upon the lower portions of the rock, eating it into -cavernous holes and leaving rugged projecting pinnacles to which the -mottled colours of the included pebbles give an additional and peculiar -effect. - -A cliff of shale seldom shows much of the picturesque, though often a -good deal of the ruinous. The rock is easily undermined by streams, -and a shale ravine usually exhibits in consequence either heaps of -crumbling rubbish, or, where the stream comes past with a more rapid -current, perpendicular walls, jointed and laminated, but without much -variety of outline. Such cliffs, however, merit the careful attention -of the observer, for from their friability they are most easily -decomposed and washed down by streams, to form new accumulations of -similar soft argillaceous matter. A shale coast-line sometimes shows -cliffs of considerable altitude, as in some parts of Skye and Pabba, -where the Lias shales may be seen piled over each other often to a -height of seventy or eighty feet, and spreading out along the shore as -low flat reefs and skerries, brown with algæ at their seaward ends, -and showing on the higher slopes of the beach the characteristic -fossils of the Lias--_ammonites_, _belemnites_, and _gryphææ_--crowded -together by hundreds. The action of the decomposing forces has operated -more effectually on the soft material of the shale than on the hard -crystalline lime of the included shells, so that the latter stand out -in relief from the dull-brown surface of the rock, and from their -numbers and prominence form one of the most marked features of the -coast-line. - -Probably few have ever visited a limestone district without marking -the manner in which that rock yields to the action of the elements, -whether in an inland part of the country where rivers have cut deep -gullies through the rock, or along some exposed shore where the stone -has been wasted by a still ruder assailant. An exposed cliff of hard -homogeneous limestone weathers into deep clefts and holes; the entire -surface assumes a pitted appearance, somewhat like a sandy beach after -a showier of rain, and the planes of stratification, or lines formed -by the parallel junction of the beds are often worn away until the -rock looks not unlike a piece of old masonry, in which the mortar has -decayed and dropped out, leaving the angles of the stones to get wasted -and rounded by the action of the weather. In many districts, too, where -the rock is richly fossiliferous, the broken joints of encrinites along -with corals and shells may be seen crowded together by myriads, their -hard skeletons protruding from the wasted rock in such a way as to show -that the stone can contain very little else. By this means we often -learn that a limestone bed is nothing but an old sea-bottom, where -the calcareous sediment was mainly derived from broken stone-lilies, -corals, and shells, though if we break off a piece of the rock the -internal fracture may show very little or no trace of any organic -structure. And hence if the geologist would form an accurate conception -of the origin and structure of many of the stratified rocks, he must -study them not in hand specimens neatly trimmed and arranged along the -shelves and drawers of a cabinet, nor even in the ponderous blocks -daily exhumed by the quarryman, but along some surf-beaten cliff-line -or down some precipitous ravine where the rock for centuries has been -exposed to the wear and tear of the elements. - -Limestones and other calcareous formations are liable to more than -ordinary decay, for, as we have seen above, percolating rain-water -constantly carries away mineral matter from their subterranean -portions. Accordingly, in some parts of the country, as for instance in -Yorkshire, the interior of such rocks has been eaten away into great -caverns by this form of decomposition. - -Some remarkable examples occur in the island of Raasay, one of the -north-eastern Hebrides. Its eastern margin shoots up from the sea to a -height of over 900 feet, the cliff-line being formed of a calcareous -grit as perpendicular as a wall, and fissured by deep chasms and rents. -The narrow table-land between the edges of this cliff and an abrupt -ridge that rises behind, is perforated by innumerable holes and clefts, -into which if a stone be thrown it may be heard for several seconds -rumbling far below. The edges of these pitfalls are often fringed -with ferns, rushes, and long grass, so as to be nearly hidden, and it -requires no little caution to traverse this elevated region in safety. -Innumerable sheep have been lost by falling into the subterranean -abysses, and even the wary natives seem to have sometimes lost their -footing. A story is told of a woman who had crossed to the other side -of the island for the purchase of some commodities, and returning -by the high grounds had got nearly within sight of her own cottage, -when by some unlucky accident she took a false step and instantly -disappeared. Unfortunately her errand had been performed alone, so that -some time elapsed ere she was missed. The search continued unremitting -for two days, but no trace of the missing traveller could be found. At -last on the third day her figure was seen creeping slowly along the -road not many hundred yards from her own door. It appeared that she had -first slid down a sheer height of about fifty feet, when her further -passage was intercepted by the sides of the fissure. During the earlier -part of her confinement she strove hard to re-ascend the chasm, and it -was not until, the effort seeming fruitless, she had begun to resign -herself to despair, that a glimmering of light from below induced her -to attempt a descent. This proved no easy matter, and occupied many -weary hours of labour and suspense; but at length she succeeded in -worming herself to the bottom, and crawled out more dead than alive -only a little way from her home. There still stand perched on some of -these precipitous cliffs the remains of a few villages, the inhabitants -of which were accustomed to tether their children to the soil, whence -one of the hamlets received in Gaelic the soubriquet of Tethertown. -Many a valuable commodity disappeared by rolling over the cliff, and -I have been assured that it was no unfrequent occurrence for a pot of -potatoes capsized at the doorway to tumble down the slope and make no -stop until safely esconced at the sea-bottom. - -The process whereby these fissures and caverns originate is the same as -that noticed already in the Spar Cave. Water containing an impregnation -of carbonic acid filters down through cracks and fissures of the -calcareous rock, dissolving out in its passage a portion of the lime -which it eventually carries back to the surface, and either deposits -there or transports into streams, and thence to the sea. Thus atom -by atom is removed wherever the percolating water reaches, until in -the course of ages an irregular cavern of greater or less extent is -produced. The decomposition of limestone at the surface results from -the same kind of action, that of carbonated water. Every shower of -rain insensibly carries away a fraction of the constituent parts of -the rock, so that the size and form of detached blocks as well as of -exposed cliffs is constantly changing. How often do we see the same -decay going on with a melancholy rapidity among the exposed marble -tombstones of our churchyards. In a few years the tablet gets worn -and furrowed as though it had stood there for centuries. Eventually, -too, the inscription becomes effaced, and perhaps ere the bones of the -deceased have mouldered away and mingled with their kindred dust, the -epitaph that recorded for the admiration of posterity his many virtues -and his vigorous talents, has faded from the stone--often, alas! only -too fit an emblem of how speedily the memory of the dead may fade away -out of the land of the living. - - - - -CHAPTER IX. - - - Mechanical forces at work in the disintegration of rocks--Rains - Landslips--Effects of frosts--Glaciers and icebergs--Abrading - power of rivers--Suggested volume on the geology of - rivers--Some of its probable contents--Scene in a woody - ravine--First idea of the origin of the ravine one of primeval - cataclysms--Proved to be incorrect--Love of the marvellous - long the bane of geology--More careful examination shows the - operations of Nature to be singularly uniform and gradual--The - doctrine of slow and gradual change not less poetic than that - of sudden paroxysms--The origin of the ravine may be sought - among some of the quieter processes of Nature--Features of - the ravine--Lessons of the waterfall--Course of the stream - through level ground--True history of the ravine--Waves and - currents--What becomes of the waste of the land--The Rhone - and the Leman Lake--Deltas on the sea-margin--Reproductive - effects of currents and waves--Usual belief in the stability - of the land and the mutability of the ocean--The reverse - true--Continual interchange of land and sea part of the - economy of Nature--The continuance of such a condition of - things in future ages rendered probable by its continuance - during the past. - -The forms of decomposition noticed in the last chapter were chiefly -of a chemical kind. Their effects were observable alike on the -surface of the earth and below ground; in the latter case we saw them -excavating caverns and long irregular chasms, in the former we noted -the production of debris which if undisturbed went to the formation -of soils. It must be borne in mind however, that in these operations -other forces than simply those of a chemical kind come into play. The -percolation of water and the removal of insoluble particles on the -exposed parts of rocks rank as mechanical processes. So also do those -by which new surfaces of mineral masses are brought within the sphere -of the chemical agencies, such as the action of frosts, rains, rivers, -and waves. In short, as already noticed, any subdivision of the forces -at work in effecting the decomposition of rocks must ever be more or -less arbitrary; but it remains nevertheless useful, if we bear in -mind that the exactly defined boundary lines are of our making, not -Nature's. With this caution we may proceed to examine what are termed -the mechanical agencies in the disintegration of mineral masses, and in -so doing, we shall find that the chemical forces are not less helpful -to the mechanical than the latter to the former. - -First, we may notice the effect of rains in washing away the -disintegrated particles to lower levels or into river-courses whereby -fresh portions of rock become exposed to the decomposing forces. Rains -also act powerfully in altering the form of cliff-lines and steep -declivities, especially where these consist more or less of friable -earthy matter. After a long continuance of wet weather, I have seen -the abrupt sides of a river-channel that were formed of a stiff blue -clay completely cut up by rents of various dimensions, whereby large -masses had subsided many feet, while others had rolled down altogether -and lay in the bed of the stream where they were undergoing a rapid -abrasion. The cause of this alteration was obvious. The rains pouring -down from the sloping grounds on either side of the river had excavated -deep channels on the abrupt face of the cliffs, while a considerable -quantity of water finding its way through the soil, had permeated -through joints and crevices in the clay some feet from the edge of the -bank. By the combined operation of these causes, masses of clay several -yards in extent lost their cohesion and either settled down a few -feet, or found their way to the bottom. Such landslips are of frequent -occurrence where large masses of rock of a hard compact nature rest -upon loose shales and clays more or less inclined. Whole hills have -been known to be hurled in this way into the valleys below. - -But these results become perhaps still more marked where to the -ordinary operations of water there are added those of intense frost. -The effects of a severe winter (such, for instance, as a Canadian one), -in loosening the particles of rocks and facilitating the breaking-up -of large masses, must be ranked among the most powerful agencies of -nature. In such a season, the percolating water with which nearly every -surface-rock is charged becomes frozen, and in the act of congelation -expands. The result of this dilatation is to exert great pressure on -the particles of the rock, and thereby loosen their cohesion. When thaw -comes the frozen liquid contracts again, but the loosened particles -have no such elastic power, and so, having lost hold of each other, -crumble down. If the season be a changeable one, frost and thaw quickly -alternating, the amount of waste produced becomes very great. Not only -is the outer surface of the stone decomposed, but the water filtering -through the joints of the rock freezes there, and thus on the arrival -of milder weather vast masses become detached from the cliffs, and -roll down, to be worn by the grinding action either of waves or of -rivers, as the case may be. Spring at last sets in with its warmth and -its showers; the snow rapidly melts away; the whole country streams -with water; every valley and hollow has its red turbid rivulet, that -bears a burden of muddy sediment into the nearest river; and thus the -loosened portions of the rocks get washed away down to sea, leaving a -new surface for the action of next winter. We can easily understand, -therefore, that in certain regions the combined effects of frost and -thaw may work in the course of ages changes of almost inconceivable -extent, and that the agency of ice must be not less varied and -important on the land than, in the case of the boulder clay, we found -it to be in the ocean. - -Besides this action in winter, which goes on more or less in every -country wherever the temperature sinks sufficiently low to permit of -the freezing of water, ice effects many changes on the surfaces of -rocks when it takes the form of glaciers and icebergs. We have already -noted the operation of a glacier during its slow progress in crushing -down large fragments of stone, scratching and abrading the rocks over -which it passes, and eventually producing a vast quantity of mud, which -is carried down by streams to form new accumulations either in lakes -or seas. We have also marked the effects of the drifting iceberg in -materially modifying the contour of submarine hills, and depositing -over the ocean-bottom mud, gravel, and boulders. Nothing further, -therefore, need be done here than simply to keep these agencies in -view, as playing an important part in the disintegration of rocks. - -Another highly interesting aqueous action is that of streams and -rivers, in scooping out for themselves channels through sometimes -the hardest and most solid rock. Such effects may be seen all over -the globe, in the old world and in the new, in the bed of the -tiniest rivulet, as well as in the course of the mightiest river. -And accordingly, in all the long list of geological agents, we find -none so well known and so often described alike by poets, historians, -and scientific writers, as well in ancient as in modern times. What -a delightful volume might be written about the geology of rivers! It -would, perhaps, begin with that "great river," the Euphrates, along -whose green banks lay the birthplace of the human race, tracing out the -features of its progress from the ravines and cataracts of Armenia, -with all their surrounding relics of ancient art, down into the plains -of Assyria, amid date-palms and Arab villages, onwards to the mounds -of Nineveh and Babylon, and thence to the waters of the Persian Gulf. -Well-nigh as remote, and perhaps still more interesting in its human -history, would be the story of the Nile. We should have to follow that -river from the mystic region of its birth,[53] marking the character -of the rocks through which winds its earlier channel, and the effects -upon them of the floods of untold centuries; it would be needful, too, -to note the influence of the waters on the lower grounds, from where -the stream flows over the cataracts of Syene, down through the alluvial -plains of Egypt; and lastly, the concluding and perhaps most onerous -part of our labour would be the investigation of the delta, marking its -origin and progress, its features in ancient times, as made known to us -in the graphic chapters of Herodotus, and the changes which the lapse -of more than twenty centuries has since wrought in its configuration. -The rivers of Europe would detain us long, not less perhaps by their -historic interest than by the variety and attractiveness of their -physical phenomena. One could scarce help lingering over the Rhine, -with its source among Alpine glaciers, its lakes and gorges, its -castles and antique towns; and when once the narrative entered the -classic ground of Italy, it would perhaps become more antiquarian than -geological. The ravine of Tivoli, for instance, would certainly lay -claim to a whole chapter for itself, with its long-continued river -action, its ancient travertin, its beautiful calcareous incrustations, -and above all its exquisite scenery. - -[Footnote 53: "Fontium qui celat origines Nilus" a description not less -true now than in the days of the Sabine bard.] - - "Domus Albuneæ resonantis, - Et præceps Anio, ac Tiburni lucus, et uda - Mobilibus pomaria rivis."[54] - -[Footnote 54: - - "Albuna's grey re-echoing home, - And Anio, headlong in his foam, - And grove of Tivoli, - And orchards with their golden gleam, - Whose boughs are dipping in the stream - That hurries to the sea." - -Hor. _Carm._ L vii. 12.] - -And when could we exhaust all, that might be said about the rivers of -our own land? - - "Of utmost Tweed, or Ouse, or gulfy Dun, - Or Trent, who, like some Earth-born giant, spreads - His thirty arms along the indented meads - Or sullen Mole, that runneth underneath; - Or Severn swift, guilty of maidens' death; - Or rocky Avon, or of sedgy Lee; - Or coaly Tyne, or ancient hallow'd Dee; - Or Humber loud, that keeps the Scythian's name; - Or Medway smooth, or royal-towered Thame." - -Passing to the new world, a vast field would spread out before us: the -Mississippi, the Atchafalaya, the Ohio, the St. Lawrence, the Amazon, -and many other rivers that in some cases rise high among the regions -of perpetual snow, and after traversing large areas of country in the -temperate zone, fall into the waters of tropical seas. By studying such -examples of river-action and delta-formation as are presented by these -gigantic streams, we should arrive at some conception of the conditions -anciently at work in producing our present coal-fields. Nor would our -researches assume aught like completion until after a scrutiny of all -the larger and more important rivers of the globe. Such a work could -be undertaken, perhaps, only by another Humboldt. Its successful -accomplishment would certainly insure the highest renown to its author, -and incalculable benefits to science. - -From what we have seen of the wide waste and decay everywhere in -progress on the solid lands of our planet, it becomes no difficult -matter to perceive what a number of agencies must be at work in the -formation of a river channel. Let the reader take his stand in some -wooded ravine, where the shelving rocks on either side are hung all -over with verdure, and a tiny streamlet murmurs on beneath with a -flow so quiet and gentle as scarcely to shake the long pendant willow -branches that dip into its surface, while the polished pebbles that -strew its bed lie unmoved by the rippling current that glides over -them. If in the midst of such a scene the question were to arise in -his mind, How came this deep, narrow ravine into existence? what -answer would in all likelihood be the first to suggest itself? His eye -would scan the precipitous walls of the dell, with their rocks cleft -through to a depth of perchance fifty feet. It would require no great -scrutiny to assure him that the beds on the one side formed the onward -prolongations of those on the other, and that consequently there must -have been a time ere yet the ravine existed, when these beds stretched -along unbroken. Satisfied with these results, his first impulse might -be to bethink him of some primeval earthquake, when the solid land -rocked to and fro like a tempested sea, and broke up into great rents -and yawning chasms. Into one of these clefts he might suppose the -little streamlet had eventually found its way, moistening the bare and -barren rocks, until at length their surface put on a livery of moss, or -lichen, or liver-wort, and the birch, the alder, and the willow, found -a nestling-place in their crevices. Such a view of the origin of the -woody dell would be certainly a very natural one, and in some instances -might be sufficiently correct, but in the present case it will not -explain the phenomena. If the reader will kindly permit me to visit the -locality in his company, perhaps we may be able to light upon the true -explanation, and see a few appearances worthy our attention. - -First, then, how can we make sure that no convulsion of nature has -produced a rent in the rocks, and so helped the streamlet to a channel? -a simple question that may be well-nigh as simply answered. We stand -in the centre of the dell on a broad ledge of stone, round whose -well-worn sides the rivulet is ever eddying onwards. The block consists -of a pale sandstone lying in a bed about three feet thick, that dips -gently down the stream and underlies a seam of dull, soft, blue shale, -full of small shells. We trace the edge of this sandstone bed across -to the left-hand side of the ravine, and away up into the precipitous -cliff, till it is lost amid the ferns and brushwood. There can be no -doubt, therefore, that the ledge on which we were but now standing is a -continuous portion of the rocks that form the left side of the ravine. -Returning again to the centre of the stream, we proceed to trace out -the course of the other end of the same bed, and find that it, too, -strikes across to the rocks on the right-hand side without a break -or fissure, and passes up into the cliff, of which it forms a part. -Clearly, then, the sandstone bed runs in an unbroken, unfissured line, -from the one side to the other, and the rocks of either cliff form -one continuous series. There occurs no break or dislocation, which, -of course, there must have been had the ravine owed its origin to any -subterranean agency. And so we come to conclude that no great cataclysm -in primeval times, no yawning abyss, or gaping chasm, has had anything -whatever to do with the formation of our deep sequestered dell. What -then? "Whither shall we turn," you ask, "to find another agency equally -grand and powerful in its operation and mighty in its results?" - -Stay, gentle reader. That craving for the grand and the sublime, that -hungering after cataclysms and convulsions, that insatiable appetite -for upheavals, and Titanic earth-throes, and all the mightier machinery -of Nature, has done no little mischief to geology. Men have reasoned -that gigantic results in the physical structure of the earth must -have had equally gigantic causes operating in sublime conflict and -in periodic paroxysms, now heaving a mountain chain to the clouds -of heaven, now swallowing up a continent in the depths of the sea. -Happily such extreme notions are fast passing away, though the old -tendency in a modified form still abounds. A closer scrutiny of Nature -as she actually shows herself, not as theorists fancy she should be, -has revealed to us that her operations are for the most part slow, -gradual, and uniform, and that she oftentimes produces the mightiest -results by combinations of forces that to us might seem the very -emblems of feebleness and inactivity. In place of sudden paroxysms -she demands only an unlimited duration of time, and with the aid of -but a few of these simple, tardy agents, she will eventually effect -results perchance yet more gigantic than could be accomplished even -by the grandest catastrophe. Nor in thus seeking to explain the past -by defining what seems the usual mode of Nature's operations in the -present, do we, as is sometimes alleged, deprive them of their high -poetic element. Assuredly there is something thrilling to even the -calmest imagination in contemplating the results of vast and sudden -upheavals, in picturing the solid crust of the earth heaving like a -ground-swell upon the ocean, in tracing amid - - "Crags, knolls, and mounds, confusedly hurl'd, - The fragments of an earlier world;" - -and in conjuring up visions of earthquakes, and frightful abysses from -which there ever rose a lurid glare as hill after hill of molten rock -came belching up from the fires below. But while far from denying that -such appearances may have been sometimes seen during the long lapse -of the geological ages, and that they give no little vividness and -sublimity to a geological picture, we claim for the doctrine of the -tranquil and uniform operation during past time of existing laws and -forces, an element not less poetic. In the former case the pervading -idea is that of unlimited expenditure of power, in the latter that of -unlimited lapse of time. In the one case the action is Titanic but -transient, in the other it is tranquil but immensely protracted. The -two doctrines in this way counterbalance each other; yet I cannot but -think that however impressive it may be to stand in some lone glen, and -while gazing at its dark jagged precipitous cliffs, to dream about the -paroxysmal convulsions of some hour far back in the distant past, the -scene becomes yet more impressive when we look on its nakedness and -sublimity not as the sudden and capricious creation of a day, but as -the gradual result of a thousand centuries. These cliffs may once have -been low-browed rocks rising but a little way out of a broad grassy -plain, and serving as a noon-tide haunt for animals of long extinct -races. Thousands of years pass away and we see these same rocks higher -and steeper in their outline, brown with algæ and ever wet with surf, -while around them stretches a shoreless sea. Ages again roll on, and -we mark still the same rocks shooting up as bleak crags covered with -ice and snow. Another interval of untold extent elapses, and rock, -snow, and ice have all disappeared beneath a broad ocean cumbered -with ice-floes and wandering bergs. Again the curtain drops upon the -scene, and when once more it rises, the cliffs stand out in much the -same abrupt precipitous aspect with that which they now present, save -that their bald foreheads look less seamed and scarred than now, and -their dark sides show no trace of bush or tree. The white cascades that -to-day pour down from their summits and sides--seeming in the distance -like the white hairs of age--are insensibly deepening the scars and -furrows on these ancient hills, and thus slowly but yet surely carrying -on the process of degradation and decay. Musing on all this long -series of stages in the formation of one single cliff-line, is there -not something more sublime, something yet more impressive than if we -pictured but the chance random result of the gigantic paroxysm of an -hour? - -Let us not be deterred then from seeking an explanation of the origin -of the ravine among some of the quieter and more unobtrusive forces -of Nature. Give them but an unlimited period to work in and they will -abundantly satisfy all our demands. - -We return again to the rocky ledge in mid-channel, and proceed to -ascend the course of the stream, marking as we go the changes in the -character and features of the stone that forms the cliff on either -hand. We come to a bare part of the ravine where brushwood and herbage -find but a scanty footing and where accordingly the rocks can be -attentively studied. The face of the escarpment shows a number of beds -of pale grey sandstone alternating with courses of a dark crumbly -shale. The sandstones being harder and firmer in texture stand out -in prominent relief while the shales between have been wasted away, -covering the bottom of the slope with loose debris. We can mark too -that, as this decay goes on, the harder beds continually lose their -support, cracking across chiefly along the lines of joint, and rolling -down in huge angular blocks into the stream. In truth we cannot doubt -that every year adds to this decay and thus slowly widens the dell, for -the broken fragments do not form in heaps over the solid rock below -so as to protect it from the weather, but are evidently carried away -by the stream and hurried down the ravine onwards to the sea. From -what has been said above relative to the disintegration of rocks by -percolating water, frosts, and other causes, the reader will easily -see how this rotting away of the sides of the ravine must be carried -on; and he will not fail to mark that here we have at work an agency -not yet considered, that of running water. The effects of the weather -are seen in the crumbling, ruinous cliffs overhead; the effects of -the streamlet are observable in the continual removal of the rubbish -whereby a fresh surface is ever exposed to the decomposing forces, -while at some points we can mark the water actually undermining an -overhanging part of the cliff from which there are ever and anon vast -masses precipitated into the channel where eventually they get worn -down and carried away out to sea. "Still," you may remark, "these -forces are at work only in widening a channel already made. How was the -ravine formed at first?" - -We continue our ascent. A scrambling walk through briars and -hazel-bushes, sometimes on rocky ledges high among the cliffs, -sometimes among the prostrate blocks that dam up the stream, brings -us at last full in front of a sparkling waterfall that dashes over -a precipitous face of rock some twenty feet high. The appearances -observable here deserve a careful attention. Our eyes have not been -long employed noting the more picturesque features of the scene ere -they discover that the dark-brown band of rock forming the summit of -the ledge over which the water tumbles is continuous all round the -sides of the dell. There is consequently no break or dislocation here. -Approaching the cascade we note the rock behind it so hollowed out that -its upper bars project several feet beyond the under ones. In this -way the body of water is shot clear over the top of the cliff without -touching rock till it comes splashing down among the blocks in the -channel. And yet this hollowed surface is never dry; the spray of the -fall constantly striking on it keeps it always dank and dripping. In -some parts the rock stands out bare and worn, while on the less exposed -portions there gathers a thick green scum which is replaced on the -drier ledges by the soft cellular leaves of the liver-wort. Now our -examination of the influence of percolating water upon even the hardest -rocks teaches us that this moist soaked surface is just the very best -condition for favouring the decay of the rock. Nay more, the green -vegetation that mantles over the stone serves to prevent the water from -running off too rapidly, and keeps the rock in a still more moist state -than would otherwise happen. So that the portion of sandstone behind -the cascade comes to be in a still more favourable situation for speedy -decay than the ledge over which the water is rapidly driven. We can -see, therefore, how in the lapse of years the corrosion may go on until -the upper projecting part of the cliff loses its support and falls with -a crash into the rocky pool below, while the form of the waterfall -becomes thus greatly altered, and new surfaces are exposed to the wear -and tear of the stream. - -But we have not yet exhausted all that the rocks at the cascade can -teach us. By dint of some exertion we climb the cliff and gain the -upper edge of the fall. The rocks that form the bed of the stream are -now seen to be deeply grooved and worn, every exposed surface having -a smoothed blunted aspect. We can mark how the stone has split up -along the natural lines of joint, whereby great facility is given to -the removing power of the current, and how large irregular angulated -blocks become detached and are swept down the stream. In not a few -parts, too, we may notice circular holes of greater or less depth, in -the bottom of each of which lie perhaps a pebble or two, that with a -constant gyratory movement, caused by the eddying water, have eaten -their way downwards into the solid rock. When the stream is in flood -and comes roaring down the rocky gorge bearing along with it a vast -amount of mud, gravel, and stones, one can easily see how the friction -of the transported material must wear down the hard bed and sides of -the channel, and how this process repeated month after month and year -after year, must aid the decomposing forces in scooping out a deep -ravine. From the cascade the ascent of the stream becomes steeper and -the run of water is consequently more rapid. Soon however we emerge -from the woody copse, and find ourselves on a flat alluvial cultivated -plain through which the rivulet winds in a tortuous meandering course, -bending back upon itself into loops that almost meet and well-nigh -form broad flat islets. Strolling along this winding route we can mark -the effects of the stream in eating away the soft clay and sand at one -part of the bend and piling them up at another. Such loose material -can present but little resistance to a stream swollen with rains, and -consequently a large quantity of the mud and gravel along with the -interspersed boulders must be swept away down into the dell at every -season of flood. The matter thus removed will of course be still -further comminuted in its passage, and at the same time will help to -grind down the hard rock surfaces over which it is driven. - -Here then may be found the whole history of the ravine. Originally the -streamlet wound its devious course through a flat alluvial country with -a channel sunk but a foot or two below the level of the plain. Such -continued its character till it reached a low bluff, down which the -water flowed more rapidly to gain a second level undulating region. -The part of this bluff crossed by the stream was ere long bared of -its covering of soil and clay, and the rock below came to be washed -by a group of little cascades. Once exposed to the decomposing and -disintegrating forces, the stone soon began to decay and the cascades -ere long merged into one. By slow degrees the rock gave way and the -waterfall retreated from the bluff. For perchance thousands of years -the same process has been going on, now with greater, now with less -rapidity, according to the nature of the rocks encountered and other -modifying causes, until the fall has eaten its way back for well-nigh -three miles and scooped out a wild rocky gorge some fifty or sixty feet -deep. This is but a solitary and insignificant instance of what may be -seen all over the world, for the process remains the same whether we -stand beside a tiny rivulet in some lone Highland glen or listen to the -roar of the falls of Niagara. - -There is but one other principal agency at work in the demolition of -rock-masses, the waves and currents of the ocean. But we have already -noted the effects thus produced, and need not now retrace our steps -further than to recall the vast amount of devastation which can be -shown to have been effected in our own country by marine causes, both -in breaching the existing shores and in scooping out valleys and -grinding down hills at former periods when the land was either rising -above or sinking below the level of the sea. - -Having now satisfied ourselves that there goes on all over the world an -incessant waste of the solid lands, that the disintegrated debris is -washed down by rains and transported seawards by rivers, and that the -waves are ever eating their way into the iron-bound coast-line as well -as into the low alluvial shore, we naturally come to ask the result -and end of all this decay. What becomes of that vast amount of mineral -matter annually removed from the land? To be able to answer this -question clearly and distinctly, let us look for a little at what takes -place in lakes, at river-mouths, and in open sea. - -The river Rhone rises among the Bernese Alps, and after a course of -about 100 miles through the Canton of Valais, it enters the upper end -of the Lake of Geneva. Its waters, where they mingle with those of the -lake, are muddy and discoloured, but where they pass out at the town of -Geneva are limpid and clear. The mud, therefore, which they bring into -the lake must be deposited there, and as the stream may have continued -to flow for thousands of years, we may reasonably expect to find some -trace of the large amount of sediment necessarily deposited during the -whole or part of that long period. Accordingly, careful examination of -the Lake of Geneva has shown that such accumulations have really been -formed, and that their progress and amount during part of the historic -period can be approximately calculated. Where the turbid current of the -Rhone enters the still water of the lake, the mud slowly sinks to the -bottom. In the lapse of centuries layer after layer has been thrown -down, rendering the lake at this part sensibly shallower, until a large -area or delta has been filled up and converted into a flat alluvial -plain. Thus, a town which in the time of the Romans formed a harbour -on the water's edge, now stands more than a mile and a half inland. -This new-formed land is entirely the work of the stream, and if we -could obtain a complete section of it from the surface to the bottom, -"we should see a great series of strata, probably from 600 to 900 feet -thick (the supposed original depth of the head of the lake), and nearly -two miles in length, inclined at a very slight angle." These strata, -which are said to have taken about eight centuries to form, "probably -consist of alternations of finer and coarser particles; for, during the -hotter months, from April to August, when the snows melt, the volume -and velocity of the river are greatest, and large quantities of sand, -mud, vegetable matter, and drift-wood, are introduced; but, during the -rest of the year, the influx is comparatively feeble, so much so that -the whole lake, according to Saussure, stands six feet lower."[55] If -the present conditions continue for a sufficient length of time, the -lake may be eventually filled up with mud, sand, and gravel, deposits -that would eventually harden by pressure into shale and sandstone. So -that the day may yet arrive when the blue waters of the Leman lake -shall have passed away, when the Rhone perchance may have ceased to -flow or found its way by some other channel, when the peasant may guide -the plough where now the boatman plies the oar, and when the geologist -shall trace out in quarries and excavations the successive deposits of -hardened sediment with their lacustrine shells and drift-wood, and, -musing on the changes of which they are the silent yet impressive -witnesses, may sit down to pen a record of the gradual extinction of -the Leman lake on that classic ground where an immortal historian -described the decline and fall of the empire of Rome. - -[Footnote 55: Lyell's _Principles of Geology_. Ninth edition, p. 252.] - -The alluvial matter deposited by the Rhone at its entrance into the -Lake of Geneva suffers perhaps no change when it once reaches the -bottom. Layer after layer accumulates tranquilly, without disturbance -from surface currents or other causes, so that the renovating effects -of the stream have here every advantage. It is otherwise, however, -where a delta gathers at the mouth of a river upon the sea-margin. -There tides and currents are ever demolishing what the stream has -piled up. Often, too, owing to the prevalence of high winds from -seawards, the river is dammed up for leagues, and the waters of -the ocean encroach far on the delta, mingling in this way marine -remains with those that are fluviatile or terrestrial. But with these -modifications the process of delta-formation remains essentially the -same, both in lakes and at the sea. The vast quantities of sand and -gravel transported by rivers during the flood-season sink to the bottom -as soon as the motion of the water will permit. This takes place at -the shore, where eventually wide tracts of low alluvial land encroach -upon the sea, covered with marshes and overgrown with vegetation. A -section of any of these deltas, obtained in boring for water, shows a -succession of sands and clays, with occasionally a few calcareous beds -and quantities of peaty matter formed of vegetation either drifted -or that grew on the spot.[56] If, now, a sufficient amount of matter -were piled over these loose incoherent strata, they would eventually -become as hard and compact as any of our ordinary building stones. The -sand would subside into a firm compact sandstone; the clay, in like -manner, would consolidate into fissile shale; the peat would become -chemically altered into coal; the calcareous seams would take the form -of layers of limestone; while the leaves, twigs, branches, and trunks, -dispersed through all the beds, would get black and carbonized, so as -precisely to resemble the lepidodendra, calamites, stigmariæ, &c., of -the carboniferous rocks. And thus might a mass of fossiliferous strata, -thousands of feet deep and thousands of square miles in extent, be -amassed by the prolonged operation of a single river. - -[Footnote 56: The structure of maritime deltas, especially their -relation to the growth and entombment of forests, will be more fully -alluded to in a subsequent chapter, when we come to inquire into the -origin of a coal-field.] - -It often happens that a delta is prevented from extending further -seawards owing to the prevalence of some marine current that comes -sweeping along the coast-line and cuts away the accumulations thrown -down by the river. The sediment thus removed is often carried to great -distances, and eventually settles down as a fine mud along the floor of -the sea, entombing any fucoids, infusoria, shells, corals, fish-bones, -or other relics that may lie at the bottom. - -He who has witnessed a storm along a rocky coast-line, has marked the -breakers battering against the weather-bleached cliffs, and heard the -thunder-like rattle of the shingle at the recoil of every wave, needs -not to be told how vast an amount of sediment must in this way be -formed. The pebbles of the beach are ground down still smaller, the -sand produced by their friction finds its way to a lower level, while -the finer particles taken up by the water are borne out to sea, and if -a current traverse the locality may be transported for leagues, till -they at last settle to the bottom. The floor of the sea is consequently -always receiving additions in the form of fine mud--the waste of the -land--derived either from breaker-action, rivers, or icebergs, so -that a series of marine deposits exactly similar to those we find -among the rocks of our hills and valleys, must be constantly in the -course of formation. If circumstances be favourable, the shingle of -the beach may eventually either be covered over or reach a part of -the sea undisturbed by currents or waves, and then consolidate into -what we call conglomerate or pudding-stone. The sand, as before, -becomes sandstone, and the mud laminated shale or hardened clay. These -deposits may go on forming for thousands of years, until at last some -slow elevation or some sudden upheaval of the ocean bed brings them to -the light of day as part of a new continent. Thus exposed they would -differ in no respect from rocks of a similar kind now visible, and -the geologist, in tracing out their origin and history, would have no -hesitation in ranking them among the ordinary marine formations of the -globe. - -In fine, we cannot quit the subject without being convinced that -these ceaseless changes afford one of the grandest examples of that -continuous series of mutations--cycle and epicycle--which has been -already alluded to as a distinguishing feature in all the operations -of Nature. We are accustomed to think and speak of "the everlasting -hills." We look on the solid lands whereon we dwell as the emblem of -all that is stable and steadfast, and on the boundless ocean as the -type of all that is unsteady and changeful. The traveller who stands -on those plains where the human race was cradled, marks still the same -valleys with their winding rivers, still the same rocks and hills, -still the same blue sky overhead. The dust of centuries has gathered -over the graves and the dwellings of the early races, yet the covering -is but thin, and if we could conjure from their resting-place some -of these venerable patriarchs, they might perhaps see little or no -change on the haunts of their boyhood. We feel it otherwise, however, -when we contemplate the ocean. In sunshine and in storm its surface -never rests. The wave that now breaks against some bald headland of -our western shores may have come sweeping across from the coast of -America, and the broad swell that rolls into surf along the shores -of Newfoundland may have travelled from the frozen seas of the North -Pole. And so it has ever been; the "far resounding sea" of Homer is the -"far resounding sea" still; and the "countless dimpling of the waves," -invoked in his agony by the chained Prometheus, remains restless and -playful as ever. - -"Firm as a rock," and "fickle as the sea," have therefore become -proverbs of universal acceptance. Yet when we investigate the matter -as we have done in this and the preceding chapter, it appears that an -exactly opposite arrangement would be nearer the truth. It is the sea -that remains constant-- - - "Time writes no wrinkle on its azure brow;" - -while the land undergoes a continual change. Hills are insensibly -mouldering away, valleys are ever being widened and deepened, rocky -coasts and low alluvial shores suffer a constant abrasion, while -even within the bowels of the earth the process of decomposition -uninterruptedly proceeds. And thus, in place of remaining unchanged -from the beginning, we know of nothing more mutable than the land on -which we dwell, so that if the waste everywhere so apparent were to -go on unchecked or unmodified, island and continent would eventually -disappear beneath the waves. Here, however, another principle comes -into operation. The debris removed from the land, as we have seen, -is not annihilated. Slowly borne seawards, it settles down at river -mouths or on the floor of the ocean as an ever-thickening deposit, -which eventually hardens into rock, as solid and enduring as that -whence it was derived. But it does not always remain there. Owing to -the action of subterranean agencies with which we are but slightly -acquainted, different parts of the sea-bottom are continually rising. -Sometimes this process goes on very slowly, as along the shores -of Sweden, where the coast has been ascertained to emerge in some -localities at the rate of about thirty inches in a century; sometimes -with prodigious rapidity, as on the coast of Chili, where the land -was upheaved from two to seven feet in a single night. There can thus -be no doubt that the mysterious agency which produces earthquakes and -volcanoes on the land affects equally that portion of the earth's crust -covered by the waters of the ocean, and must be ceaselessly employed -in elevating large areas of sea-bottom into new continents, that will -ere long become clothed with vegetation and peopled with animals. -In contemplating, therefore, the constant decay in progress on the -surface of the land, we see not a mere isolated process of waste, but a -provision for future renovation. The sandstone cliffs of the shore are -battered down and their debris carried out to sea, but when sea-bottom -comes to be land-surface, they may be sandstone cliffs again, lashed -once more by the breakers, and once more borne as sediment to the -depths of the sea. And thus, in what may seem to us sublime antagonism, -land is ever rising in the domain of ocean, and ocean ever encroaching -on the regions of land. No sooner does a new island, or mountain peak, -or wide area of continent, appear above the waves, than the abrading -agencies are at work again. Rain, air, frost, rivers, currents, -breakers, all begin anew the process of destruction, and cease not -until the land has utterly disappeared, and its worn debris has sunk -in mid-ocean to be in process of time once more dry land, and suffer -another slow process of obliteration. - -Such is the economy of nature around us now, and that such will -continue to remain the condition of things in the future, we can affirm -with probability from a consideration of the history of the past. The -geologist can point to masses of rock several miles in thickness, and -occupying a large area of the globe, formed entirely of the worn debris -of pre-existing formations. The very oldest rocks with which he is -acquainted are made up of hardened sediment, pointing to the existence -of some land, even at that early period, worn down by rivers or -wasted by the sea. During all the subsequent ages the same principles -were at work, and now well-nigh the only evidence of the geological -periods is to be gathered from the layers of sediment that successively -settled down at the sea-bottom. The records which it is the task of -the geologist to decipher, are for the most part written in sand and -mud--the deposits of the ocean, for in by far the larger number of -formations into which the stratified part of the earth's crust has been -divided, and which form his only guide to the history of the past, he -can detect no trace of land. Hill and valley have alike disappeared, -and the character of their scenery and inhabitants he can often but -dimly conjecture from the nature of the sediment and of the drifted -terrestrial relics that may chance to be found among strata wholly -marine. - - - - -CHAPTER X. - - - The structure of the stratified part of the earth's - crust conveniently studied by the examination of a - single formation--A coal-field selected for this - purpose--Illustration of the principles necessary to such - an investigation--The antiquities of a country of value in - compiling its pre-historic annals--Geological antiquities - equally valuable and more satisfactorily arranged--Order - of superposition of stratified formations--Each formation - contains its own suite of organic remains--The age of - the boulder defined by this test from fossils--Each - formation as a rule shades into the adjacent ones--Mineral - substances chiefly composing the stratified rocks few - in number--Not of much value in themselves as a test of - age--The Mid-Lothian coal-basin--Its subdivisions--The - limestone of Burdiehouse--Its fossil remains--Its probable - origin--Carboniferous limestone series of Mid-Lothian--Its - relation to that of England--Its organic remains totally - different from those of Burdiehouse--Structure and - scenery of Roman Camp Hill--Its quarries of the mountain - limestone--Fossils of these quarries indicative of an ancient - ocean-bed--Origin of the limestones--Similar formations still - in progress--Coral-reefs and their calcareous silt--Sunset - among the old quarries of Roman Camp Hill. - -Among the standard jokes of ancient Athens was that of the simpleton -who, with the intent of selling his house, carried about a brick as a -specimen. In this and the following chapter I propose to follow his -example, and, for the purpose of giving my reader a correct notion of -the structure displayed in the stratified portion of the earth's crust, -to select therefrom a single formation whose details will connect -together the subjects discussed in the previous pages. And in so doing -it will, I trust, be found that what was ludicrous in the hands of -the Greek becomes sober sense in those of the geologist. The "brick," -then, which I would humbly present to the thoughtful consideration of -the reader as really a specimen of the house of which it forms a part, -has been termed the "Carboniferous System," and consists of a series -of stratified rocks sometimes nearly 15,000 feet thick. The plants -and animals found in these strata have been already described somewhat -in detail, and we have turned aside to look at the processes whereby -such masses of sedimentary rock came to be accumulated. But we shall -probably better understand the habits of the animals and the general -aspect of the vegetation, as well as the agencies at work in depositing -vast beds of mineral matter, if we take a coal-field and analyse it -stratum by stratum, marking as we go their varied and ever-changing -character, and the corresponding diversity of the included organic -remains. Such an examination will bring before us some of the more -striking and important laws of geological research, and while of use -to the young observer, may be not without some share of interest -to the general reader. Before beginning, however, let me endeavour -to illustrate the principles that will guide us by a simple though -hypothetical story. - -Suppose the bed of the Firth of Forth were raised above the level of -the sea and covered over with verdure, and that, in ignorance of the -previous topography of the locality, a mason were to excavate on the -lately-born land the foundation for a dwelling-house. Immediately -below the grass he would com? upon layers of hardened mud containing -oyster-beds, with detached valves of cockles, mussels, fish-bones, and -perhaps the tooth of an anchor or the timber of some old herring-boat. -Now, if he were gifted with but ordinary intelligence, what would he at -once conclude from these remains? Plainly, that the spot on which he -stood had once been the bed of the sea. And if in place of appearing -as dry mud and sand these deposits had got hardened into shale and -sandstone, and the shells, too, had become hard and stony, this would -not alter his convictions. He would still assert positively that he -stood upon an old sea-bottom. And suppose further, that all this were -far away from any sea, still such a circumstance could make no change -in his opinion; he would rightly assert that the place of sea and -land might vary, and that the ocean's being now many miles distant -could be no argument against the waves having once rolled over the -site of the intended dwelling-house. Let us further imagine that he -continues his trench, and in sinking deeper comes to a bed of dark -peat with snail-shells and bones of sheep, deer, and oxen. What will -he infer from these? Clearly that they represent an old land-surface, -once covered with vegetation and browsed over by ruminant animals, and -that this old land-surface has at some distant period been submerged -beneath the sea. Suppose, moreover, that below the peat there were a -thin bed of reeds and rushes intermingled with the mouldering remains -of fresh-water shells. He would in that case infer that before the -formation of the peat the locality was occupied by a lake. - -Putting now all these deductions together, our mason would have evolved -a very interesting history. He would have ascertained that in a bygone -age the spot on which he stood was the site of a lake, tenanted by -delicate shells and fringed with reeds and rushes, where the coot and -the mallard may have reared their young; that in process of time the -vegetation gained upon the water, choking up the lake, so as gradually -to form a soil firm enough to support sheep, deer, and oxen, and -yielding shady coverts whither the antlered stag could retire and lay -him down to die; that in after years the sea had encroached upon the -peat-moss, and oyster-beds begun to form where cattle had been wont to -browse; that again the ocean receded, and the land emerged to assume -new verdure and receive new inhabitants. - -Now, in all this reasoning there is no hypothesis or speculation. The -mason proves himself an intelligent, honest fellow, and uses his eyes -and his head where many other men would perchance see very little need -for the use of either. There can be no setting aside of his story; -he can appeal to facts. "There," says he, "is a layer of peat with -the rush-stalks and moss-fibres matted together in the soft brown -mouldering substance, exactly as I have seen them a hundred times in -the peat-cuttings on the moors, and I cannot but believe that they -must both have had the same origin, that is, that they grew in swampy -hollows of the land. There, too, lies a stratum of fresh-water shells -identical with those that occur in our ponds and marshes. Although -mouldering now, they are evidently not fragmentary, but entire and -unbroken; some of them are young, others full-grown, and they lie -grouped together as in our present lakes. Such shells could only live -in fresh water, therefore the spot where I stand must have been at one -time a fresh-water lake. There, again," he continues, "is a bed of -oysters which cannot have been transported hither, for their valves are -together, lying just as they do in our present oyster-beds. This green -field, therefore, must have been at one period a muddy sea-bottom." - -After this manner and upon this kind of evidence must all inquiries -into the past changes of the earth's surface be conducted. And provided -only we proceed cautiously, reasoning from positive facts, and -striving as far as possible to exhaust what Bacon calls the "negative -instances," our deductions possess all the certainty of truth. For in -much the same fashion do we derive no small part of our acquaintance -with the early history of our own land, as well as with the arts -and customs of other nations. The scattered relics turned up by the -operations of the farmer--wooden canoes, flint hatchets, gold torques, -bronze pots, fragments of pottery, and rusty coins--all have their -bearing upon the annals of the country, and so clear is the evidence -which they read out that an eminent antiquary has divided the early -ages of Scotland into three periods, distinguished, from the character -of their relics, as the "Stone Period," the "Bronze Period," and the -"Iron Period."[57] But in such a classification the historian has -little to guide him save the nature of the relics themselves. He places -the rudest first, and groups the rest in succession, according to the -degree of advancement in civilisation which they respectively indicate. -And the grouping seems just, though in some cases objects belonging -to two of these periods may have been to some extent contemporaneous, -just as thatched roofs gave way to tiles, tiles to slates, and slates -partly to lead, though at the present day a walk of half an hour in -some localities will bring before us specimens of all these styles -still in use. If, however, the relics of geological history lay -scattered about like those of early Scottish history, all hope of -ever attaining to anything like a correct chronology and arrangement -would have to be abandoned in despair. In truth, it would then be -impossible to conjecture whether any succession of ages preceded man, -during which other tribes of plants and animals lived and died, or -whether the whole mass of fossiliferous rocks had been accumulated -since the human era, or perhaps created just as we find them. But all -this uncertainty and confusion has been obviated simply by the fossils -being ranged in beds vertically above each other, the oldest at the -bottom and the latest at the top. So that if we find in a low cliff -along the shore blown sand and broken whelks immediately beneath the -vegetable mould, and oyster-valves in a clayey bed three feet below, we -pronounce the oysters to have lived before the whelks, and that between -their respective lifetimes a sufficient interval must have elapsed to -allow three feet of sand, clay, and gravel, to accumulate. What is thus -true on the small scale holds equally so on the large. The stratified -formations in which organic remains occur are found to be grouped -regularly over each other in a settled invariable order. If A be below -B in England it will be below B all over the world, and if C be above -D at the North Pole it will be so at the South Pole too, and at every -locality where the two rocks lie together. This order of superposition -forms one of the grand tests for the age of different rock masses. By -means of this simple rule the geologist has been enabled to arrange -the different stratified formations, supplying the missing portions of -one locality from the more complete series of another, so as to form a -chronological table of no small part of our planet's primeval history. - -[Footnote 57: See Dr. Daniel Wilson's deeply interesting work _The -Pre-historic Annals of Scotland_.] - -But this is not all. We must attend to the character of the organisms -as well as to their order of occurrence. We must distinguish the animal -from the vegetable, the terrestrial from the marine, and scrupulously -examine the peculiarities of each so as to recognise them again in -other strata. By such careful scrutiny we may trace out the successive -changes in the physical aspect of a district during past times, -viewing in terrestrial plants (when clearly occupying their original -site) evidence of an old land-surface; in _cyprides_, _unios_, and -_paludinæ_, traces of a former lake; and in corals and marine shells, -unmistakable proofs of an ancient sea-bottom. Still further, by marking -the specific character of such fossils we obtain a key to the age of -many rocks that otherwise would be unintelligible, for it is found that -each of the stratified formations, from the oldest upwards, has its own -peculiar and characteristic organisms recognisable all over the world. -This test of the geological position and age of any fossiliferous rock -has a peculiar value, for it can be applied with infallible success -where every other fails. The order of superposition is often obscured -by dislocations and other causes, and the mineralogical texture of a -formation may change entirely in a short space; but if the imbedded -fossils remain, we can be at no loss as to the relationship of the rock -which contains them. And hence, if in some lone island of the Hebrides, -haunted only by the screaming sea-fowl, we find a patch of shale -containing ammonites, belemnites, and a host of other shells in large -measure identical with those occurring among the clays and limestones -of Gloucestershire, we infer that they must all belong to one series -and be of the same age; that, as we know the English beds to form part -of a formation called Lias, of which, the exact place in the geological -scale has been ascertained, so in like manner the Scottish beds must -occupy a position in the same series; and that consequently there was -a time when the site of Cheltenham and part of the Hebrides lay each -beneath a sea which teemed with ammonites, belemnites, and many other -mollusca, along, too, with the bulky saurians of the Lias. And yet -no study of the surrounding rocks in the northern locality, even if -carried on for a thousand years, could ever have thrown one ray of -light upon the subject. In an earlier page our grey rounded boulder -was introduced to the reader as a mass of sandstone belonging to the -Carboniferous group of rocks. How could one be sure of the precise -geological age of a loose water-worn block that might have journeyed -all round the world? Simply by its included fossils. The calamite, -lepidodendron, and stigmaria, revealed the date of the stone as clearly -and unmistakably as if we had seen it lifted from its original bed -by the lever and crane of the quarryman. These plants are peculiarly -characteristic of the Carboniferous strata, and they consequently -stamp as undoubtedly of carboniferous age the rock which contains -them, whether it be sandstone or conglomerate, limestone or shale, and -whether we meet with it among the newly-raised blocks of the quarry, -or among the pebbles of the sea-shore. Each geological formation, I -repeat, beginning at the oldest known to us, and ending with those that -are still forming in our lakes and seas, has its own set of organic -remains whereby we can detect it wherever it may chance to occur, from -the equator to the poles. Each has its _style_, so to speak, just as -we can at once tell whether a drawing represents a Hindoo, Egyptian, -Assyrian, Greek, or Gothic temple, simply from the general _style_ of -the architecture. - -Could we but voyage back in time as we can sail forward in space, -we should find each of the geological formations not less clearly -defined than are the different nations and countries of the present -day.[58] Were the reader suddenly set down in an out-of-the-way street -of Paris, he would probably not be long in discovering that he stood -on French ground. Or if spirited away in his sleep he should awake on -the banks of the Nile, he would soon ascertain himself to be in the -land of the Ptolemies. And so if you transported a geologist blindfold -into a quarry where ammonites and belemnites abounded, mingled here -and there with bones of ichthyosaurs and plesiosaurs, he would tell -you at once that the quarry lay among liassic strata. Or if he were -placed in a ravine where the rocks on either hand displayed fern-stems, -lepidodendra, stigmariæ, and sigillariæ, he would tell you that the -surrounding district was one of carboniferous rocks, and that probably -at no great distance there might be found smoking engines and dozens -of coal-pits. Or could you set him down in some dark night upon a -wild coast-line, and show him, perchance by the flare of torch-light, -bones and scales of osteolepis, pterichthys, and dipterus, lying on -the rocks around, he would tell you that the grim crags which shot up -into the gloom were as ancient as the era of the Old Red Sandstone. In -any case the character of the rock would signify nothing, nor would -he care about the general features of the landscape, though these too -become important characteristics in certain cases. Show him but a few -recognisable fossils, and you give him, as it were, an "Open Sesame" to -which the rocks unfold their gates and reveal a store of wonders yet -more varied than those in the cave of Ali Baba. - -[Footnote 58: See _ante_, pp. 31, 32, and the Table of Rocks at the end -of the volume.] - -But though the geological systems stand thus strongly marked off from -each other when viewed as a whole, their boundary lines can often be -only approximately drawn, thereby reminding us that the divisions are -of man's device, and can have had no place in the plans of Him who -needs not to chronicle His working by years and ages, but with whom -there is no past and no future. One formation insensibly passes into -another just as one nation merges into those around it. There are -sometimes gaps, however, between the formations, serving to mark out -strongly the limits of each,[59] precisely as intervening seas and -mountain-chains serve to mark put the boundaries of different peoples -and tribes. - -[Footnote 59: Such cases, however, are probably merely local, and may -have originated from some features in the ancient physical geography -of the districts where they occur. For instance, it has always been -thought that palæozoic ages were marked off by a strong line of -demarcation from succeeding secondary times. But the gap which occurs -in England, France, and Germany, is being slowly filled up from the -evidence furnished by other countries, and we shall probably find in -the end that the Permian dovetailed with the Trias as closely as the -Silurian with the Old Red, or the Lias with the Oolite. In truth, the -longer we study the past history of our planet the less do we see of -hiatus and chasm and sharp clearly defined boundary line; while the -doctrine of a uniform system of laws and arrangements in the physical -world, first philosophically propounded in the immortal "Principles" of -Sir Charles Lyell, is ever receiving fresh confirmation.] - -The mineral substances of which these formations consist are -comparatively few in number, being chiefly varieties of sandstone, -shale, conglomerate, and limestone. One sandstone can often be scarcely -distinguished from another, and so also with the other rocks; hence -such tests as mineralogical texture supplies can seldom be relied -on to determine the age of rocks. We can prove, for example, that a -series of limestones in England may be identical in age with a set of -sandstones in Sweden, and with a group of shales in America, because -they all contain the same or representative genera and species of -organic remains. They occupy the same position in the geological scale; -that is, the animals whose fossilized remains lie buried in these rocks -were all living at the same time, while lime was gathering at the -sea-bottom over the site of part of England, and sand was being thrown -down upon a portion of what is now Sweden, and mud was accumulating -over a submerged area of America. In such cases the differences of -mineralogical character go for nothing in determining the age of the -rocks; we have to rely solely on the embedded fossils, and on the order -of superposition. - -Keeping in view, then, that the formations into which the geologist -has grouped the stratified portion of the earth's crust have a settled -and invariable order of occurrence, that each of them contains its own -peculiar and characteristic group of organic remains whereby it can -be recognised in any part of the world, and that such remains form -often the sole test at once of the geologic age and of the origin of -the rocks wherein they lie, we may return to the plan above proposed -and endeavour to understand the structure of a coal-field. For this -purpose it may be well to select one of the northern coal-fields of -Britain, since these perhaps display a greater variety in their organic -contents, and bear evidence of more diversified changes in their mode -of formation than can be seen in those of the south. The strata that -compose the coal-basin of Mid-Lothian will probably best suit our -purpose, as they are free from the disturbing effects of those igneous -intrusions which play so important a part among similar rocks to the -north and west. - -The Mid-Lothian coal-field comprises a mass of stratified beds of -sandstone, shale, coal, ironstone, and limestone, the united depth of -the whole being above 3000 feet. By reference to the annexed Table it -will be seen that the lowest beds of the section are chiefly sandstones -and shales, extending downwards to an unknown depth, without any coal -that can be profitably worked. These under-strata form the Lower -Carboniferous group. Above them comes a middle zone in which the -characteristic beds are of limestone, comprising the middle portion or -Mountain Limestone of the Scottish Carboniferous rocks. The third and -highest subdivision forms the Upper Carboniferous group or true Coal -Measures, and constitutes the whole of what is properly the Mid-Lothian -coal-field. For the sake of noting some of the remarkable changes -exhibited in the character of the rocks, it may be well to begin our -survey among the upper beds of the under group. Let us take as our base -the famous limestone of Burdiehouse, and work our way upward through -the four thousand feet of strata that lie piled above it. - - -VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD. - - UPPER CARBONIFEROUS - OR COAL-MEASURES. - / / | | A series of sandstones, shales, - | | | | and fire-clays, with interbedded - | =Flat Coal Group.= / | | seams of coal occupying the - | (Above 1000 feet.) \ | | central area of Mid-Lothian - | | | | coal-field. - | \ | | - | |—————————| - | / | | - | | | | - | | | | A great series of sandstones - | | | | and shales with three seams of - / =Roslyn Sandstone / | | marine limestone (marked here - \ Group.= \ |·········| by dotted lines), With the - | (About 1500 feet.) | | | exception of one or two thin seams - | | |·········| it contains no coal, and serves in - | | | | this way to mark off the coal- - | | | | bearing beds above from the still - | | | | richer coal-bearing beds below. - | \ |·········| - | |—————————| - | / | | - | | | | A group of sandstones and - | =Edge Coal Group.= / | | shales similar to those at the - \ (800-900 feet.) \ | | top, and like them abounding in - | | | coal seams, some of which are - \ | | thick and valuable. - ———————————————————————————————————————————————————————————————————————— - CARBONIFEROUS LIMESTONES. | | - / =Roman Camp / | | A set of marine limestones - | Limestones.= | | | intercalated with sandstones, - \ (150-200 feet.) \ | | shales, and a few seams of coal. - ———————————————————————————————————————————————————————————————————————— - LOWER CARBONIFEROUS. | | - / | | - | | | - | =Burdiehouse / | | Sandstones and shales extending - | Limestone.= | |·········| to an unknown depth, often - | (27 feet.) \ | | with seams of dull-grey compact - / | | limestone, rarely of coal. The - \ Thickness of Lower | | beds become very red towards - | Carboniferous Rocks | | the base, and wholly devoid of - | unknown, but | | fossils. - | probably greater | | - | than that of the | | - \ upper. | | - -The Burdiehouse limestone is twenty-seven feet thick, of a yellowish -or bluish-grey colour, very compact, splintery, and often fissile -in structure, with a finely striped and laminated appearance, which -probably indicates a slow and tranquil origin. It is Crowded with -fossils, every fragment when taken up showing its seed-cone, fern-stem, -fish-scale, or minute _cyprides_. All the plants seem to belong to -terrestrial species, and have a broken and often a macerated look. -Manifestly they never grew where we now find their remains; they must -have come drifting down from swamp, or jungle, or hill-side. And so -we come to know that during the later ages of the Lower Carboniferous -period, there lay somewhere in the neighbourhood of Burdiehouse a land -clothed with ferns and club-mosses, and through whose swampy hollows -there spread a network of stigmariæ, while sigillariæ waved their -fronds high overhead. From what has been said on a previous page we may -infer that the climate of the old land was moist and equable like that -of New Zealand, nourishing a prolific growth of ferns and other plants -comparatively low in the botanical scale. The scenery of the vegetation -displayed perhaps no great variety of outline, but exhibited rather an -endless succession of the same graceful forms. - -But the limestone presents us with other remains than merely those of -terrestrial plants. It displays in abundance the minute dissevered -cases of _cypris_, the small crustaceous animal described above. -Recent species of this genus inhabit stagnant ponds or the bottoms of -gently-flowing rivers, and we hence infer that the ancient species must -in like manner have possessed a similar habitat, and consequently that -the rocks which preserve their remains must have been deposited in -fresh (or, perhaps, brackish) water. Tried by this test the Burdiehouse -limestone must be regarded as a lacustrine, or more probably a -fluviatile formation, which gathered slowly on an undisturbed bottom -swarming with crustaceans and plentifully covered with leaves, -branches, rootlets, and other fragments of terrestrial plants brought -down by streams from the adjoining land. Thus the inferences drawn from -the numerous plants, and from the countless multitude of cypris-cases, -come to be mutually corroborative. The former tell us of some -neighbouring forest-covered country; the latter lead us, as it were, -into its river-mouths, whence we can descry the waving woods on either -side. - -Still we have not exhausted all the fossil remains of the Burdiehouse -rocks. Mingled among the stems of ferns and lepidodendra, and the -scattered valves of the cyprides, lie the scales, teeth, and bones, -of several large ganoidal fishes, along with entire specimens of the -smaller genera. The scales of holoptychius are especially abundant, -often crowded together by dozens, and probably not far out of the -arrangement they had when grouped on the body of the living animal. -Detached teeth of the same fish also frequently occur along with -disjointed internal bones. The remains of the contemporary megalichthys -likewise abound, more particularly the scales, which have a fine -nut-brown colour, and dot the surface of the rock with their bright -glittering enamel. Several other smaller ganoids may be met with, -especially a small and elegant species of Palæoniscus (_P. Robisoni_), -and one of Eurynotus, a fish remarkable for the great size of its -dorsal fin. Not uncommon, too, are the ichthyodoralites of a gigantic -placoid--the _Gyracanthus formosus_--with all their delicately-fretted -ornament and a peculiar crystalline glistening surface when broken -across, whereby the smallest fragment can be easily distinguished from -any other bone in the limestone. Such are the ichthyic remains of the -Burdiehouse beds; what deductions can be legitimately drawn from them? - -As before, we must have recourse to the analogy of living nature. The -existing ganoidal fishes chiefly inhabit lakes and rivers, especially -near the confluence of the latter with the ocean. They feed on -the decaying matter brought down from the land, or on the minute -Crustacea that swarm upon the river-bottom. If, as seems probable, -the ancient ganoids had habits similar to those of their present -representatives, then the rocks wherein their remains occur abundantly -may have originated on river-bottoms, and such may have been the case -at Burdiehouse. So that here again we have corroborative evidence -of the fluviatile origin of the limestone in question. But besides -the remains of ganoidal fishes there occur the defensive spines of -placoids. Now, the placoids are emphatically marine fishes, and the -sole living representative of the most ancient genera of this order -is the Port-Jackson shark, that haunts the seas round Australia. The -ichthyodorulites of Burdiehouse, therefore, if we would apply analogy -consistently, must be regarded as the relics of marine species. And -this conclusion, too, will be found in entire harmony with those -already obtained, for if we are right in assuming the Burdiehouse -strata to have originated at a river-bottom, particularly near the sea, -we may expect to find the remains of marine predaceous fishes imbedded -in the sediment that gathered there, just as the teeth of the shark may -be preserved among the mud forming in the upper reaches of many British -estuaries, seeing that not a few instances are known where that fish -has been stranded on such shores as those of the higher parts of the -Firth of Forth. These Burdiehouse ichthyodorulites give positive proof -that the limestone could not have originated in a lake, and the only -explanation left is that of a river-bottom. - -But it may perhaps be objected that, after all, these fish-remains -are for the most part fragmentary, and may consequently be drifted -specimens, so that no conclusion as to the source of the rock can be -based on their occurrence there. The imbedded land-plants confessedly -came from some distance, why may not the same have been the case with -the bones and scales of the river-haunting ganoid fishes? And, indeed, -did we regard these fish-bones and scales merely in themselves, the -argument might not perhaps be very easily answered, although the great -numbers and perfect outline of the bones, teeth, and scales, afford -pretty strong evidence that the owners lived and died in the locality -where their remains are found. But there is a curious kind of evidence -to be gleaned from the rocks around them whereby this objection can be -at once set aside. In the limestone itself, and especially in some of -the shales above, there occur vast numbers of small oblong coprolitic -concretions of a dirty yellow or brown colour, full of scales and -fragments of bone. There can be no doubt that these are the excremental -remains of predaceous animals, while their great number and perfect -preservation assure us that they could not have been drifted from a -distance, but must rather have been deposited on the spot where we now -find them. And thus we conclude that the site of Burdiehouse must have -been a favourite haunt of these bone-covered fishes; that the bulkier -forms, armed with pointed teeth or barbed-spines, preyed upon their -humbler congeners, while these in turn may have fed on the cyprides -that swarmed by millions at the bottom of the estuary. I have often -detected in these coprolites the peculiarly-sculptured scales of the -palæoniscus. These graceful little animals must, therefore, have died -that their lordlier brethren might dine. - -On a survey, then, of the whole evidence from fossils, we are led to -conclude that the Burdiehouse limestone was slowly elaborated at the -bottom of an estuary, into which the remains of terrestrial plants were -drifted from the land, while bone-covered fishes haunted the waters, -and into these busy scenes huge sharks ascended from the sea to share -in the decaying putrescent matter ever brought down from the interior. - -The upper part of the limestone is shaly and argillaceous, and rests -below a series of shales and thin sandstones. If the question were -asked, what caused the change from limestone to shale, from the -deposition of a calcareous to that of a muddy sediment, several -answers might be given. The most probable seems to be the following. -The limestone on weathered surfaces displays the mouldering casts of -cypris-cases sometimes in such abundance as to show that the rock must -be largely made up of them. The cyprides of the present day probably -cast their shells annually; the integuments thus thrown off forming -under favourable circumstances a thin mouldering calcareous marl at -the bottom of the pond or marsh, along with the decaying shells of -_paludina_, _planorbis_, _limnea_, or other fresh-water molluscs. We -may conceive the Burdiehouse limestone to have had a similar origin. -The cyprides, inhabiting water that contained little argillaceous -matter, must have propagated by myriads, and during a long period of -repose, in which the conditions of land and sea, and the directions of -tidal currents and river-courses, appear not to have greatly varied -in the neighbourhood of Burdiehouse, the calcareous exuviæ of these -minute animals, along perhaps with the remains of other estuarine or -fluviatile organisms,[60] would form each year a scarce appreciable -stratum, until by slow aggregation a bed twenty-seven feet deep was -elaborated. Each successive annual layer would hardly settle down more -perceptibly or more rapidly than "the flickering dust that mottles the -floor of some old haunted chamber." - -[Footnote 60: Though I have never observed molluscan remains in the -limestone of Burdiehouse, they are abundant twelve miles to the west, -in the equivalent strata around Mid-Calder, one little gastropod -being especially plentiful near the base of the calcareous rock in a -seam known to the quarrymen as the "Buckie fake." I have not met with -specimens sufficiently perfect for identification, the hard splintery -nature of the rock seldom allowing anything but a cross-section to be -seen save on weathered specimens, where the general contour of the -shells has sometimes reminded me of _Paludina multiformis_ grouped -together in a recent fresh-water marl. In the shales above the -Burdiehouse limestone, Dr Hibbert states he found a _unio_ (?), called -by him _U. nuciformis_. _Trans. Roy. Soc. Edin._ vol. xiii. p. 245.] - -At last, however, this condition of things came to be modified. The -direction of the river channel along some part of its course had -varied, or some analogous change had taken place, so that muddy -sediment transported from the land sank down amid the cyprides at -the bottom. In circumstances so uncongenial these tiny denizens -of the estuary diminished in numbers until the silt and sand came -down so rapidly and in such abundance that they eventually died out. -Alluvial matter still darkened the water and covered the river-bottom, -enveloping now the fronds of a delicate fern that had waved along -the margin of some sequestered lake far inland, anon a seed-cone -that had been shaken by the breeze from the spiky branches of some -tall club-moss. Among these muddy beds occur numerous coprolites and -fish-scales, along with cypriscases and a few shells of unio (?), -showing that though the cyprides were decreasing, the water still -presented the old estuary conditions and still swarmed with life. - -Eventually there came other changes in the direction or rapidity of -river currents, and the accumulations of mud and silt were succeeded by -a long protracted deposition of yellow sand, now forming the sandstone -of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c., -and in certain limited areas these plants matted together in such -quantities that their remains now form thin irregular seams of coal. -It would appear, therefore, that notwithstanding these changes in the -matter transported and deposited at the locality in question, the -estuary character of the locality remained essentially the same. The -sand was at length replaced by fresh accumulations of mud and sandy -silt, which went to form the beds of shale and shaly sandstone now -found above the Straiton rock. - -When in the course of many long centuries a depth of strata amounting -to fully 300 feet had been amassed, the area of Mid-Lothian underwent -a total change. Owing to a depression of the earth's crust, that seems -to have been general over the whole of central Scotland, the estuary -in which the Burdiehouse limestone and superincumbent strata were -deposited became open sea. As the evidence of this change rests solely -on the character of the imbedded organic remains, we shall pursue our -induction by examining the beds somewhat in detail. - -Rather more than 300 feet above the limestone of Burdiehouse there -occurs in the Mid-Lothian coal-field a series of shales and seams -of limestone. The former are sometimes black and hard, sometimes -bluish-grey, soft, and frequently imbedding the remains of several -genera of mollusca and other organic remains. The limestones vary -considerably in the thickness and general aspect of their several -seams, some being highly crystallized and about two or three feet in -depth, others dull, compact, and ranging up to twenty and thirty feet -thick. The shales and limestones are intercalated with and sometimes -pass into each other, through the gradations of shaly limestone and -calcareous shales. The whole series may measure 150 to 200 feet, -resting on the Straiton sandstone below, and passing upwards into -the under part of the coal-bearing strata of Mid-Lothian known as -the _Edge series_. These limestones form the northern _marine_ -equivalents of the mountain limestone of England, while the sandstones -and shales on which they rest, including the Burdiehouse beds and -all the Lower Carboniferous group, must probably be regarded as -_estuarine_ equivalents of the same formation. That is to say, while -marine limestones were accumulating over the site of central England, -sandstone, shale, and drifted plants, were slowly gathering in a wide -estuary over what is now central Scotland, and only at the close of the -period did marine limestones form simultaneously at both localities. - -In examining these Mid-Lothian beds we are struck at once with the -great dissimilarity that obtains between their organic remains and -those of the underlying strata. All the land-plants disappear--ferns, -lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer -occur, though the shales seem, at a first glance, to differ in no -respect from those underneath, in some of which the cypris-cases were -seen to abound. Neither can we detect the glittering scales and teeth -that stood out in such strong relief upon the rocks below. Yet the -fossils are scarcely less numerous than they were in the lower beds. -Nay, in some of the limestones they lie so crowded together that the -rock seems entirely made up of them. Plainly such a total renovation -of organic life points to some equally extensive change of a physical -kind. Let us examine for a little some of the fossil remains occurring -in the mountain limestone series of Mid-Lothian, and read off, if we -can, the revolutions which they chronicle. - -[Illustration: Fig. 32.--Section from Gilmerton to Crichton; _a_, Lower -Carboniferous; _b_, Mountain Limestone; _c_, Edge Series; _d_, Roslyn -Sandstone Group; _e_, Flat Coals; _y_, Drift.] - -The neighbourhood of Edinburgh affords many facilities for the study of -these rocks. They can be seen, for instance, at many points along the -ridge of the Roman Camp Hill, near Dalkeith, exposed in the operations -of quarrying. That ridge is formed by what is known technically as -an anticlinal axis (Fig. 32); in other words, the lower beds of the -coal-measures rise up here into a sort of broad wave-like undulation, -round the sides of which the higher parts of the series are folded. The -elevated area has either been pushed up from below, or the more level -country around has subsided into two trough-like hollows, so that now -the strata, which geologically speaking are lowest, come to occupy the -highest ground in the district. Seated on some of the opener spots -of this woody eminence the observer has a noble prospect on which to -expatiate. The ground around him is rich in historic associations, -and links itself to many a varied page in the annals of Scotland. The -hill on which he rests is crowned by the mouldering mounds of what -tradition reports to have been a Roman station, but which may perhaps -belong to a still earlier era. A few hundred yards north rise the -wooded slopes of Carberry Hill, where the hapless Mary surrendered to -her rebel lords, and whence she was led into her own capital amid the -insults of an infuriate rabble. Northward, too, lies the fatal field -of Pinkie, and eastward the less deadly but not less decisive field -of Prestonpans. To the west the eye can mark the grey smoke of the -Scottish metropolis, with its dusky towers and its lion-shaped hill, -and then the blue waving outline of the Pentlands that sweep away -south and lose themselves among the distant hills which girdle in the -coal-basin of Edinburgh and Haddington. The course of the Esk--that -_fabulosus amnis_--passes by many a time-honoured spot, from Habbie's -Howe and the scene of the Gentle Shepherd down by the haunted scenery -of Roslyn, the cliffs of Hawthornden, the grounds of Newbattle, and -the old Roman station of Inveresk. East, west, and south, the broad -expanse of green field and clustering wood swells upward to the distant -hills that encircle the landscape with a wavy line of softest blue. -Northward the eye rests on the Firth of Forth with its solitary sails, -bounded by the bosky heights of Fife, and opening outwards by the May -Island and the Bass Rock into the far-off hazy ocean. On every side -objects of historic interest lie crowded together, about which many -pleasant volumes have been and might still be written. If the observer -be a lover of geological science he will find an examination of the -structure of the hill to impart an additional interest to the scene. -From the wide panorama of hill and dale, river and sea, with all its -battle-fields, castles, and abbeys, and all its memories of the olden -time, let him turn into one of the quarries that indent the flanks -of the hill, and try to decipher there the records of a still older -history. An hour or two thus spent will pass swiftly and pleasantly -away, and on quitting the quarry he will have gained a new light in -which to look on the landscape that lies spread out below. - -The mountain limestone of Mid-Lothian consists, as has been mentioned, -of several seams interbedded with black and calcareous shales. The -quarries on Roman Camp Hill have been opened in several of the thickest -of these seams. Let us enter one of the excavations. A vertical face -of rock forms the background, overhung above by long dangling tufts of -withered grass, and washed below by a pool of water having that milky -green tint peculiar to old lime-quarries. The lowest rock visible is -a dull grey limestone with a yellowish weathered surface. Above it -rests a mass of hard yellow calcareous shale, known to the workmen -as "bands." This rock is worthless as a source of lime, nor from its -irregular laminations and shivery structure has it much value in any -other way. A few inches of surface-soil form the upper part of the -section. It requires but a glance over the weathered surface of the -limestone to mark that the rock abounds in fossils. Of these by far -the most numerous are the joints of the stone-lily, for the most part -of small size, and when broken across, with their minute central -apertures, looking like so many fractured stems of tobacco-pipes. Other -organisms also occur, such as a small delicately-plaited productus, -a larger and more boldly-ribbed spirifer, a small cyathophyllum or -cup-coral, and the fragile interlacing meshes of one of the net-like -bryozoa--the fenestella. Of rarer occurrence are the whorled shells -called bellerophon, the long chambered shells of orthoceratites, and -the grooved tapering shells of pinnæ. Many of the same fossils can be -detected in the beds above, which thus evidently all form part of one -series with the rock below. What, then, were the circumstances under -which these strata originated? - -The answer to such a question is not far to seek. The corals and -crinoids are exclusively marine families, and so any stratum in which -their remains occur must have had a submarine origin. It matters not -in this case though the specimens be fragmentary, showing a broken and -drifted appearance. For even supposing that they did not live at the -spot where their petrified relics are now exhumed by the operations of -the quarryman, granting that they were drifted from a distance, still -they could only have been drifted from one part of the sea-bottom to -another. The state of keeping of the specimen often tells vastly on the -value of its evidence when it belongs to a land or fresh-water tribe. -Thus, in one of the limestones of West-Lothian I have found a black -carbonized stem of sigillaria. Now, the sigillaria was a land-plant -as much as any of our hazels or willows, and where the evidence from -the associated organisms coincides, furnishes its own testimony as -to the origin of the rock which imbeds its remains. But the stem in -question was a mere fragment, and showed moreover a worn macerated -surface. Such a fossil had evidently no value as a test of the origin -of the limestone, which might have been elaborated either in an inland -lake or in open sea. That it had really a marine origin, and that the -sigillaria actually was, as it seemed to have been, a drifted plant, -I ascertained beyond a doubt by detecting on the same slab hundreds -of encrinal stems along with the shells, and thin, delicate, silvery -spines of productus. Thus, then, the organisms of the land may be -carried into the sea, and in dealing with their fossilized remains -in the deposits of former ages we must be very careful in the use of -evidence derived from fragmentary and drifted specimens. But no such -caution is needed in regard to the productions of the sea. If they be -fragmentary and drifted, we may believe they were rolled about by tides -and currents previous to their final entombment; but still they remain -as good a test as ever of the marine character of the rock in which -they occur.[61] - -[Footnote 61: The exceptional instance, of the accumulation on the land -of blown sand imbedding the broken remains of marine shells, needs only -to be noticed here.] - -The fossils of Roman Camp Hill are not drifted specimens. They must -have lived and died where the quarryman now finds them. We recognise -them as all unequivocally marine; corals, crinoids, and brachiopodous -molluscs, are all clearly the denizens of the sea, and hence we -conclude that they mark the site of an ancient ocean. The snail-shells -that swarm about the fruit-trees of our orchards not more unmistakably -indicate a land-surface than do these petrified relics evidence an old -sea-bottom. We can argue, too, from the crowded way in which they lie -grouped together, that life must have been prolific in these primeval -waters. Every fragment of the rock shows its dozens, nay, hundreds, -of stone-lily joints, disjointed indeed, yet easily recognisable. -They must have swarmed as thickly along the floor of the sea as the -strong-stemmed tangle that darkens the bottom of many a picturesque -bay along our western coasts, yet with a gracefulness of outline such -as none of our larger sea-weeds can boast. Less numerous but not less -markedly _in situ_ are the shells of productus and spirifer, the former -with its finely-striated surface fresh as if the creature had died but -yesterday, while the slender spines with which it was armed lie strewed -around. In short, the whole suite of organisms points to a period of -tranquil deposition in a sea of probably no great depth, where the -lower forms of the animal kingdom flourished in abundance, contributing -by their calcareous secretions to form continuous layers of limestone. - -Such a condition of things finds a parallel in many parts of the globe -at the present day. Thus, the shores of the islands of the Pacific -are white with fine calcareous mud, that results from the action -of breakers on the surrounding coral-reefs. This mud, enveloping -fragments of coral, shells, sea-weed, drift-wood, and other extraneous -substances, hardens on exposure, and becomes eventually a limestone, -travertine, or calc-sinter. We may believe that the same process goes -on out at sea, around the edges of atolls or circular coral-reefs, -and that the sediment thus thrown down will enclose any zoophytes -or molluscan remains that may lie at the sea-bottom, along perhaps -with _fuci_, and occasional water-logged fragments of wood that have -been drifted from land. Along the shores of Guadaloupe a bed of this -calcareous silt has formed since America was colonized by man, for it -has been found to contain fragments of pottery, arrow-heads, and other -articles of human workmanship.[62] The same rock has yielded, besides, -the partially-petrified bones of several human skeletons, one of which, -though without the head, forms a prominent object among the fossil -treasures of the British Museum. The rock in which these remains are -embedded is described as harder than statuary marble, notwithstanding -its recent origin. By supposing the same process to be carried on over -a large area and for a long period, we may see how a continuous stratum -of limestone could be elaborated, full of fossil relics of corals, -molluscs, and other marine productions. And in some such way, we may -be permitted to believe, the seams of limestone on Roman Camp Hill -were accumulated. The billows of that old carboniferous ocean may not -have sent up their white surf against the margin of snowy coral-reefs, -but the currents below did their work of demolition as effectually, -and by sweeping through the submarine groves of stone-lilies and -cup-corals, as the night winds of autumn sweep athwart the heavy-laden -fields, would prostrate many a full-grown stem and scatter its -loosened joints among the thickening lime that covered the bottom. -Stone-lily, cup-coral, net-coral, productus, spirifer, pinna, nautilus, -orthoceratite, all would eventually be entombed amid the decaying -remains of their congeners, and thus produce a slowly-increasing seam -of limestone. - -[Footnote 62: Lyell's _Manual of Elementary Geology_, p. 121. Fifth -edition.] - -We still linger in the old quarry on Roman Camp Hill, but the day -draws rapidly to a close, and the long level beams of the setting sun -lighten up the higher grounds with a golden flush, while the valley -below lies deep in shade. The rays fall brightly on the abrupt face of -limestone at the further end of the quarry, every prominence standing -out in bold relief, and casting its shadow far behind. Our eye, in -passing over the sunlit rock, can detect the fractured joint of many an -encrinite glancing in the light; along, too, with the strongly defined -outlines of some of the lesser and more abundant molluscs--spirifers -or producti. Some of them, sorely effaced by the rains, have begun -to yield a scanty nestling place for creeping fibres of moss; others -yet bare, afford a rest to the _Vanessa_ whereon to spread its wings -in the mellow sunset ere flitting homewards among the dewy herbage. -The bushes overhead scarcely rustle in the light-breathing air that -comes fitfully across the land, and the long grass nods dreamily on -the margin of the pool below. There rests a calm stillness on all the -nearer landscape, and the distant ground blends away into the shades -of evening. The scene, in short, has about it that solemn impressive -repose which irresistibly arrests the fancy, and sets it to dress up -into fantastic shapes the massive clouds that float in the western -sky, to picture grim forms amid the misty shadows of the valley, or to -dwell half dreaming upon the memories of the past, that come crowding -through the mind in quick succession. Our labours among the fossils of -the old quarry, however, enable fancy to draw her stores from another -source. We muse on these petrified relics, gilded by the last rays -of the setting sun, when slowly, like a dissolving view, sunset and -herbage melt away, and the bottom of the old carboniferous ocean lies -before us with its corals and shells and stone-lilies, stretching out -their quivering arms, or expanding and contracting their flower-like -petals amid a scene of ceaseless animation and activity. Geology -delights in contrasts, and assuredly the contrast presented to us this -evening between the present and the past of Roman Camp Hill, will not -rank among the least striking of those which she has to reveal. There -is now spread over us the blue sky, richly hung with tinted clouds, -and melodious with the evening songs of the lark, the blackbird, and -the thrush. Not less surely did a wide expanse of sea during the -Carboniferous era roll over the hill on which we stand. And yonder -silvery moon that mounts up amid the violet twilight of the east, has -witnessed each scene and all the countless changes that have intervened -between them. The same pale light that now begins to steal through the -woods and athwart the fields, must have streamed down upon that old -sea and illumined its green depths. Oceans and continents, islands and -lakes, hills and valleys, have come and gone with all their successive -races of living things, and that same planet has marked them all. She -has seen, too, as but a thing of yesterday, the appearance of man upon -the scene, with all the successive centuries that have elapsed since -then. Truly the "goddess of the silver bow" would have a strange story -to tell us could we interrogate her about the past. But the days of -Endymion have gone by, and she now no longer visits in a personal form -the seat of beings who gaze at her crescent orb and daringly pronounce -it a scene of blasted ruin and desolation. - - - - -CHAPTER XI. - - - Intercalation of coal seams among mountain limestone beds of - Mid-Loihian--North Greens seam--Most of our coal seams - indicate former land-surfaces--Origin of coal a debated - question--Erect fossil trees in coal-measures--Deductions to - be drawn therefrom--Difference between the mountain limestone - of Scotland and that of England--Coal-bearing character of the - northern series--Divisions of the Mid-Lothian coal-field--The - Edge coals--Their origin illustrated by the growth of modern - deltas--Delta of the Nile--Of the Mississippi--Of the - Ganges--Progress of formation of the Edge coals--Scenery - of the period like that of modern deltas--Calculations of - the time required for the growth of a coal-field--Why of - doubtful value--Roslyn Sandstone group--Affords proofs of - a general and more rapid subsidence beneath the sea--Its - great continuity--Probable origin--Flat coals--Similar - in origin to the Edge coals below--Their series not now - complete--Recapitulation of the general changes indicated by - the Mid-Lothian coal-field. - -Among the old quarries of Roman Camp Hill and down the course of -several streams in the same county, the limestone beds of the mountain -limestone series are seen to be associated with strata of shale, some -of which are highly calcareous, and charged with the same organic -remains that occur in the limestones. Such shaly intercalations mark -as before the transport and deposition of muddy sediment around and -above the corals and stone-lilies of the sea-bottom. All these beds -must undoubtedly be regarded as marine. But there occur, besides, -seams of sandstone and black partially-bituminous shale, with layers -of coal and fire-clay. To this singular intermixture it may be well -to advert more particularly, since it forms one of the distinguishing -features of these northern rocks, as contrasted with those of central -and south-western England, and more especially since it will lead us -to mark again the value of fossil remains as evidence of the ancient -changes of land and sea. - -The southern part of Mid-Lothian consists of a broad heathy moorland, -that slopes northward into the more cultivated country, and swells -upward to the south into the series of undulating ridges that form the -Moorfoot Hills. It is traversed by several streams which rise high -among the pasture grounds of the south, and flow some into the valley -of the Esk, and thence into the sea at Musselburgh; others past the -ancient fortalices of Borthwick and Crichton, and so by the valley of -the Tyne into the sea at Tyningham. In their upper course they traverse -a broad belt of the mountain limestone that stretches across this -part of the country from east to west, and dips away north under the -coal-field. Where the streams have been able to cut through the thick -mantle of heath, sand, gravel, and clay, by which these higher grounds -are covered, we sometimes obtain a complete section of the strata -displayed in regular sequence along the bottom of the channels. Thus, -one of the rivulets that trickles slowly through the swampy ground of -Middleton Muir, on approaching the line of limestone begins to descend -more rapidly, and has excavated its course through several feet of the -rock below. The limestones are well exposed along each side of the -stream, forming in some places steep walls tapestried with moss and -overhung with scraggy furze, and offering to the student an instructive -series of sections. Near the farm of Esperston, where the stream flows -through a narrow secluded valley, the limestones form a floor which the -water in the course of centuries has worn smooth, so that the rock with -its included encrinal stems and shells, polished by the ceaseless flow -of the current, shows like a sheet of variegated marble. At one point -on the side of the water-course the observer may notice a low ledge -of rock jutting out for a short way along the edge of the stream. The -upper part is a hard compact limestone, full of small crinoidal joints. -The bed underneath it has been greatly eroded by the rivulet, but -enough remains to show that the stratum is one of coal. It rests upon -the series of limestones and sandstones seen in the upper part of the -water-course, and is surmounted by the thick limestones of Arniston and -Middleton. A similar seam nineteen inches thick has been worked among -the limestone about three miles to the west at Fountain. The same bed -occurs among the quarries on Roman Camp Hill already mentioned, and I -have seen an equivalent stratum intercalated among sheets of cup-corals -and stone-lilies on the shore at Aberlady, where the waves have laid -open perhaps the finest section of Carboniferous limestone strata in -Scotland. In West-Lothian, too, the same intercalation of coal-seams -among the mountain limestone beds can be seen in many places. Thus, in -the bed of the River Almond, near Blackburn, the following section is -laid bare:-- - - Calcareous shale. - Limestone (marine), eight feet. - Calcareous shale, with _spirifers_, &c. - Coal, six to eight inches. - Fire-clay. - Sandstone. - -A short way further down the stream another bed of limestone occurs -with several seams of coal below it, one of them attaining a thickness -of six feet. - -In addition to the thin seam at Esperston, the Mid-Lothian field -contains several others. Of these by much the most important is that -known as the North Greens Seam. It varies in thickness from only a -few inches to fully 5 feet, and has been extensively worked for the -_parrot_ or gas-coal which it contains. It rests upon a pavement of -shale, sometimes of fire-clay, and occurs about midway between two -thick marine limestones, being from 80 to 90 feet distant from each. -I have laid open many a block of the parrot-coal at the pit mouth, -and marked the well-defined outlines of the stigmaria covered with a -yellowish efflorescence of iron pyrites, like gilded figures upon -a black velvet ground. The plants lie with their divergent rootlets -spread out regularly along the stem like teeth on the back of a comb, -thus seeming to indicate no hurried agglomeration by some tidal wave or -turbid river, but rather a slow and tranquil deposition. - -The fossils of the coal-seams consist for the most part of the -plants above described, which we saw to belong to _terrestrial -species_. But the reader will now understand that in dealing with -organic remains we cannot infer, because a certain stratum contains -nothing but land-plants, that it must necessarily by consequence be a -land-formation. For we have seen that the plants of the Burdiehouse -limestone, though all terrestrial, gave no support to the idea that the -rock had originated on land. In all such cases regard must be had not -only to the nature of the imbedded organisms, but their condition and -mode of occurrence, and to the character of those associated with them. -Especial care must be taken to distinguish what has been transported -from what is _in situ_, otherwise, by attending only to one part of -the evidence, we shall miss the import of the whole, and altogether -misinterpret the records which we seek to decipher. - -For years the subject of the origin of coal formed one of the many -battle-fields on which geologists delighted to break lances. They -ranged themselves under two banners, the "drift"-theory men and the -"growth"-theory men, the former maintaining strenuously that coal -was simply vegetation transported from the land and deposited in -large troughs at river-mouths or sea-bottoms, the latter as eagerly -contending that the vegetation had not been drifted, but grew on -the very locality where its remains are now exhumed. Neither party -lacked plausible arguments in support of its doctrines. The "drift" -combatants stoutly affirmed it to be contrary to all experience that -a land-surface should be so oscillating as their opponents required, -that in short it was absurd to hold each coal-seam as marking a period -of elevation, for there were often dozens of seams in as many yards -of strata, some of them scarcely an inch thick, and yet, according to -the "growth" theory, each would have required for its accumulation -a special uplifting of the land above the sea-level. These and many -other difficulties were thought to be triumphantly overcome by the -hypothesis of transport and deposition. The vegetation borne down -by some ancient Mississippi would collect in vast rafts, and these -becoming water-logged would sink to the bottom, where, by getting -eventually covered over with silt and sand, they would in process of -time be chemically altered into coal. This explanation was, however, -vigorously resisted by the opposite side. They alleged that the "drift" -theory could account neither for the wide extent of coal-seams nor -for their remarkable persistency in thickness. If the vegetation had -really been hurried out to sea by river-action, it seemed natural to -expect that the coal-seams should occur in sporadic patches of very -unequal thicknesses, according as the drifted plants had been more -densely or more loosely packed. But this was found not to be the case -in point of fact. The coal-seams were ascertained to be generally -singularly continuous, and to retain for the most part a pretty uniform -thickness over considerable areas. And what was still more worthy of -note, they were, as a whole, markedly free from extraneous matter, -such as sand and mud. Where these impurities did occur, it was usually -in the form of intercalated seams or partings, often quite as regular -and extensive as the coal itself. Had the vegetation, therefore, been -transported into the sea, it could hardly fail to get mixed up with -the fine impalpable mud which, like that of the Ganges or Mississippi, -might have discoloured the ocean for leagues from the river-mouth, -and settled down as a thickening stratum at the sea-bottom. And many -other arguments, derived from the nature and arrangement of the strata -interbedded among the coal-seams, were urged to prove that the latter -had originated from vegetation which grew on the spot. - -[Illustration: Fig. 33.--Section from Cape Breton coal-field, showing -four planes of vertical stems, and seven ancient soils with their -covering of vegetation. - -_a_, sandstones; _b_, shales; _c_, coal; _d_, fire-clays; _e_, -arenaceous shales.] - -The warfare seems now pretty nearly at an end, and as often happens in -such cases, it has been found that each party was to some extent in the -right and to some extent in the wrong. It has been ascertained that -some coal-seams must have originated from the deposition of drift-wood -in the mud and ooze of the sea-bottom, while others undoubtedly arose -from the decay and entombment of vegetation in swampy plains of the -land. That the latter mode of formation has been the usual one in most -of our coal-fields has been generally acknowledged since Sir William -Logan's announcement that each coal-seam, for the most part, rests -upon a bed of fire-clay, which, with its embedded roots, marks the -site of an ancient soil. This fact has been abundantly confirmed in -every part of this country, and indeed wherever an extended series of -coal-seams has been examined. Not only have the underlying fire-clays -been found, but in not a few instances erect stems of trees, passing -down through the coal-seam and spreading out their divergent roots in -the clay below, exactly as they must have done when they flourished -green and luxuriant in the times of the Carboniferous system. This was -especially the case in the Parkfield Colliery, Wolverhampton, where -seventy-three trunks were laid bare in the space of about a quarter of -an acre, each with its roots attached. The same appearance was observed -some years ago in the Dalkeith coal-field, where a group of erect -trees was encountered covering a space of several square yards. Some -instructive sections of such fossil-forests are given by Mr. Brown from -the Cape Breton coal-field.[63] In one of them (Fig. 33) no fewer than -four planes occur, each supporting its group of erect steins. Now, no -one can glance over this and the other sections illustrative of the -same paper, or the descriptions given by Sir Charles Lyell and others -of the Nova Scotian coal-field, without being compelled to admit that -the trees in question grew just where their upright stems can still be -seen, and consequently that the accompanying coal-seams originated not -from vegetation drifted by river-action, but from vegetation that grew -upon the spot. And though erect stems do not exist in every coal-field, -we seldom fail to detect the not less important occurrence of the -fire-clays and hardened shales that support the coal-seams and prove -by their embedded rootlets their identity with ancient soils. Thus we -arrive at the inference that while in certain localities coal-seams -have resulted from drifted vegetable matter, they have nevertheless for -the most part been formed from plants that flourished where the collier -now excavates, amid damp and dripping caverns, their carbonized remains. - -[Footnote 63: _Quart. Jour. Geol. Soc._ vol. vi. pp. 120, 130. The cut -given above (Fig. 33) is taken from one of these sections as modified -by the late Sir Henry de la Beche (_Geological Observer_, p. 582). In -the original the beds are inclined at a considerable angle, but for the -sake of clearness they are here reduced to horizontality.] - -Applying, then, this deduction to the strata occurring on the horizon -of the mountain limestone in Mid-Lothian, we are led to believe that -the North Greens coal-seam marks the site of a former land-surface. -It shows no vertical stems, but has all the other accompaniments of -an ordinary seam, such as the underlying fire-clay and shale, with -their included stigmariæ. And this conclusion has more than ordinary -interest, for if it be true, we have evidence of a terrestrial -formation among strata unequivocally marine; in other words, we see -proofs either of an elevation or a filling-up[64] of the sea-bottom -carried slowly on until land-plants grew up in matted swamps where -once there swarmed corals and encrinites, and then of a gradual -subsidence, so that marine organisms flourished again in abundance -over the site of the submerged vegetation. It is not insisted that -each of the thin coal-seams among the limestone strata marks a former -terrestrial area. Some of them may possibly have resulted from the -transport and deposition of plants borne from the land. Yet there are -others of wide extent resting upon beds of fire-clay which contains -stigmaria rootlets, &c. These I cannot but regard as the remains of -plants that grew upon the spot. And so, while we recognise in the beds -of limestone undoubted evidence of a former sea-bottom, I am persuaded -we must equally admit that at least several of the coal-seams bear fair -evidence of a former land-surface, scarcely raised above the sea-level -indeed, but nourishing nevertheless a thickly matted vegetation. In -this way we shall see the mountain limestone series of the Lothians -to be not a purely marine formation, but one partly marine and partly -deltoid, showing in the succession of its strata proofs of a gradual -submergence, interrupted by movements of elevation, so that the area -which at one period formed the ocean-bed became at a later time low -delta-land, and after continuing perhaps for ages to stretch out its -verdant surface beneath the open sky, sank again amid the corals of a -wide-spread sea. - -[Footnote 64: If it be correct to set down the North Greens coal-seam -as really representing a terrestrial surface, that is, of course, a -flat delta or plain scarcely raised above the sea-level, we must, I -suspect, call in the aid of a slight elevatory process, or else hold -that the depth of the sea at the locality where the lower limestone was -forming did not exceed 80 or 90 feet, and may have been considerably -less, and that this space came to be eventually filled up by the -detritus of the river. But the wide extent and sometimes the great -thickness of the limestone beds seem to indicate a greater depth, and -thus favour the idea of an elevation of the sea-bottom to form the -North Greens coal-seam.] - -Now this condition of things differs entirely from what is presented -by the Mountain Limestone group of England. That formation, when -typically developed, attains a thickness of from 1000 to 2000 feet, and -gives rise to that green hilly kind of scenery whence it has derived -its name. It is unequivocally a marine deposit, since it abounds in -corals, echinoderms, brachiopodous molluscs, and other productions of -the deep. Northward, however, it undergoes a gradual change, getting -greatly thinner, and split up by a series of intercalations of shale -and sandstone. This alteration goes on until, on the border-land -between the two countries, the massive limestone of Derbyshire has -dwindled down into a series of thin beds, often widely separated by -intervening strata, which contain many seams of coal. After crossing -the Silurian district, and descending the northern slopes of the -Lammermuir Hills, we get into the Carboniferous system again, and find -its limestone series still farther diminished. With this decrease -of marine formations, we can detect an augmentation of coal-bearing -strata. Thus the Berwickshire coal-field lies in this lower set of -beds, far under the coal-measures of Newcastle. In the Lothians, too, -as has been shown, coal is extensively worked in the same series, and -these seams also find their representatives in Fife and Lanarkshire. -The gradual change from the kind of strata found on the horizon of -the Burdiehouse limestone, to those occurring on the horizon of the -Mountain limestone, indicates, as we saw, a gradual change of the -conditions of deposition; and the nature of this alteration is shown by -the difference in the character of the imbedded fossils. The passage -of the massive Derbyshire limestone into the thin limestones and -coal-bearing sandstones of the north, as decidedly marks another change -in the relative position of sea and land. The former was a succession -in time, the latter was one in space, but the mode of reasoning remains -the same for both. In the former case, we saw estuarine strata passing -upward into others wholly marine, and the order of superposition told -us that the locality was first an estuary, and then slowly became open -sea. In the latter case, we see marine beds not succeeded by estuarine -strata, but becoming estuarine strata themselves. The thick limestones -gradually thin out horizontally into a great series of sandstones and -shales, with interbedded coal-seams, so that what bears evidence of a -deep sea at the one end, gives proof of a muddy and sandy delta at the -other. In other words, during the ages represented by what we call the -Mountain Limestone, the central and south-western portions of England -lay far below a wide breadth of ocean, and nourished a luxuriant crop -of stone-lilies, mingled with the other denizens of the deep, while the -Border district, and the whole of central Scotland, exhibited all the -conditions of a vast delta, sometimes spreading out as broad verdant -jungles, anon showing only scattered irregular groups of low, bare -mud-banks and sand-spits, which at other times disappeared altogether -beneath the dun discoloured waves. Now the reader will not fail to -mark that this curious and interesting fact in the past history of our -country, is ascertained solely from a comparison of fossil remains. -The stone-lilies and shells of Derbyshire, and the lepidodendra and -land-plants of the Lothians, form our sole basis of evidence, and we -may rest on them with as perfect certainty as if they were so many duly -attested documents deposited among the archives of our State-Paper -Office. - -In our survey of the coal-field of Mid-Lothian, we have passed from -the Lower Carboniferous estuary beds of Burdiehouse to the Middle -Carboniferous marine beds of Roman Camp Hill, and their associated -terrestrial strata,--the coal-seams and fire-clays. We come now, in our -upward progress, to the Upper Carboniferous group, or Coal-measures -proper.[65] These strata rest immediately upon the limestones, and -attain a depth here of over three thousand feet. They consist of a -great series of sandstones, shales, coals, and fire-clays, that vary -in thickness from less than an inch to many feet, or even yards. The -coal-seams are especially variable, many of them existing as mere films -of carbonaceous matter; others varying up to a depth of fourteen feet. -There are from fifty to sixty that exceed a foot, but the average -thickness throughout the whole series is about three and a half -feet.[66] They are nearly all underlaid by fire-clay or shale, and very -generally have a roof of the latter material. - -[Footnote 65: These terms--Lower, Middle, and Upper Carboniferous, are -used for want of others, and for the sake of clearness. They must not -be regarded, however, as equivalent to similar groupings of the English -carboniferous rocks, for the Scottish series is probably much older -than the greater part of the English, and coeval, to a considerable -extent, with the mountain limestone and millstone grit of the latter -country.] - -[Footnote 66: See Milne on Mid-Lothian Coal-field. _Trans. Royal Soc. -Edin._ vol. xiv. p. 256, whence the above details are taken.] - -By referring to the diagram of this coal-field, given above at p. -196, the reader will notice that the series is divisible into three -groups:--1_st_, and undermost, a considerable depth of coal bearing -strata known as the _edge series_, because they lie along the western -limits of the coal-basin at a high angle, and sometimes even on -edge; 2_d_, A great thickness of sandstones nearly barren of coal, -but containing at least three beds of limestone this may be termed -the Roslyn sandstone group; 3_d_, and highest, another series of -coal-bearing strata, commonly called the _flat coals_, because they -occupy the centre of the basin where the beds repose at a low angle, -and are in places quite flat. It will be convenient to keep in mind -this three-fold division, for it will point us to some important -changes in the ancient conditions of this coal-field. - -The edge series, which forms the lowest, and of course oldest of the -above groups, averages from 800 to 900 feet in thickness. It contains -about thirty seams of coal above a foot thick, and many more of less -size. They occur irregularly, some lying only a few inches apart, -others from eighty to ninety feet, the intervening space being occupied -by sandstone or shale. - -Now as each coal-seam, with its associated under-clay, appears to mark -a former land surface, it will follow that there must be as many old -land surfaces in this series of strata as there are such coal-seams, -and that for every intervening mass of sandstone or shale, the area -of vegetation must have been submerged. This conclusion would have -been violently resisted by the supporters of the "drift" theory. They -would have roundly asserted that such an unsteady surface was a mere -supposition to suit a hypothesis, unsupported by fact, and contrary -to the analogy of existing nature; and they would not perhaps have -hesitated to maintain, that such an oscillating land could be little -fitted to nourish so rich and luxuriant a vegetation as that of the -Carboniferous period. But it will not be difficult to show that our -conclusion, so far from being contrary to analogy, is amply borne -out by the processes of existing nature, and that its opponents, and -even its original asserters, failed to perceive that what it demands -is not a rapidly oscillating crust, but one as steady and uniform as -that of many of the least disturbed countries at the present day; -and that we do not require to call in the aid of a special elevation -and submergence for every coal-seam, but that for the most part the -hypothesis of a steady sinking of the area of a coal-field, interrupted -perhaps by occasional elevatory movements, along with an active and -constant deposition of sediment by the varying currents of a large -river, is sufficient, if not thoroughly to explain, at least to -throw great light upon the origin of those enormous masses of strata -composing our present coal-basins. The oft-recurring variations in -the nature of the strata that form our coal-measures, sandstones -alternating with shales, these again with coals and fire-clays, -together also with the terrestrial origin of the coal-seams, and the -occasional presence of true marine organisms, make it evident that, -to obtain any modern analogue to such a condition of things, we must -examine those localities where large bodies of fresh water, carrying -sediment and vegetation from the land, mingle with the sea. Let us then -look for a little at the operations now in progress at the mouths of -the larger rivers, and mark how far they elucidate the structure and -history of a coal-field. - -"Egypt is the gift of the Nile." Such was the conclusion arrived at by -one of the most diligent observers of ancient Greece--the venerable -Herodotus.[67] He sailed up the river marking all the leading features -in its scenery, and noting the more apparent evidences of ancient -physical changes. His remarks on these subjects form one of the -earliest specimens of scientific reasoning that have come down to us, -and are remarkable for their correctness and the truly inductive mode -of thought which they evince. Modern travellers have amply confirmed -the opinions of the father of history, and we now know that but for its -central river, Egypt would be a vast dreary expanse of arid sand like -the neighbouring deserts of Lybia. The Nile, by annually inundating -the country, deposits over it a stratum of rich loam, and thus not -only waters the land, but continually renews the soil. The sediment -in this way brought down has gradually encroached upon the waters of -the Mediterranean, being heaped up at the river mouth into shifting -sand-banks, islets, and great tracts of low, swampy ground, slightly -raised above the sea-level. Through this series of silting deposits, -the river sends a number of branches, often winding in labyrinthine -convolutions, and ever changing their course, by wearing away the silt -at one place, and throwing it down at another. The area traversed by -the mouths of the Nile was called by the Greeks the Delta, from its -similarity in form to the Greek letter, and the name has since been -given to all such fluviatile deposits, whether they have this general -form or not. - -[Footnote 67: _Euterpe_, 5.--His words are very emphatic. "To one of -ordinary intelligence, who has not heard of it before, but sees it, -Egypt is manifestly land acquired by the inhabitants, and a gift from -the river--δωρον τον ποταμον." The 10th and 12th chapters of the same -book deserve especial study for the admirable inductive style in which -the historian compares the phenomena observable in Egypt with what were -well known as the results of river action in other lands. The passages -might be quoted word for word in the most rigid scientific argument of -any modern geologist.] - -The sediment annually deposited by the Nile varies in thickness in -different years. The mean thickness of the annual layers at Cairo has -been calculated not to exceed that of a sheet of thin pasteboard, so -that "a stratum of two or three feet must represent the accumulation -of a thousand years."[68] Such thin laminæ must resemble greatly some -of the more fissile shales in the Carboniferous system, which were, -perhaps, formed by as slow a process, and in their aggregate depth -probably took many thousand years to accumulate. But those fluviatile -depositions of the Nile vary little in kind, for when cut through they -are found regularly stratified down to their base, which rests upon the -great underlying sand. They show us how the argillaceous seams of the -coal-measures may have originated; but the diversity of character in -these Carboniferous rocks indicates a more varied kind of sediment, and -probably more rapid and active transporting currents. A closer analogy -to such a condition of things meets us on the shores of the New World. - -[Footnote 68: Lyell's _Principles_, p. 262.] - -The Mississippi, so magnificent in all its proportions, has raised -a delta which covers a tract of about 14,000 square miles, equal to -almost half the area of Ireland. The lower parts of this delta are -formed of low, shifting banks, traversed by innumerable streams that -diverge from the main river, and alternately throw down and remove vast -quantities of earthy sediment, intermingled with rafts of drift-wood. -These swamps are covered with a rank growth of long grass and reeds, -and for about six months of the year are more or less submerged below -the waters of the river, while liable at the same time to continual -inundation and encroachment from the sea. The higher parts of the -delta, though also subject to a similar periodical submergence, -nourish a more luxuriant vegetation. Vast tracts of level sandy soil -are densely overgrown with pine, which is used extensively for making -pitch. Large districts of the swampy ground are covered with willows, -poplars, and thickets of the deciduous cypress, an elegant tree that -rises more than 100 feet above the soil. When in hot seasons these -swamps get dried up, "pits are burnt into the ground many feet deep, -or as far down as the fire can descend without meeting with water, -and it is then found that scarcely any residuum or earthy matter is -left. At the bottom of all these 'cypress swamps' a bed of clay is -found, with roots of the tall cypress, just as the underclays of the -coal are filled with stigmaria."[69] In this way a thick accumulation -of vegetable matter goes on forming for years, until either the river -changes its course, and inundating the swamp gradually covers it over -with sand and mud, or until, owing to oscillations of the earth's -crust, the district is either permanently submerged, so as to be silted -over, or elevated to nourish a new and different kind of vegetation. - -[Footnote 69: Lyell's _Elements_, p. 386.] - -That such changes have taken place in the past history of the river we -have several interesting proofs. Thus, owing to the great earthquakes -of 1811, 1812, an area of more than 2000 square miles was permanently -submerged.[70] Since then it has gone under the name of the "Sunk -Country;" and Sir Charles Lyell, who visited the locality in 1846, that -is, thirty-four years afterwards, tells us that he saw innumerable -submerged trees, some erect, others prostrate. Now, it is easy to see -how such an area may, when the climate suits, become the receptacle of -vast accumulations of peat, which, by pressure and chemical action, -will ultimately pass into coal. If we suppose the submergence carried -on more rapidly at some periods, the plants might have been unable to -keep pace with the ever-increasing inroads of sand and mud. In such -cases the layer of vegetation would become eventually entombed beneath -succeeding deposits of earthy matter. Were the amount of sediment thus -thrown down sufficient in the end to counteract the downward motion of -the earth's crust, and so raise the bottom of the river or lake to the -level of the water, vegetation would spring up afresh and clothe the -new raised surface as densely as in former years. This alternation, -according as the amount of sinking or the amount of sediment -predominated, might go on for thousands of years, until a series of -strata many thousand feet thick were accumulated, and tranquilly -carried down bed after bed below the level of the waters. - -[Footnote 70: See Sir Charles Lyell's _Second Visit to United Stales_, -chap, xxxiii.] - -It is interesting to know that the case supposed here has actually -been realized in the delta of the Ganges. Some years ago an Artesian -well was attempted to be made near Calcutta, and the auger was sunk -to a depth of 481 feet.[71] The material passed through consisted of -sand, clay, and nodules of argillaceous limestone, and at various -depths, from 50 to 380 feet, several seams of decaying wood and peat -were found, along with bones of various animals, such as deer and -fresh-water tortoises, and fragments of lacustrine shells. Each of -these vegetable layers evidently formed at one time a forest-covered -swamp like those of the surrounding delta at the present day; and -hence it follows, that during the accumulation of the Gangetic delta, -the ground in that locality must have undergone a depression of more -than 300 feet, and that this sinking has been interrupted by slight -elevations, or by periods when the ground remained stationary, so as -to admit of a dense and prolonged growth of vegetation, at successive -intervals, upon the swampy flats and shifting islands. The general -appearance of these old forests is pretty well shown by the mangrove -swamps along the mouths of the river. These trees flourish in dense -jungles on the banks, and extend even below high water mark, being -covered in places by shell fish. So that were these maritime parts -of the delta inundated by the ocean, and buried beneath a mass of mud -and silt, the peaty layer that would be formed would display trunks of -trees still occupying their original erect position, and spreading out -their roots in the clay below, exactly as the sigillaria is found to do -in the coal-seams of the carboniferous rocks, while clustered round the -carbonized stems, or scattered among the decayed leaves and branches, -there might be detected limpets and barnacles (as lingulæ and pectens -occur in the coal-seams), showing, by their mode of occurrence, that -they lived and died upon the spot. - -[Footnote 71: See Lyell's _Principles_, p. 280.] - -If my reader will now suppose this sand of the Indian river to be -hardened into sandstone, the mud in like manner compressed into -shale, and the peat beds chemically altered into coal, can he fail to -perceive the striking analogy between the section thus displayed and -those already given from the Mid-Lothian and Cape Breton coal-fields? -The differences between the ancient and modern strata are not in kind -but in degree. The Scottish series reaches to more than six times the -thickness of the Indian one, and the coal-seams in the one exceed in -individual thickness the peat-beds in the other. We must remember, -however, that the climate of Hindustan is not remarkably favourable to -the accumulation of vegetable matter, the heat being so great that the -plants decay almost as rapidly as they grow. And it should likewise -be borne in mind, that were the conditions of subsidence and of the -gradual accumulation of sedimentary matter to continue even in the same -ratio as heretofore, the Ganges might, in the course of ages, heap up -a series of stratified sands, clays, and peat-beds, many thousand feet -in thickness, and many thousand square miles in extent, rivalling, -or perhaps surpassing in depth, the largest coal-field in the world. -The parallelism between this delta and an ordinary coal-field holds -singularly close, not merely as regards the nature of the stratified -deposits. The alluvial plain of Bengal has undergone a process of -subsidence to an unknown depth, whereby successive areas of terrestrial -vegetation have been carried down to be entombed beneath fluviatile -sand and mud. It is likewise subject to the more sudden operation -of earthquakes, whereby large tracts of country become permanently -altered, and changes are effected on the direction, rapidity, and -detritus of the streams. It is, moreover, liable to wide-spread -inroads of the sea, which sometimes covers cultivated districts to -a depth of several feet, laying waste the fields and destroying the -inhabitants. These and other features help us to understand the origin -of such vast masses of sedimentary strata as those of our coal-fields, -where terrestrial, fluviatile, and marine remains alternate in rapid -sequence, or sometimes occur together. - -The origin of the constant succession of coal seams, sandstones, and -shales, of the Edge series may be thus accounted for. The area of -Mid-Lothian formed part of a great delta, which, like that of the -Ganges, was undergoing a gradual subsidence during the Carboniferous -era. The rate of this movement probably varied at different times, and -might even be occasionally interrupted by short periods of elevation. -When the ever-increasing accumulations of silt brought down by the -river reached or nearly reached the surface of the water, they would -become the site of wide tracts of swampy vegetation that flourished -for hundreds or thousands of years. Eventually, however, these -jungles, invaded by the changing currents of the river, were buried -beneath a thick deposit of fluviatile sediment, or more probably the -vegetation might become unable to keep pace with an accelerated rate -of submergence, and the forests would then be tranquilly carried -down beneath the water, and soon covered over with sand and mud. The -detrital matter might in like manner continue to be deposited over the -sunk forest for many years, perhaps centuries, until the muddy bottom -again reached the surface, and once more waved green with sigillariæ, -calamites, and lepidodendra. Another long interval might here elapse, -in which a thick bed of vegetable matter might accumulate, much after -the manner of the formation of peat among the bogs and mosses of our -own country. The periodical inundations of the river probably gave -rise to wide marshes and lagoons, often tenanted by lacustrine shells, -and thickly overgrown with aquatic vegetation. The decaying plants -decomposed the red ochreous matter with which the water was charged, -and re-deposited it among the mud and rotting leaves at the bottom as -a carbonate of iron. Such ferruginous accumulations, often entombing -fern-stems and other plants, with scales and teeth of ganoidal fishes, -sometimes _conulariæ_ and _lingulæ_, and, in certain localities, whole -acres and miles of fresh-water shells, are known now as our _clay-band_ -and _black-band ironstones_. We can easily conceive that, in shallower -parts of the lagoons, a dense growth of marshy plants might spring -up, preventing any deposition of iron, and when the whole came to be -covered over with later accumulations of sand or mud, the deeper parts -of the old lake would be covered with a seam of ironstone, and the -shallower portions would display a bed of coal. In some such way we -may account for the frequent passage of ironstone into coal, and coal -into ironstone in many of our coal-fields. If undisturbed by the ever -changing currents of the river, these wide expanses of marsh and lake -might continue for many long years, the constant evaporation being -counterbalanced by continual supplies of water from the main stream. -Eventually, however, owing perhaps to another period of more rapid -submergence, the water gained the ascendency, and once more rolled over -prostrate stems and matted thickets of ferns, that sank slowly down -beneath a deepening sheet of sand and mud. Often, too, the sea must -have flooded, perhaps for years, the flat delta-lands, carrying with it -its own productions, such as the lingulæ and cardiniæ, which we find -among the coal seams. And thus the process went on during the long ages -of the Carboniferous system. Forest after forest spread its continuous -mantle of green athwart the low swampy lands of that old delta, and -each in succession foundered amid the muddy waters, now of the ocean -and now of the river, that strewed over its site a rich detritus which -went to form the soil of new jungles and forests. - -The Edge series measures from 800 to 900 feet in depth, so that the -depression must have been carried on till the forest that once grew -nearly on the sea-level had sunk 800 feet below it This process was -undoubtedly a very slow and tranquil one. Yet geologists used to regard -these frequent changes of sedimentary matter as so many proofs of -repeated catastrophic submergences, when the ocean came rolling over -the land, prostrating forests, uprooting the hugest trees, and leaving -the scattered bones and scales of fishes amid vast accumulations of mud -and sand, where but lately there had bloomed a luxuriant vegetation. -But the sober and diligent student of geologic fact will read in these -rocks no such record of cataclysms. He will see in them evidences of -the same gradual and sure operation which marks the processes of Nature -at the present day. He will note how during a tranquil and probably -imperceptible submergence of the river-bottom, forest after forest -sprang up, flourished perhaps for ages, and eventually settled down -beneath the waters of the river and sometimes of the ocean, amid ever -increasing accumulations of mud and sand. Musing on these ancient -changes he will be lost in wonder at the immense duration of the period -during which they were in progress; and he will try in some measure -to realize the features of their scenery. He will picture the delta -with its ever-varying islets and sand-banks, its lakes and submerged -forests, its leafless trunks peering above the water and sticking along -the shoaling mud, and its crowded jungles that cover every drier spot. -He will cast his eyes to where the delta opens out into the ocean, and -mark how the waves encroach upon the mud-banks, cutting away what the -river has piled up, and washing the roots of gigantic trees that wave -their green coronal of fronds above, and overshadow the rippling of -the green sea below. He will try to thread the windings of the stately -river through brakes of ferns and calamites, and banks richly hung -with tree-ferns and sigillariæ, and then upward through dark shaggy -pine-woods, silent and gloomy, with the water creeping lazily through -the shade or dashing in white cascades over dripping rocks, and onward -still, far away among the distant hills till the fountainhead of the -great stream is reached, gushing from the splintered sides of some -lone rock, or pouring perchance out of the glimmering caverns of some -massive glacier high amid the regions of perpetual snow. - -Many attempts have been made to estimate the amount of time which -some of our coal-fields may have required for their accumulation. -But so large a number of conjectural elements must necessarily enter -into such calculations, that the results come to be of very doubtful -value. By estimating the amount of sediment annually transported by -such rivers as the Ganges or Mississippi, we may ascertain how long a -mass of similar sedimentary strata would take to form under similar -conditions. And if our calculation had to do merely with such detrital -accumulations, we might hope to arrive at some approach to accuracy. -But besides these sedimentary strata, the formation of which must have -been wholly analogous to that of similar deposits at the present day, -we have to deal with the problems suggested by the coal-seams. We know -nothing of the climate of the Carboniferous period save what may be -conjectured from the analogy of existing climates; and in a question -regarding the accumulation of decaying vegetable matter climate is a -subject of the first importance. We are ignorant, too, of the rate of -growth peculiar to the carboniferous flora; and even if we hold that -it was probably rapid, the process of decay may have been equally -speedy, and so a forest might go on shooting up fresh trees as the old -ones rotted away, yet at the end of a thousand years there might be a -scarcely greater thickness of vegetable matter on the ground than at -the commencement. A seam of coal two feet thick might thus represent, -say the accumulation of a hundred years, and another of exactly the -same thickness might stand as the accumulation of a thousand years. -Until we know more of the vegetation and climate of the coal period, -the thickness of a coal-seam can hardly be held as a certain guide to -the lapse of time required for its formation. - -For the sake of illustration, let me take the following fragment of a -coal-measure section:-- - - Shale, 20 feet. - Coal, 4 " - Fire-clay, 6 " - Sandstone, 40 " - -Beginning at the bottom, we may compute the period of the forty feet -of sandstone variously, according to the river selected as the type -of a transporting agent. Tried by the standard of the Nile, all other -conditions being similar, such a deposit would require perhaps not -less than 14,000 years; by that of the Mississippi, 5000; and by that -of the Ganges, nearly 2000.[72] We come, then, to the superincumbent -fire-clay and coal, representing an ancient soil and the forest that -grew on it. The occurrence of these seams shows us that the river-bed -had become a swampy tract clothed with vegetation; but who shall -say how long it may have continued so? Like the sunk country of the -Mississippi, it may have been submerged, and to some extent cut off -from the sediment-transporting channels of the river, and thus, as -a vast lake, have nourished a prolific growth of marshy and aquatic -plants. If the temperature resembled that of our own country, the -growth of peaty matter, other circumstances being favourable, might -be comparatively rapid. If, however, as seems probable, the climate -were more warm and humid, giving rise to a more luxuriant vegetation, -and at the same time to a more rapid decay, a long interval might have -elapsed without adding materially to the thickness of the vegetable -accumulations, and the eventual entombment of peaty matter sufficient -to consolidate into four feet of coal, might be owing in some measure -to the submergence of the swamp beneath the waters of the river, -whereby a quantity of detrital matter was deposited that arrested the -process of putrefaction, and entombed the thickly matted plants which -were growing on the spot at the time. Hence, until we know more of -the conditions under which vegetation may accumulate at river-mouths -in such a climate as the coal plants are conjectured to have enjoyed, -calculations of the amount of time required for the formation of a -great series of coal-bearing strata must be regarded as premature. -In the present instance, we can but affirm that the growth of the -four-foot coal-seam probably occupied many long years, even at the most -rapid rate of accumulation known to us. The forest-covered swamp on -which the plants grew was eventually invaded by muddy detritus brought -down by the river; and during another period of indefinite extent--five -hundred years or five thousand years--fine mud continued to settle down -over the foundered forest, hardening eventually into twenty feet of -shale. - -[Footnote 72: Some observers have pointed to the occurrence of vertical -and inclined trunks of trees in the Carboniferous sandstones, and -deduced therefrom what has seemed to them a triumphant argument in -favour of the rapidity wherewith our coal-fields must have formed. A -foundered tree, they say, sank with its heavy-laden roots among the -sand at the bottom, its stem pointing up into the water like the snags -of the Mississippi, so that the sand must have come rapidly down to -entomb the whole before it had time to decay, and thus thirty or forty -feet of sediment must have been deposited in a few years, perhaps even -months. But this is somewhat like a begging of the question. We have -yet to learn how long a water-logged trunk will resist decomposition.] - -The Edge coals of the Mid-Lothian coal-field are succeeded by a group -of sandstones and thin shales, with three or more seams of limestone. -This group of strata, which we may call the Roslyn Sandstone Series, -reaches a thickness of from 1200 to 1500 feet, and serves as a middle -zone to divide the Edge coals below from the Flat coals above. It -contains only a few thin laminations of coal, and these chiefly at its -upper and under portions. Such a great intercalation of beds, without -coal-seams, points, we might readily conjecture, to some change in the -physical conditions of the ancient delta. The nature of this change can -be easily made out from an examination of the rocks, and the reader -will see that here again we are indebted to fossil remains for the most -conclusive and satisfactory evidence of these old physical revolutions. - -The absence of coal-seams suffices to indicate that during the -formation of the middle group that part of the delta occupying the -site of Mid-Lothian was continually submerged, and never rose to the -surface so as to allow a covering of vegetation to form upon it.[73] -The large beds of sandstone prove a continued transport and deposition -of detritus during undisturbed periods of considerable length. The -intercalations of shale, pointing to local changes in the currents or -other modifying causes, are usually of small thickness and extent, -while the sandstone beds sometimes attain a depth of 150 or 200 feet, -and extend over wide areas of country. So far these mechanical rocks -indicate the deposition of sand and mud under water, but whether at -river-mouth or sea-bottom is left uncertain. From the fossil remains, -however, we learn that the deposition took place in the sea, but at -no great distance from land; in other words, the area of Mid-Lothian, -which, during the accumulation of the edge coals, had been alternately -clothed with vegetation and inundated by the river, sank down many -fathoms, so that the sea rolled over it and all its submerged forests. -The proof is two-fold, first, from the character of the organic remains -in the limestones; and second, from that of those in the sandstones and -shales. - -[Footnote 73: Of course, this deduction is founded, as the reader -will notice, on the assumption that we have now the series, as it was -deposited, and that no peaty swamp or forest was denuded away, and its -site occupied by sand and silt. But the assumption is rendered probable -from the conditions of formation indicated by the Roslyn group.] - -In some of the streamlets that flow into the beautifully wooded vale -of the Esk, south of Penicuik, these limestones can be well seen, -worn in the water-channel, or crusted over with moss along the banks. -Their organisms are singularly abundant, and consist of cyathophylla, -encrinites, spirifers, producti, &c., all exclusively marine. In a -picturesque brook that falls into the Esk near a saw-mill in the -grounds of Penicuik House, I have seen the little cup-corals clustered -by dozens on the weathered rock, showing their delicate striated -wrinkles in high relief among the scattered valves of productus and -innumerable joints of the stone-lily. They were all well preserved, and -in their grouping and general appearance differed in no respect from -similar organisms in the mountain limestone of Roman Camp Hill. The -inference to be drawn from them must accordingly correspond with what -has been deduced from the mountain limestone fossils, viz., that they -mark the site of a sea-bottom which remained free from mud and sand for -considerable periods, during each of which there abounded corals and -shells, whose exuviæ went to form several seams of limestone. But that -this sea-bottom was at no period very far distant from land, is proved -by the drifted plants that occur in the sandstones and shales both -below and above, and which often show so little trace of maceration, -that we can hardly believe they were carried far, or floated for a long -while previous to being enveloped in the sand or mud at the bottom. I -have never detected vegetable remains in the limestones themselves, but -there seems no reason why they should not be found there. - -One of the most remarkable and difficult phenomena presented by these -limestones is their great persistency. I have traced them over a -large part of Mid-Lothian, from the highly inclined beds at Joppa to -the contorted and faulted strata near Carlops. I have found them, -too, in many parts of West-Lothian and Stirlingshire, from the sea at -Borrowstounness southwards into Lanarkshire. They likewise occur in -Fife, and seem to sweep away through Lanark and Ayrshire. The area -in which I have found them cannot be much under 700 square miles, -yet they are probably spread over a much greater extent of country. -Throughout this region they appear to continue on the whole at pretty -much the same vertical distance from each other, and average three or -four feet thick each. They vary in number, three being found in parts -of Mid-Lothian, in other parts only two. Throughout West-Lothian there -seem to be but two seams in the middle or moor-rock series, and the -same two seams are found passing over into Perth near Culross. There -are differences, too, in the structure and composition of the seams, -one running sometimes as a single bed of dull blue limestone, and -then gradually splitting up into three layers of a greyer and more -earthy texture, with soft shale between them. But making all these -abatements, the observer cannot fail to be struck with the general -regularity and continuity of these limestones. And the fact becomes -all the more remarkable when we consider the great irregularity, and -continual intercalations, and repetitions of the strata, both above and -below. Marine beds are usually persistent over large areas, especially -where extensively developed. As they decrease in thickness, their -continuity for the most part lessens, so that the rule is, on the -whole, a safe one, the thinner any particular stratum, the less likely -are we to trace it to a considerable distance. Yet, not only are these -Mid-Lothian limestones thin, but they occur in regular sequence among -a set of continually alternating and very irregular beds, and extend -over several hundred square miles of country. And this, too, not in a -single seam, but in two, three, or even more, so that the difficulty of -accounting for such intercalations is proportionately increased. - -We have seen above that the area of a delta is often partially -submerged below the sea, and that such changes may become of the most -marked kind where the country is liable to be depressed by earthquakes. -There can accordingly be no difficulty in understanding how the ancient -carboniferous delta of Mid-Lothian may have likewise subsided. But the -limestones are unmistakable evidence that not only was the area of -the delta submerged, but that for a while no sediment was deposited -over it, and hence marine animals peculiar to clear water flourished -so long and so abundantly as to form by their remains several beds of -limestone. Had these beds been merely local we might have regarded them -as having been deposited in lagoon-like portions of the delta, shut out -from the detrital matter of the river on the one side and open to the -sea on the other. But their wide extent and nearly uniform thickness -preclude such a supposition. The following explanation appears to me -the most probable:-- - -After the series of the Edge coals had been brought to a close, the -coal-fields of Scotland underwent a complete submergence below the sea. -This depression was probably very gradual, yet more rapid than that -long-continued one which had been going on during the earlier part of -the Carboniferous series, and the consequence of this greater rapidity -was to prevent the growth of stigmaria swamps or reedy jungles, by -keeping the alluvial surface continually sunk to some depth below the -water. The amount of subsidence until the deposition of the lowest -limestone may not have been great, but even a slight depression would -tell vastly on an area of flat delta land. Mud banks would be brought -down into the region of waves and surface-currents, and speedily be -spread out over the floor of the sea. Forest-covered islands would in -like manner be levelled down, and their trees sent drifting seaward -or submerged amid the re-formed silt. Thus altered, the delta would -sink below the sea, and the sediment borne down by the river would -be scattered out over the older deposits as a slowly-forming sheet. -By degrees this detrital matter must have been carried less and less -farther out to sea; in other words, the area of deposit or delta must -have crept gradually nearer to the land--a result owing partly to the -recession of the ancient coast-line, and partly perhaps to a greater -amount of depression inland than at the coast, which would of course -lessen the velocity of the streams and cause them to deposit their -burden of sediment at higher levels than before. The consequence -of this retreat of the delta from the sea would be to purify the -water over the site of the old swamps, and render it fitted for the -habitation of corals, molluscs, and other marine animals. A medium thus -prepared would not be allowed to remain long untenanted, and so we find -that it came to be densely peopled with the organisms peculiar to such -a station. Stone-lilies, cup-corals, net-like bryozoa, molluscs of many -kinds, and large predatory fish, swarmed in these old waters, and their -calcareous shells and skeletons are now broken up by the quarryman and -the collier as hard compact limestone. - -After these animals had lived and died in successive generations, -perhaps for thousands of years, the downward movement of the earth's -crust seems to have ceased for a while or to have become greatly less. -The effect of this would be just to reverse what had been previously -done, especially if a slight elevatory movement took place. The streams -would in such circumstances descend from the uplifted ground with -renewed velocity and transport their detritus to gradually increasing -distances. The muddy and sandy sediment thus borne seawards would -slowly silt over the coral-banks at the bottom, and in conditions so -ungenial the organisms would dwindle down and finally die out. A great -thickness of sand and mud would be spread out over their remains so -long as the currents from the land continued to carry sediment out to -sea, and thus probably originated the sandstones and shales superposed -above the lowest limestone. - -Eventually the old steady downward movement returned, and with it the -corals and stone-lilies. The detritus again sank to the bottom much -nearer the land, forming great banks and shoals that choked up the -river-mouth. Seaward the water regained its purity, and the bottom -once more swarmed with living things. Another lapse of many thousand -years may have here intervened during which the marine exuviæ gathered -into another seam of limestone, until again the process of subsidence -either ceased for a time, or what is perhaps more probable, became -considerably feebler. Detrital matter began to creep seaward as -before, and eventually entombed the corallines and crinoids to a great -depth. The calcareous bed thus formed is the second limestone, and the -superincumbent silt-beds represent the sandstones and shales that rest -above it. - -In some such way as this does the Roslyn sandstone series appear to -have originated. I have indicated what seems to have been the main -features in the process, but it was probably a very complex one. There -may have been a great many oscillations of level of variable effects, -some of them raising the disturbed area to a much greater height at -one point than at another. This inequality would of course produce -marked effects along a low flat country such as that at the mouth of a -great river. New currents would be produced and the direction of old -ones changed; great shoals and banks of silt would be worn down only -to be thrown up again at some new point, where another oscillatory -movement would expose them afresh to destructive denudation. The -variations in the amount of elevation and depression would likewise -modify the transport of detritus to the sea, and give rise to a varied -and ever-changing sea-bottom. In short, the alternations and variations -must have been endless, for to the ordinary multiplied interchanges of -a delta we must add those induced by a constant and unequal oscillation -of the earth's crust. - -The Roslyn sandstone series comes to a close, and passing onward in -ascending scale we meet with another great group of coal-bearing -strata. They occupy the central area of the Mid-Lothian coal-field, -and from their gentle inclination as compared with the lower strata -that rise up from under them on either side of the basin, are known as -the _Flat Coals_. Their total thickness--that is to say, all that has -escaped denudation--amounts to a thousand feet or more. They consist -chiefly of sandstones, shales, ironstones, and fire-clays, with from -twenty to twenty-five seams of coal, of which sixteen are thick enough -to be worked. Their similarity to the Edge coals below points to a -similarity in the conditions of formation. The frequent alternations -of sandstone and shale show how the delta gradually pushed outwards -again and re-occupied its ancient site above the successive forests of -the Edge series and the successive coral-beds of the Roslyn group. The -coal-seams indicate the further progress of the detrital accumulations, -and the eventual formation of vast swampy flats that nourished a rank -growth of stigmariæ, and tracts of drier ground waving with ferns, and -shadowed by the spiky foliage of the club-moss and the broader fronds -of the tree-fern. - -The Flat coals are not succeeded by any other palæozoic strata. Above -them stretches the drift already described: sometimes in the form -of a stiff blue clay resting on a striated rock-surface; sometimes -as a coarse gravel containing fragments of all the rocks in the -neighbourhood; and sometimes as a fine white sand diagonally laminated, -and often showing dark partings of coal-fragments. From the section -given above (Fig. 32) at p. 196, the reader, will see that as the -upper limit of the Flat coals is formed by the drift, a large part of -that series may have been borne away by denuding agencies. Had there -been even a seam of limestone above them, it would have sufficed to -show their true thickness, for we should then have seen, that how -much soever had been removed in later times from above the limestone, -nothing had been removed from below it; and so it would mark the true -original limit of the series. We cannot now tell how much thicker the -upper part of the Mid-Lothian carboniferous system may have been. -Probably, during the long ages that intervened between palæozoic and -post-tertiary times, many hundred feet were borne away and carried to -other sites, there to grow up into new islands and continents, clothed -with other types of verdure, and peopled by other races of animals, and -fitted to become, in a long subsequent period, the dwelling-place of -man. - -In fine, the evidence of these ancient changes in the history of the -Mid-Lothian coal-field is derived, as we have seen, from two sets of -facts; first, those of a mechanical, and, second, those of an organic -kind--the one class explaining and confirming the other. Beginning our -investigation at the horizon of the Burdiehouse limestone, we saw the -curtain rise slowly from off a wide estuary, in which there gambolled -large bone-covered fishes, while huge pine-trees--branchless and bare, -seed-cones, fern-fronds, and twigs of club-moss, floated slowly away -out to sea. The panorama moved on, and brought before us the ocean-bed -of the Roman Camp limestone, with its groves of stone-lilies and -bunches of coral; its tiny shells moored to the bottom, or creeping -slowly athwart the limy floor, or spreading out their many arms, and -rising or sinking at will. This picture passed slowly away, and then -came the delta of the Edge coals, with its sand-banks and ever-shifting -currents, its stigmaria swamps, and its forest-covered islets. We saw -the delta gradually sink beneath the sea, and corals and stone-lilies -cluster thick over its submerged area, to form the limestones of the -Roslyn group. Again, the mud-bars of the river crept out to sea, and -tangled forests waved green as of old, washed by the sea or inundated -by the river. How this last period came to a close, we shall probably -never know, and have no possible means of conjecturing. We pass at one -step from the ancient era of the coal to the comparatively modern one -of the drift--from a verdant palæozoic land, to an icy post-tertiary -sea. It is like a leap in history from the days of Pericles and Aspasia -to those of King Otho, or from the tents of Runnymede to the Crystal -Palace of Sydenham. - - - - -CHAPTER XII. - - - Trap-pebbles of the boulder--Thickness of the earth's crust - unknown--Not of much consequence to the practical - geologist--Interior of the earth in a highly heated - condition--Proofs of this--Granite and hypogene - rocks--Trap-rocks; their identity with lavas and - ashes--Scenery of a trappean country--Subdivisions of - the trap-rocks--Intrusive traps--Trap-dykes-intrusive - sheets--Salisbury Crags--Traps of the neighbourhood of - Edinburgh--Amorphous masses--Contemporaneous trap-rocks of - two kinds--Contemporaneous melted rocks--Tests for their age - and origin--Examples from neighbourhood of Edinburgh--Tufas - or volcanic ashes--Their structure and origin--Example - of contemporaneous trap-rocks--Mode of interpreting - them--Volcanoes of Carboniferous times--Conclusion. - -In the previous pages, allusion has been made to the trap-pebbles -imbedded in the boulder, to the various forms of decay exhibited by -granitic and trappean rocks, and to the elevation and depression of the -solid crust of the earth. Will the reader bear with me for but a few -pages more, while I seek to indicate one or two points of interest in -a branch of geology that would abundantly reward a diligent observer? -Since the days of Hutton, the investigation of what are called -_igneous_ rocks has fallen somewhat into the background, and geologists -have given themselves, perhaps too exclusively, to the study of organic -remains, so that while the palæontology of the British islands has -enjoyed an extensive exploration, but little has been done towards the -elucidation of our igneous formations and their accompanying phenomena. -Much remains to be accomplished, even in those districts usually -regarded as in a manner thread-bare, and he must be but an indifferent -observer who cannot add a few gleanings to the general stock of -information upon this branch of British geology. - -Many conjectures have been formed, and many theories propounded, as -to the nature of the internal parts of our globe. Some have supposed -that there is an outer solid film or crust, some ten or twenty miles -thick, enveloping a vast ball of intensely heated matter; others have -attempted to show that the interior must be nearly solid throughout, -with, however, great lakes, or vesicles of gas and melted rock, -somewhat after the fashion, we may suppose, of the oil-holes in a -Gruyère cheese. But whether the heated material occupy the whole or -only parts of the internal area, is not of much consequence to the -practical geologist; he is content to believe that it exists, and in -sufficient quantity, too, to produce the most momentous changes on -the surface of the earth. We see the effects of this subterraneous -agent in earthquakes and volcanoes, and the geologist can tell us -of similar, as well as of other changes, effected by it during past -ages. Granite hills, and mountainous districts of mica-slate and -gneiss, bear evidence of what is termed _metamorphism_--a change in -the mineral structure of rocks, believed to have taken place through -the agency of heat deep in the interior of the earth; for no analogous -appearances have been detected in progress at the surface. Such rocks, -known as _metamorphic_, or _hypogene_, still form a difficult problem, -not likely to be satisfactorily solved until the chemist shall have -thoroughly investigated the subject; for it seems likely to be found, -after all, that long-continued chemical action, without a very alarming -degree of heat, may have produced even the most intense metamorphism. -But dropping this part of the subject, in which so much yet remains -to be discovered, let us look for a little at another branch of the -geologist's evidence, where we meet with no such hampering hypotheses -and doubtful conjectures, namely, the _trap_-rocks. - -Every one knows that basalt, lava, pumice, scoriæ, and ashes, are -the various matters ejected from volcanoes. When these materials are -found interstratified among the various geological formations, they -are termed _trap_-rocks,--a name derived from the Swedish _trappa_, a -stair, in allusion to the step-like or terraced appearance which they -often present. They are of all ages, having been detected in the lower -Silurians of Wales, and in the deposits of all subsequent periods up -to the volcanic eruptions of the present day; thus evidencing, that -from the remotest times there have been Ætnas and Vesuvii slumbering -perhaps for ages, and then awakening to lay the surrounding districts -in ruins. I have already said that the rocks from which the geologist -has to compile his history, are mostly relics of the sea; hence most of -the trap-rocks which he meets with in his explorations are the products -of submarine eruptions. Far away down among the Silurian rocks, he can -trace the floor of a primeval ocean thickly covered with stone-lilies, -trilobites, and molluscs, and in following it out he marks how ashes -and lapilli, ejected from some submarine orifice, settled down amid the -organisms and well-nigh destroyed them, while at other times streams of -molten matter were poured out along the sea-bottom, and hardened into -masses of solid rock. He sometimes even encounters what seems the vent -whence these eruptions proceeded, filled up now by a boss or plug of -hardened trap, but he never can detect any trace of land. Some of these -oceanic volcanoes may, like Graham's Island in the Mediterranean, have -raised their tops above water, sending clouds of steam and cinders far -and wide through the air, but the waves would eventually wear down the -new-born land, and scatter its broken fragments along the floor of the -sea. Among the carboniferous rocks of Scotland, however, we meet with a -different condition of things. There, too, we can trace out submarine -lava-streams, and mark how showers of ashes destroyed the delicate -organisms of the deep; but we encounter, besides, undoubted traces -of a land, not parched and ruinous as though the igneous forces had -laid it waste for ever, but thickly clothed with vegetation of a more -luxuriant type than that which clusters over Vesuvius and Calabria, or -lies spread out across the "level plains of fruit-teeming Sicily."[74] -We have looked at the plants and animals of the Carboniferous era; its -rivers and deltas; its slow elevations and depressions of the ground. -It may, perhaps, complete the picture of that ancient period, if we -examine, though but briefly, its igneous eruptions, the more especially -since these may be regarded as, to a considerable extent, typical of -trap-rocks belonging to every age and every country. - -[Footnote 74: Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch. _Prom. -Vinct._ 369--a passage graphically descriptive of an ancient eruption -of Ætna.] - -Unless when deeply buried beneath drift-sand and clay, trappean regions -usually possess scenery of a marked kind. A green undulating country -stretches out as far as the eye can reach, diversified here and there -with bold abrupt crags and conical hills. The lower grounds show in the -winter season their rich brown loam, that waxes green as the spring -comes on, and ere summer's close spreads out its heavy crops of golden -grain. The higher ridges are for the most part thickly wooded, yet -the soil is often scanty, and, among the white stems of the beech, or -the matted roots of the fir and the elm, we may not unfrequently see -the rock protruding its lichen-crusted face, mottled with mosses and -liverworts, while some sluggish runnel collects in stagnant pools, -or trickles over the blocks with a thick green scum. Sometimes the -hill has never been planted, but stands up now, as it has done for -centuries; its western face craggy and precipitous, with bushes of -sloe-thorn and furze, and stray saplings of mountain-ash clinging to -the crevices, while its eastern slope sinks down into the rolling -country around with a green lumpy surface, through which, at many a -point, the grey time-stained rock may be seen. The whole district -suggests to the fancy a billowy sea, and, as one casts his eye from -some commanding hill-top athwart the wide expanse of hill and valley, -sweeping away in endless undulations, he is apt to bethink him of -some day far back in the past, when the verdant landscape around lay -barren and desolate, while the solid earth rocked and heaved in vast -ground-swells like a wide tempested ocean. Such is the aspect presented -by some of the more trappean regions of Scotland. But the origin of -this kind of scenery must be ascribed to the effects of denuding -currents in scooping out the softer strata into clefts and valleys, and -leaving the harder trap-rocks in prominent relief, rather than to any -great inequality of surface produced by the eruption of igneous matter; -for we shall find that the throwing out of sheets of lava and showers -of volcanic ashes was often a very quiet process after all. - -Trap-rocks generally may be variously classified according to the -aspect under which we view them. Mineralogically they are _augitic_, -when the mineral _augite_ enters largely into their composition; -_hornblendic_, when the _augite_ is replaced by _hornblende_; and -_felspathic_, where _felspar_ forms the most marked constituent. -The first class includes all the dark homogeneous compounds called -_basalts_; the second, the hornblendic _greenstones_, or _diorites_; -and the third, the _felstones_, _porphyries_, and _tufas_. -Geologically, they are _beds_ when they are interstratified with the -contiguous rocks; and _dykes_ or _veins_ when they penetrate them like -walls, or in an irregular manner. The former class may be either of -the same age with the rocks among which they lie, or of a later date, -just as in a pile of books the centre one may either have been placed -there originally with the rest, or thrust in long afterwards. The -latter class must always be later than the rocks which they traverse, -for it is plain the rocks must have been in existence before trap-dykes -and veins could be shot through them. Hence geologists are accustomed -to speak of contemporaneous and subsequent trap-rocks: the one list -including all the tufas, and those melted rocks which can be shown to -have been erupted during the time when the limestones, sandstones, or -shales around them were forming; the other embracing all the dykes and -veins along with those beds of melted rock which have been intruded -between the strata. These and other distinctions will be better -understood from a few examples collected chiefly from the carboniferous -district of central Scotland. - -The trap-rocks seen there exhibit a wide range of structure, texture, -colour, and general aspect. There are two pretty marked kinds--the -augitic or hornblendic, and the felspathic; the former being usually -of a more or less crystalline aspect; the latter, commonly dull, and -often without any crystals.[75] In the augitic traps, the crystals -are sometimes of large size and well-defined, so that the rock could -hardly be distinguished at first sight from an ordinary grey granite, -while at other times, and not unfrequently even in other portions of -the same mass, the stone assumes a black appearance without distinct -crystals. The former variety would be called a _greenstone_, the -latter a _basalt_; the chief components in either case being felspar -and hornblende, or felspar and augite, with a variable admixture -of other minerals, the shade of colour varying from a pale blue or -leek-green, through the different hues of grey, to a deep velvet black. -There are other traps, however, consisting entirely, or nearly so, -of felspar, whence they are known as _felstones_. Such rocks enjoy -a wide range of colour, some of them being pure white, others of a -bluish grey or dingy brown; and they may be seen graduating from a -pale yellow, or flesh-colour, to a brick-red or deep purple. When a -trap displays distinct disseminated crystals, usually of felspar, it -becomes a _porphyry_; when it shows rounded cavities, like those of -furnace-slag, it is said to be _vesicular_; and when these globular or -almond-shaped cavities are filled with carbonate of lime, chalcedony, -or other minerals, the rock forms an _amygdaloid_. Such peculiarities -of structure indicate to some extent the origin of the mass, and may -be found in any kind of trap. Thus we have porphyritic greenstones, -basalts, or felstones, and the same rocks may be likewise vesicular -or amygdaloidal. Some of them, such as many greenstones, display on -weathered surfaces that curious spheroidal structure already alluded -to; others are built up into geometric columns. - -[Footnote 75: This distinction, though a sufficiently safe one in -some localities, must not be held as by any means universal in its -application, the felspathic traps being often as crystalline in aspect -as the augitic, and the augitic, on the other hand, as dull as the -felspathic.] - -Such peculiarities of composition and structure form the basis of -a mineralogical classification of the igneous rocks, which is of -use in working out the geology of a district. The most convenient -subdivision for our present purpose, however, is that which proceeds -upon the origin and mode of occurrence of the trap-rocks. Viewed -thus, they resolve themselves into two great groups, the _intrusive_ -and _contemporaneous_, both of which contain greenstones, basalts, -&c.,--the sole distinction between those of the one class and those of -the other, being the relation of age and mode of occurrence which they -bear to the surrounding rocks. - -I. The _intrusive_ traps occur in the form of walls and veins, -sometimes in that of flat parallel beds, and often as huge amorphous -masses, to which no definite name can be given. But whatever shape they -may assume, they generally agree in presenting well-marked features, -whereby their origin can be readily ascertained. The rocks through -which they pass are more or less hardened, often contorted, and -sometimes traversed by innumerable cracks and rents, into some of which -the trap has penetrated in the form of veins. - -A trap-dyke is a long wall of igneous matter, cutting more or less -perpendicularly through the surrounding rocks. Sometimes these dykes -attain a breadth of many yards, and may not unfrequently be traced for -miles running in a nearly straight line over hill and valley, easily -recognisable by a long smooth ridge, with the rock protruding here and -there from below where the soil is thin. It is interesting to follow -out one of these long ramparts from its beginning to its close, and -mark how undeviatingly it cuts through the rocks. No matter what may -be the nature of the stone encountered, hard conglomerate, friable -shale, compact limestone, or jointed fissile sandstone, all are broken -across, and the right line preserved throughout. Nay, I have seen a -still more curious instance of this persistency, where the dyke ran for -four miles through a set of mountain limestone and lower coal-measure -strata, and several enormous sheets of greenstone and basalt. Even -when passing through these traps the dyke remained perfectly distinct, -its crystalline structure and external configuration presenting a -well-marked contrast with those of the surrounding eminences. Of -course it must have been formed after all the rocks through which -it passed. The sandstones and shales must have settled down long -previously on some estuary bed or sea-bottom; the corals and shells -of the limestones, and the matted plants of the successive coal-seams -must have lived and died, perhaps thousands or millions of years -before, and their remains have hardened into stone, ere the continuity -of the strata was broken across by the long deep wall of greenstone. -Trap-dykes are accordingly appropriately termed _intrusive_. They -have been intruded among and must always be later than the rocks in -which they occur. In tracing out their character, more especially in a -trappean district, such as that of Linlithgowshire, where they abound, -we soon find other evidence of their intrusive nature. Where they pass -through limestone, they sometimes convert it into a white saccharine -marble; shales they bake into a sort of porcelain or burnt pottery; and -sandstones become semi-fused into a hard homogeneous quartz-rock. Nor -are the changes confined to the rocks traversed; the dykes themselves, -along their sides, become fine grained and hardened; occasionally, too, -the colour alters from the usual bluish or greenish-grey to black, or -to a brick-red, or dull-brown, similar to that of the altered shale -and sandstone, of which detached portions may be found adhering to -the outer walls of the dyke, or even embedded in its substance. -The central portion of the dyke may thus be markedly crystalline, -forming what we should call a greenstone, while the outside parts, -where the trap comes in contact with the adjacent rocks, are fine -grained and homogeneous, so as to become a true basalt. Sometimes, -too, these exterior edges are highly vesicular and amygdaloidal, -detached fragments closely resembling the slag of an iron-furnace, and -occasionally the dyke presents a columnar arrangement, the ends of -the hexagonal or polygonal columns abutting against the sandstone or -other rock on either side, and losing themselves towards the centre in -the general mass of the trap. Where the strata traversed are broken -and jointed, the dykes which cut them through may be seen in some -places throwing out lateral veins that accommodate themselves to all -the irregularities of the fissures. These minor portions exhibit for -the most part the same leading features with the parent mass, and the -result of the whole is a general baking of the beds, with sometimes not -a little contortion, and an amount of irregularity and disturbance, -apparent at once to the most inexperienced observer. (See Fig. 34.) - -If the reader will verify these statements by actual exploration in -the field, he will probably not be long in arriving at the following -conclusions: trap-dykes must once have been in a melted state, as is -shown by their vesicular cavities and divergent veins; this liquid -condition must have been attended with the most intense heat, as may be -gathered from the burnt and baked appearance of the contiguous rocks; -they have, for the most part, especially where of large size, risen -from below along previously-formed dislocations--a circumstance which -may be inferred from their persistency in a straight line through beds -of very different resisting power, for had the liquid matter forced -a way for itself, it would have squirted between the beds along the -lines of least resistance, and not directly and for miles across them; -and hence, trap-dykes must be regarded not as themselves the agents -in dislocating and contorting a district, but merely as signs of the -parent force at work below. - -All the features of these trap-dykes here stated may be observed in the -central district of Scotland, among rocks of Carboniferous age. But he -who would study trap-dykes on the great scale without quitting Britain, -should visit some of the more trappean islands of the Hebrides. He -will there find them intersecting glen and hill-side, in an intricate -network, standing up through the heather like ruined walls, and running -often for considerable distances up bald cliff-line, and across -precipitous ravine. In some localities, among such limestone districts -as that of Strath, detached eminences may be seen with congregated -dykes coursing their sides and summits, while the heathy interspaces -are cumbered with grey and white protruding blocks of marble, that give -to these green knolls the aspect of old time-wasted abbeys with their -clustering tombstones. The magnificent sections laid open in these -localities by the action of mountain streams, and by the waves of the -Atlantic, leave the student of igneous rocks nothing to desire save a -long lease of leisure. - -Another form frequently assumed by the intrusive traps, is that of -wide beds or sheets intercalated with greater or less regularity among -stratified rocks (Fig. 34 _b_). They may be regarded as horizontal -dykes, the igneous matter, in place of cutting across the strata, -having forced a way for itself between them. Viewed in this light they -will be found exactly to correspond with ordinary dykes; the rocks on -which they rest, and those which lie above them being both altered like -those on either side of a dyke or vein. A well-known example of this -form of trap is that of Salisbury Crags, where a bed of greenstone -twenty to eighty feet thick is intercalated among sandstones, shales, -and coarse limestones, belonging to the Lower Carboniferous series. Its -under surface presents a remarkably even line, broken at intervals, -however, where the truncated ends of sandstone beds protrude up -into the greenstone, or where the latter cuts into the sandstone -below, occasionally enveloping detached fragments, and sending veins -through them. Along the line of contact both rocks undergo a change. -The greenstone becomes reddened, finer grained, and of a dull earthy -aspect. The sandstones and shales are also red, and excessively hard, -the former resembling a quartz rock, and the latter passing into a sort -of flinty chert or chalcedony. The sandstones above the trap, where -they can be examined, are also found to present the same hardened, -baked appearance, the most intense metamorphism being observable -in those parts which are completely surrounded by igneous matter. -These points were noted many years ago during the famous controversy -between the disciples of Hutton and Werner, the former viewing them -as demonstrative evidence of the igneous origin of the trap-rock, the -latter, on the other hand, professing to see nothing in the section -of the Crags at all militating against the theory that the rocks had -originated from deposition in water. Many a battle was fought in this -locality, and not a few of the trap-dykes and hills possess to the -geologist a classic interest, from having been the examples whence -some of the best established geological opinions were first deduced. -The contest between the Huttonians and Wernerians terminated long ago -in the acknowledged victory of the former; Hutton's doctrines are now -recognised all over the world. It is interesting, however, to walk -over the scenes of the warfare, and mark the very rocks among which it -raged, and from the peculiarities of which it took its rise. Basalts -and greenstones, sandstones and shales, with all their crumplings and -contortions, still stand up as memorials of powerful igneous action, -and of physical changes in the primeval past; and they have become to -the geologist memorials, too, of changes in the onward progress of his -science, where, out of conflicts perhaps yet more tumultuous than those -of ancient Nature, there emerged at last the clear demonstrable truth. - -[Illustration: Fig. 34.--Intrusive Trap.] - -In the accompanying section (Fig. 34), the more marked characters of -intrusive traps are exhibited. The main mass of igneous rock is the -dyke (_d_), rising through a dislocation or fault, which has thrown -down the beds on one side several feet below those on the other, as -is shown by the interruption of the shale and ironstone beds (_sh_). -The dyke gives off two ramifications, one of them cutting across the -beds obliquely as a vein (_v_); the other passing along the planes -of the shaly layers as a horizontal bed (_b_). The vein, it will be -noticed, produces considerable alteration in its progress, carrying up -and baking a portion of the shale (_sh_), and turning up the edges of -the beds on both sides, which get cracked and hardened along the line -of contact. The bed runs with some regularity for a short distance -through the shales, which show marks of great alteration at their -junction with the trap. Its under surface at one point is seen to have -involved a portion of the shale which has become in consequence highly -metamorphosed, while along the upper surface the bed has sent out a -short irregular vein that twists and otherwise alters the shales -above. These circumstances would suffice to show that even though we -did not find this bed in connexion with a mass of intrusive trap, it -must, nevertheless, have been thrust among previously-formed strata, -and could not have been contemporaneous, that is, poured out along the -sea-bottom before the shales above it were deposited. - -But one other form needs to be mentioned here as characteristic of the -Carboniferous intrusive trap-rocks--that of great amorphous masses -which cut through the strata irregularly. They have not the wall-like -form of dykes, nor do they conform to the line of bedding of the rocks -among which they occur. They are sometimes irregular lumps, lying -above or among the strata, and probably connected with some vein or -dyke below. In other localities they look like the upper ends of vast -pillars which may descend into the very depths of Tartarus, as though a -great hole had been blown through the crust of the earth, and a column -of melted matter had risen to fill the cavity. Such masses are often -called _bosses_, and seem not unfrequently to have been the craters -of eruption whence great sheets of lava and showers of ashes were -ejected far and wide over the neighbourhood. They serve to connect -the intrusive traps, whose age is always more or less uncertain, with -the bedded traps properly so called, the geological date of which can -usually be sufficiently ascertained. - -II. The bedded or contemporaneous trap-rocks consist of two well-marked -kinds. There are, 1st, the melted rocks, such as greenstones and -basalts and 2d, the tufas and volcanic ashes. - -Those of the first-named class differ in no respect from the traps -already noticed, so far as regards mineralogical texture, general -structure, and appearance. In hand specimens the intrusive and bedded -greenstones and basalts cannot be distinguished, nor even when examined -in the field and in masses extending over considerable areas is it -always possible to say to which division any particular hill or crag -should be assigned. The reason of this resemblance is obvious. Where a -trap has either cut through or insinuated itself among rocks of earlier -date it is called intrusive, in relation to the rocks so traversed, -and of course we cannot be sure to what geological period it should be -referred, nor how long an interval may have elapsed between the time -when these rocks were forming and the time when the trap was intruded -among them. If, however, the igneous rock passed upward through these -same strata and then spread out as a flat sheet along the sea-bottom, -the part that came to the top would be termed contemporaneous -with the deposits going on at the time. Hence it follows that all -contemporaneous lava-form trap-rocks are at the same time intrusive as -regards the strata passed through in their progress to the surface. If -the sheet of melted matter that spread out below the water were in the -course of ages worn completely away, along with the strata subsequently -piled above it, so as to leave merely a neck or dyke filling up the -cavity through which the lava rose, we should pronounce the remnant -intrusive, and could form no certain conclusion as to its age or -as to whether its site had ever been a crater actively at work in -throwing out lava and ashes. The sole difference, therefore, between a -contemporaneous and an intrusive greenstone is simply this: the former -rose through a fissure until it reached the surface, and then rolled -out as a flat parallel sheet; the latter may have been erupted from -below at the same time, yet, owing to different circumstances, never -reached the surface, but spread out among or cut through the strata -underneath. And so, when we come to examine in quarries, ravines, and -other exposures, the remains of two such eruptions, we soon ascertain -the relative age of the former from that of the strata among which it -occurs, but as to the date of the latter we are wholly at a loss, for -it gives us no clue by which we can show whether it was erupted before -or after the other. We can but compare the mineralogical character -of the intruded with that of the contemporaneous masses in the same -district, and, from the resemblance which may be traced between them, -draw at the best but a doubtful inference as to their relative dates. - -The contemporaneous traps always assume a bedded form, the intrusive -occasionally do so; and the question naturally arises here, what are -the tests whereby a bed of trap may be known to be contemporaneous and -not intrusive? The answer is happily a simple one. An intrusive mass -is found to alter to a greater or less extent the rocks in contact -with it; if it occur as a dyke, then the beds on either side have -been cut through and probably otherwise affected; if it take the form -of a bed or sheet, the strata lying above and below it will be found -to be both altered, showing evidently that a heated mass has been -interposed between them, and consequently that the igneous rock is of -later date than any of the strata among which it occurs. In the case -of a contemporaneous melted trap, however, the appearances presented -are different; it always takes the form of a flat bed corresponding -to all the inclinations and curvatures of the sandstones, shales, -limestones, or other strata among which it lies. If examined carefully, -it may be found not unfrequently baking and contorting the bed that -forms its pavement, but producing no change whatever on that which -composes its roof. It may be capped and underlaid by layers of shale, -and in such a case we might not improbably find the shale below it -highly baked, so as to resemble a sort of rude pottery, while the shale -above would present no sign of such metamorphism, but on the contrary -might display its delicate plants or shells down to the very surface -of the trap, and were the latter concealed from view we should never -suspect, from the aspect of this shale, that any igneous rock existed -in the neighbourhood. The inference to be drawn from such appearances -seems very obvious. Had the upper shale been in existence when the -greenstone or basalt was erupted, it would have suffered an alteration -similar to that produced on the shale below; and the fact, plain and -palpable, that it has undergone no such change, shows pretty clearly -that it was deposited at the bottom of the water after the trap had -cooled and consolidated, and that consequently the trap must be -intermediate in age between the beds on which it rests and those which -lie above it; in other words, that it is a _contemporaneous_ rock. -Hence, if we know the exact geological position and age of the shales, -we know also those of the associated trap, and can thus ascertain -that at a certain definite period in the past history of our planet a -particular district was the scene of volcanic action. - -Examples of such contemporaneous traps abound among the carboniferous -rocks of central Scotland, especially in Fife and the Lothians (Fig. -35). I may refer again to the vicinity of Edinburgh as affording some -excellent illustrations. The eastern part of Arthur's Seat displays -a series of basalts and greenstones which can be proved to have been -thrown out during the times of the Lower Carboniferous rocks, at a -period long anterior to that of the Burdiehouse limestone. The Pentland -Hills exhibit on a much greater scale vast sheets of felspathic traps, -such as felstones and tufas, traceable in some cases for six or seven -miles, which were erupted at a still earlier period.[76] The trap -pebbles in our boulder consisted of light yellow and pink felstone, -and were derived, I make no doubt, from these Pentland Hill beds, when -what forms now the cone of Carnethy, rising well-nigh 1900 feet above -the sea, existed as one of a scattered archipelago of islets, or as a -sunken rock battered by the waves that scattered its shingle along the -floor of what may have been either a shallow sea or a shoaling estuary, -where eventually the sand and pebbles hardened into that bed of coarse -grey sandstone whence our boulder was derived. - -[Footnote 76: The geology of Arthur's Seat and Pentland Hills was -admirably worked out more than quarter of a century ago by Mr. M'Laren. -His work (already referred to) is unfortunately now out of print.] - -The second class of contemporaneous trap-rocks are the tufas or -volcanic ashes. They differ entirely in their aspect and origin from -any of the rocks already described. Greenstones, basalts, felstones, -and such like, were all melted rocks, thrust up from below as we see -lava thrown out by a modern volcano, being styled contemporaneous when -poured out along the sea-bottom or the land, and intrusive when they -never reached the surface but cut through the strata below. The tufas, -however, point to a totally different origin. They are of various -shades of colour, according to their chemical composition. In East -Lothian they assume a deep red hue; among the Pentland Hills they -are often flesh-coloured, while in Linlithgowshire they range from a -dull-brown to a pale leek-green, green being the prevailing tint. They -always show a dull uncrystalline surface, irregularly roughened by -included fragments of various rocks, such as trap, sandstone, shale, -and many others. These fragments or _lapilli_ vary in size from less -than a pin-head up to large bombs of several hundredweight, and from -being generally abundant give to the tufas one of their best-marked -characteristics. The smaller pieces are usually more or less angular, -and throughout the carboniferous series of Linlithgowshire consist -chiefly of a pale felspathic matter, lighter in shade and commonly -harder in texture than the matrix or paste in which they lie. In -some localities, where the included pieces are larger, they have a -rounded form, and often show a honey-combed vesicular surface, like -balls of hardened slag. Fragments of sandstone have not unfrequently -a semi-fused appearance, and plates of shale sometimes look like the -broken debris from a tile-work, although in many instances these -fragments may be found showing no trace whatever of alteration, being -undistinguishable from the neighbouring sandstones and shales from -which they probably came. I have seen in some of the coarser tufas, or -rather volcanic conglomerates, enormous masses of basalt and greenstone -buried deep in the surrounding green or red felspathic paste, and -showing on their more prominent edges the usual vesicular cavities. -In such conglomerates there is usually no division into beds; the -whole mass, indeed, forms a bed between lower and higher strata, but -internally it shows for the most part no trace of stratification. In -these confused assemblages one may occasionally light upon detached -crystals of augite or other mineral scattered irregularly through -the tufa. Their angles will be found often blunted, and the crystals -themselves broken, appearances which have likewise been noticed among -the ash of modern volcanoes. When the tufas are finer grained they -usually exhibit a well-marked stratification, and can often be split -up into laminæ like an ordinary fissile sandstone. Organic remains not -unfrequently abound in such laminated beds, and vary in their character -as widely as in any other stratified rock, being sometimes land-plants, -sometimes sea-shells. - -Such are some of the more obvious characters of the volcanic ashes or -tufas, as developed among the carboniferous rocks of central Scotland. -Their great varieties of composition and general aspect render them -a somewhat difficult set of rocks to master, but when fairly and -fully understood they soon prove themselves to be by far the most -interesting section of the traps, for one needs seldom to hesitate a -moment as to their origin or date, while their fossil contents impart -to them an interest all their own. By comparing such rocks with the -consolidated ash or fine dust and _lapilli_ of a modern volcano, a -remarkable resemblance of external characters is found to subsist; and -this likeness holds sufficiently close, when pursued into details, to -show that the ancient and the modern rocks have resulted from the same -source, that, namely, of volcanic eruption. The ash of active burning -mountains falls down their sides loosely and incoherently, every -successive shower of dust or scoriæ settling without much regularity -on those that have gone before. The ash of the old carboniferous -eruptions, however, was showered for the most part over the sea or -across wide shoaling estuaries, at least it is only such portions of -it as fell there that have come down to our day. Settling down among -the mud and sand at the bottom, the volcanic matter accumulated in -wide horizontal beds, every marked inequality being smoothed down by -the currents until a series of regularly stratified layers came to be -formed, entombing any organisms that might find their way to the bottom -or be lying there at the time. The ash of terrestrial volcanoes has no -marked stratification because thrown out in open air, while that of -the carboniferous rocks of central Scotland is distinctly bedded from -having been deposited under water. - -Tufas and contemporaneous melted traps are very generally found -together interstratified regularly with each other, and the inference -to be drawn from their juxtaposition is of course simply this, that -at one time liquid lava rolled along the bottom of the water, while -at another showers of volcanic dust and cinders settled down in -successive beds. This active play of the igneous forces took place -at the mouths of estuaries or farther to sea; and it is accordingly -sometimes not a little interesting to trace, amid the sediment that -accumulated below the water during the pauses between the eruptions, -well-preserved remains now of plants that had come drifting from the -land, anon of slim spirifers, and producti that swarmed upon the -hardened lava-streams, and amid the thickening volcanic mud that -slowly sank to the sea-bottom. Such a sequence of events will be made -plain from the following section, the materials of which are derived -from different parts of the trappean region of Linlithgowshire. The -undermost bed here shown (1) is one of marine limestone, abounding -with encrinal joints, corals, spirifers, and other undoubtedly marine -organisms. Above it comes a layer of tufa or volcanic ash (2) of a -dull green aspect, the boundary line between the two rocks lying as -clear as if the quarryman had marked it off with his foot-rule. The -upper part of the ash, however, does not show an equally clear line -of demarcation with the stratum above. On the contrary, it gradually -changes its character, becomes more calcareous as it goes up, with -here and there a stone-lily joint or a stray productus, until these -organisms increase so much in number that the rock insensibly passes -into an ordinary limestone (3) like that below. Next succeeds a thin -seam of ash (4) resting sharply on the limestone and overlaid by a bed -of shale (5) containing the same marine organisms. Another stratum of -ash (6) resembling those below follows the shale, and is surmounted by -a close compact greenstone (7) that hardens the ash on which it rests, -but produces no apparent alteration on the soft fissile shale (8) -above it. Next is a fourth seam of volcanic ash (9) resembling those -below it, but without any shells or crinoidal joints, the only fossils -observable being a few carbonized stems apparently of calamites and -lepidodendra. Above it comes a bed of white quartzy sandstone (10) with -similar vegetable remains, and then a layer of white stiff fire-clay -(11) with rootlets of stigmaria, above which lies a seam of coal (12). -A thin layer of soft blue shale (13) here intervenes, somewhat baked -along its upper portions by another bed of compact vesicular greenstone -(14), which displays in places a well-marked columnar structure. It is -surmounted by a highly characteristic ash (15) in which there occur -numerous large bombs chiefly of trap of different kinds, some of them -highly vesicular. Fragments of shale also occur, mingled here and there -with black carbonized fragments of coal-measure plants, but without any -of the shells and other marine organisms so abundant below. The topmost -bed is a grey carbonaceous sandstone (16), underlying a thin covering -of vegetable mould. - -[Illustration: Fig. 35.--Contemporaneous Trap.] - -Such is the skeleton, as it were, of the section; the mere dry bones -which remain to the geologist, and which he must study closely to -be able to give them life again. The lowest bed visible, with its -stone-lilies and molluscs, we readily recognise as marking an old -ocean-bed, so that the little episode in the primeval records of -our planet here presented to us opens, like the two great epics of -antiquity, within sound of the wide-roaring sea. The seam of ash which -follows shows, from the sharpness of its line of demarcation with the -limestone, how the denizens of the sea-bottom were suddenly destroyed -by a thick shower of volcanic dust that settled down over their -remains. The waters, however, soon cleared, and ere long stone-lilies -and producti were plentiful as ever, mingling their remains among the -upper layers of the soft muddy ash, and giving rise therefrom to a -sort of calcareous ash or ashy limestone, until in the course of time -the volcanic matter became wholly covered over by a seam of ordinary -limestone. The corals and stone-lilies were, however, anew destroyed -by the deposition of volcanic dust that settled over them as a seam -of ash, after which the water was again rendered turbid and muddy by -the inroad of foreign matter, which, brought down by rivers or by the -changing currents of the ocean, sank to the bottom and eventually -consolidated into a seam of shale. Thereafter the volcanic forces -began once more to eject a quantity of dust and scoriæ that fell into -the water and spread along the bottom as a stratum of ash, and to -pour out a current of lava which hardened into a great sheet visible -now as the undermost greenstone of the section. The emission of the -lava seems to have terminated the eruption, for the next stratum is -one of shale like that below the ash, so that the muddy sediment, -the deposition of which was interrupted for a while by the volcanic -products, began afresh to settle down along the sea-bottom. This -last condition of things seems to have continued for a considerable -period, seeing that the shale bed is relatively thick, and from its -fissile laminated structure indicates a slow and tranquil deposition. -Another eruption of volcanic dust and ashes again interrupted the -detrital deposits, and gave rise to another seam of tufa. This last -subterranean movement seems to have considerably altered the general -contour of the sea-bottom, and so elevated it, at least at one part, -that a thick accumulation of sand, and subsequently of clay, filled -it up to the level of the water or nearly so, giving rise to a dense -growth of the stigmaria and other coal-measure plants whose roots are -still seen imbedded in the fire-clay on which, as a soft muddy soil, -they originally grew. It is probable, however, that, notwithstanding -such elevations of the sea-bed, there was a general subsidence of the -ground during the accumulation of these strata, for we see that the -peaty morass, represented now by the coal-seam, ere long sank beneath -the waters, with the inroads of which it was unable to keep pace, while -there slowly silted over it a muddy sediment that hardened at length -into what is now a seam of shale. But this order of things had been in -existence for but a comparatively short period when the igneous forces -broke out again, ejecting a stream, of molten lava that spread along -the bottom of the shallow waters and hardened as before into a sheet -of greenstone. This was followed by an abundant shower of dust and -lapilli, along with numerous large masses of greenstone and basalt. -These falling into the water accumulated on the upper surface of the -lava-stream, then somewhat cooled, and formed in the end a stratum of -ash of a rubbly conglomeritic aspect. That the sheet of greenstone -really spread out along the sea-bottom before the ejection of the ash, -and was not intruded among the beds at a later period,--that, in short, -it must be regarded as a contemporaneous and not as an intrusive rock, -seems sufficiently shown by its great regularity and evenness, and by -the unaltered condition of the fine soft felspathic matter which covers -its upper surface. It was assuredly in a highly-heated condition when -poured out, as may be gathered from the baked aspect of the mud over -which it rolled; but it had cooled and solidified, at least along its -upper surface, ere buried beneath the shower of ashes. The last bed -exhibited in the section is a grey sandstone, with many carbonaceous -streaks and traces of land-plants, showing a pause in the volcanic -activity of the district, during which the streams from the land -brought down sandy sediment, with an abundant admixture of macerated -leaves, branches, and other drift-wood. - -It thus appears that not only were the plains and hills of the -Carboniferous era richly clothed with vegetation, and its waters -crowded with animals, but that then, as now, subterranean forces were -at work, sometimes elevating, sometimes depressing the area alike of -the land and of the sea; while, not unfrequently, melted lava rose -from below, rolling along the bottom of the waters, and showers of -ashes were flung far and wide through the air, settling at last as a -thickening sediment along the floor of the sea, or athwart the marshy -swamps of the delta. Whether the interior of the land had burning cones -among its pine-covered hills we know not yet. Such, however, probably -existed; nay, there may have been among the higher peaks some "snowy -pillar of heaven," like the Ætna of Pindar, raising its smoking summit -among everlasting crags of ice in solitudes lifeless and bare.[77] - -[Footnote 77: The highest points of New Zealand, nearly 10,000 feet -above the sea, are said to be clothed for two-thirds of their height -with ice and snow. If, therefore, during Carboniferous times, there -existed somewhere to the west of what is now central Scotland, a chain -cf hills 5000 or 6000 feet high, their summits might perhaps have been -as wintry us that of Mont Blanc.] - -Our boulder has served us like the minstrels in modern Gothic -poetry, who appear between the cantos, and give an air of unity and -completeness to what would otherwise be often rambling and unconnected. -And now, at the close, it comes again before us, lying in its bed of -clay, clustered with mosses of brightest green, and overshadowed by its -flickering canopy of beechen leaves. Silent and senseless, the emblem, -seemingly, of calm repose and unchanging durability, what could we have -conceived it should have to chronicle, save the passing, perchance, of -many a wintry December and many a sultry June. Such, indeed, would be -the character of its records of the centuries that have passed away -since the birth of man, did any such record survive in its keeping. -But it rests there as the memorial of far earlier centuries, and of -an older creation; and though now surrounded with all that is lovely -or picturesque--the twinkling flowers on every side, the wide arch of -boughs overhead, and the murmuring streamlet in the dell below--and -though forming itself no unimpressive object in the scene, the boulder -looks out upon us unconnected with anything around. Like a sculptured -obelisk transported from the plains of Assyria to the streets of -London, it offers no link of association with the order of things -around it; its inscriptions are written in hieroglyphics long since -extinct, but of which the key yet remains to show us that the rocks of -our planet are not masses of dead, shapeless matter, but chronicles of -the past; and that all the varied beauty of green field and waving wood -is but a thin veil of gossamer spread out over the countless monuments -of the dead. We have raised one little corner of this gauze-like -covering, and tried to decipher the memorials of bygone creations, -traced in clear and legible characters on the boulder. First, there -lies spread out before us a wide arctic sea, studded with icebergs -that come drifting from the north. Here and there a bare barren islet -rises above the waste of waters, and the packed ice-floes often strand -along its shores, while at other parts great towering bergs, aground -in mid-ocean, keep rising and falling with the heavings of the surge, -and seem ever on the verge of toppling into the deep. But this scene, -so bleak and lifeless, erelong fades away, and we can descry a wide -archipelago of islands, green well-nigh to the water's edge, and -looking like the higher hill-tops of some foundered continent. The -waves are actively at work wearing down the shores, which present for -the most part an abrupt cliff-line to the west. This picture, too, -gets gradually dim, and when the darkness and haze have cleared away, -the scene is wholly new. For miles around there spreads out an expanse -of water, like a wide lake, thickly dotted with islets of every form -and size, clothed with a rich vegetation. Here a jungle of tall reeds -shoots out of the water, clustering with star-like leaves; there a -group of graceful trees, fluted like the columns of an ancient temple, -and crowned by a coronal of sweeping fronds, spread out their roots -amid the soft mud. Yonder lies a drier islet, rolling with ferns of -every shape and size, with here and there a lofty tree-fern, waving its -massive boughs high overhead. The vegetation, rank and luxuriant in the -extreme, strikes us as different from anything visible at the present -day, though, as our eyes rest on the muddy discoloured current, we -can mark, now and then, huge trunks, branchless and bare, that recall -some of the living pine-trees. The denizens of the water seem to be -equally strange. Occasionally a massive head, with sharp formidable -tusks, peers above the surface, and then the gleam of fins and scales -reveals a creature some twenty or thirty feet long. Glancing down into -the clearer spots, we can detect many other forms of the finny tribes, -all cased in a strong glistening armature of scales, and darting -about with ceaseless activity. Beyond this scene of almost tropical -luxuriance, on the one side, lies the blue ocean, with its countless -shells and corals, its stone-lilies and sea-urchins, and its large -predaceous fish; on the other side stretches a far-off chain of hills, -whose nether slopes, dark with pine-woods, sweep down into the rich -alluvial plains. And then this landscape, too, fades slowly away, and -thick darkness descends upon us. Yet through the gloom we feel ever -and anon the rambling earthquake, and see in the distance the glare -of some active volcano that throws a ruddy gleam amid the pumice and -ashes, ever dancing along the surface of the sea. And now this last -scene melts away like the rest, and dark night comes down in which we -can detect no ray of light, and beyond which we cannot go. The record -of the boulder can conduct us no further into the history of the past. - -The same principles which have been pursued in the previous pages in -elucidating the history of the Carboniferous system, will conduct -the reader to the true origin and age of any group of rocks he may -encounter, whatever its nature, and wheresoever its locality. Let -him, therefore, in his country rambles, seek to verify them in valley -and hill-side, by lake and cataract, and along river-course and -sea-shore. Let him not be content with simply admiring the picturesque -grouping of rock-masses, but rather seek to interpret their origin -and history, tracing them step by step into the past, amid ages long -prior to man. Such a process will give him a yet keener relish for -the beauties of their scenery, by ever calling up to his mind some of -those striking contrasts with which geology abounds. In the stillness -of the mountain-glen, he will see on every side traces of the waves of -ocean, and when dipping his oars into the unruffled sea among groups -of wasted rocks, miles from shore, he will bethink him, perchance, -of some old forest-covered land, of which these battered islets are -the sole memorials. His enjoyment of the scenery of nature is thus -increased manifold, and he carries about with him a power of making -even the tamest landscape interesting. Cowper, in one of his exquisite -letters, remarks,--"Everything I see in the fields, is to me an object; -and I can look at the same rivulet or at a handsome tree, every day of -my life, with new pleasure." Had the sweet singer of Olney lived to -witness the results obtained by the geologists whom he satirized, he -would perhaps have sauntered along the Ouse with a new pleasure, and -have felt a yet more intense delight in casting his eyes athwart the -breadth of landscape that spreads out around-the "Peasant's nest." - -Such, however, are after all only secondary incentives to the study of -the rocks. As a mental exercise, geology certainly yields to none of -the other sciences, for it addresses itself at once to the reasoning -powers and to the imagination, and may thus be made a source both -of intellectual training and of delightful recreation. Of none of -the sciences is it so easy to get a general smatter, yet none is so -difficult thoroughly to master, for geology embraces all the sciences. -In so wide a field, the student will therefore find ample room to -expatiate. In beginning the study, he may perhaps think it, as Milton -pictured the other paths of learning, "laborious, indeed, at the first -ascent; but else so smooth, so green, so full of goodly prospects and -melodious sounds on every side, that the harp of Orpheus was not more -charming." If time and taste disincline him to travel over the whole -of the broad field, there are delightful nooks to which he may betake -himself, replete with objects of beauty and interest, where he may -spend his leisure, and by so doing not merely delight himself, but -enlarge the bounds of human knowledge. No part of the domain can be too -obscure or remote to reward his attention; no object too trifling or -insignificant: for the march of science, though a stately one, proceeds -not by strides, but by steps often toilsome and slow; and she stands -mainly indebted for her progress not to the genius of a few gigantic -intellects, but to the united efforts of many hundred labourers, each -working quietly in his own limited sphere. - -But the highest inducement to this study must ever be that so quaintly -put by old Sir Thomas Browne: "The world was made to be inhabited by -beasts, but studied and contemplated by man: 'tis the debt of our -reason we owe unto God, and the homage we pay for not being beasts; -without this the world is still as though it had not been, or as it -was before the sixth day, when as yet there was not a creature that -could conceive or say there was a world." Geology lifts off for us the -veil that shrouds the past, and lays bare the monuments of successive -creations that had come and gone long ere the human race began. She -traces out the plan of the Divine working during a vast cycle of ages-, -and points out how the past dovetails with the present, and how the -existing condition of things comes in as but the last and archetypal -economy in a long progressive series. By thus revealing what has gone -before, she enables us more fully to understand what we see around us -now. Much that is incomplete she restores; much that is enigmatical -she explains. She teaches us more fully man's true position in the -created universe, by showing that in him all the geologic ages meet -that he is the point towards which creation has ever been tending. How -far the facts brought to light by geology may bear upon the future, -will not, perhaps, be solved until that future shall have come. There -is, nevertheless, in the meanwhile, material enough for solemn and -earnest reflection, and as years go by the amount will probably be -always increasing. For we must ever be only learners here, and when all -earthly titles and distinctions have passed away, and we enter amid the -realities of another world, we shall carry with us this one common name -alone. It will, perhaps, be then as now, that only - - "In contemplation of created things - By steps we may ascend to God." - -And it can surely be no unmeet preparation for such a scene, in humble -faith to read the records of His doings which the Almighty has graven -on the rocks around us. Many problems meet us on every hand problems -which it seems impossible for us now to solve and as the circle of -science ever widens, its enveloping circumference of difficulty and -darkness widens in proportion. It is, doubtless, well that it should be -so; for we are thus taught to regard our present state as imperfect and -incomplete, and to long for that higher and happier one promised by the -Redeemer to those that love Him, when "we shall know thoroughly even as -we are thoroughly known." - - -TABLE OF FOSSILIFEROUS ROCKS. - -LYELL'S _Elements_, p. 109. - - 1. RECENT. } POST-TERTIARY. - 2. POST-PLIOCENE. } - } - 3. NEWER PLIOCENE. } PLIOCENE. } - 4. OLDER PLIOCENE. } } - } TERTIARY - 5. UPPER MIOCENE. } MIOCENE. } or - 6. LOWER MIOCENE. } } CAINOZOIC. - } - 7. _a_ UPPER EOCENE. } EOCENE. } - 7. _b_ MIDDLE EOCENE. } } - 8. LOWER EOCENE. } } - - 9. MAESTRICHT BEDS. } - 10. UPPER WHITE CHALK. } - 11. LOWER WHITE CHALK. } - 12. UPPER GREENSAND. } CRETACEOUS. } - 13. GAULT. } } - 14. LOWER GREENSAND. } } - 15. WEALDEN. } } - } - 16. PURBECK BEDS. } } - 17. PORTLAND STONE. } } SECONDARY - 18. KIMMERIDGE CLAY. } } or - 19. CORAL RAG. } JURASSIC } MESOZOIC. - 20. OXFORD CLAY. } } - 21. GREAT or BATH OOLITE. } } - 22. INFERIOR OOLITE. } } - 23. LIAS. } } - } - 24. UPPER TRIAS. } } - 25. MIDDLE TRIAS, } } - or } TRIASSIC. } - MUSCHELKALK. } } - 26. LOWER TRIAS. } } - - 27. PERMIAN, } - or } PERMIAN. } - MAGNESIAN LIMESTONE. } } - } - 28. COAL-MEASURES. } } - 29. CARBONIFEROUS } CARBONIFEROUS. } - LIMESTONE. } } - } PRIMARY - 30. UPPER DEVONIAN. } DEVONIAN, or } or - 31. LOWER " } OLD RED SANDSTONE. } PALÆOZOIC. - } - 32. UPPER SILURIAN. } SILURIAN. } - 33. LOWER " } } - } - 34. UPPER CAMBRIAN. } CAMBRIAN. } - 35. LOWER " } } - - - * * * * * - - -Transcriber Note - -Minor typos corrected. Some images moved to nearest paragraph break. A -paragraph break was added to page 74 and 105 to accommodate placement -of Figures 17 and 26 respectively. 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