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+*** START OF THE PROJECT GUTENBERG EBOOK 77091 ***
+
+[Transcriber's note: This article has been extracted and prepared from
+_The Geographical Journal_, v. 70, 1927.]
+
+
+
+
+                     PROBLEMS OF THE LIBYAN DESERT
+
+   John Ball, O.B.E., D.Sc., M.Inst.C.E., F.G.S., Director of Desert
+                             Surveys, Egypt
+
+                         _Map following p._ 96.
+
+
+THERE can be few tracts of the Earth’s surface which present such a
+number and variety of interesting geographical problems as the Libyan
+Desert. The following are some of the more puzzling of the questions
+which have been asked at various times since I made my first
+acquaintance with the desert thirty years ago:
+
+  1. Did the Nile, or a branch of it, ever flow through the Libyan
+  Desert to the west of the present Nile Valley?
+
+  2. By what natural agency were the great depressions in the Libyan
+  Desert excavated?
+
+  3. Can other depressions in this region, besides the much-discussed
+  Wadi Rayan, be considered possible of utilization in connection with
+  irrigation, flood-protection, or drainage of the Nile Delta, or as
+  sources of water-power for Egypt?
+
+  4. Whence comes the artesian water of the oases?
+
+  5. Why have not certain lakes, such as Sittra and Bahrein, situated as
+  they are in an almost rainless region, long since dried up?
+
+  6. Can the present scanty supplies of drinkable water derived from
+  local rainfall along the Egyptian Mediterranean Littoral be
+  supplemented by artesian borings?
+
+  7. Are the artesian water-supplies of the oases gradually diminishing?
+
+  8. Where shall we look for the mysterious “Zerzura,” or “Oasis of the
+  Blacks”? Are there any other “lost” oases remaining to be discovered?
+
+  9. Can the present difficulties of travel in the Libyan Desert, which
+  are chiefly due to scarcity of water, be alleviated by the sinking of
+  new wells?
+
+  10. Where are the “Tortoise Marshes” of Ptolemy?
+
+  11. What determined the peculiar distribution of the sand-dunes, some
+  of which extend in straight lines for hundreds of kilometres?
+
+  12. What is the full extent of the distribution of flint implements
+  and pottery, of which accumulations have been found in what are now
+  among the most desolate parts of the desert, and what light does this
+  throw on human history and climatic changes?
+
+To most of these questions no completely satisfactory answers have as
+yet been given, though several of them have been the subject of
+painstaking investigations by many workers. Our knowledge of the facts
+bearing on them has, however, increased considerably during the past few
+years; and in the present paper I propose briefly to review the various
+questions in the light of the latest available data concerning them.
+
+The problems, though they concern a variety of subjects, are very much
+interwoven with each other, and they are mostly alike in that the first
+difficulty in any attempt to solve them has always been the
+incompleteness of our geographical knowledge concerning the Libyan
+Desert as a whole. If we possessed a series of detailed contour-maps
+covering the whole of the desert, most of the problems would be in a
+fair way of solution. But the surveys which would be necessary for such
+a result are impracticable at present, mainly on the score of expense.
+
+Some fifteen years ago I essayed the construction of a contoured map of
+the Libyan Desert, in connection with the International “Million” map of
+the world. But I had to give up the task, finding that there were vast
+areas without a single observation for altitude, while such altitude-
+data as did exist were open to errors of a magnitude intolerable for
+even a preliminary contouring of the areas with which they were
+concerned. The attempt however was not without its value, because it
+drew attention to the great lack of reliable levels in the Libyan
+Desert, and to the necessity of remedying this defect before even an
+approximately true picture of the general relief could be obtained.
+
+In the interval which has elapsed since my former attempt, a great deal
+of surveying has been carried out in the Libyan Desert, and in most of
+this work special attention has been paid to the matter of levels. Two
+long chains of triangulation have been run westward from the Nile to
+Siwa Oasis, one _viâ_ the Wadi Natrun and the other _viâ_ Baharia Oasis;
+another chain has been completed along the coast westward from
+Alexandria to Sollum, and a triangulation is now being carried out which
+will eventually connect the oases of Kharga and Dakhla with the Nile
+Valley. Small local triangulations have also been made in certain areas,
+such as Dakhla Oasis and in the neighbourhood of Owenat. In all these
+triangulations vertical as well as horizontal angles have been observed,
+so that reliable trigonometric altitudes are now available for almost
+all the points occupied or sighted. In the southern and central parts of
+the desert, we are still dependent on barometric levels; but the old
+determinations have mostly been replaced by later and better ones; and
+the use of motor-car transport, by facilitating exploratory journeys in
+hitherto untrodden regions, has permitted of a large number of
+additional determinations being made in tracts where no previous
+observations of any kind existed.[1]
+
+Owing to the methods employed in the latest barometric determinations,
+the resulting altitudes are of a much higher order of accuracy than
+could be hoped for in previous measurements of the kind; in fact, it is
+believed that they are not so very far behind trigonometric levels in
+precision. This increased accuracy has been rendered possible by the
+collaboration of the Egyptian Meteorological Service, which, thanks to
+the abundant weather-data now received and dealt with by it, is in a
+position to furnish a close approximation to the sea-level pressure at
+any point in the deserts at any given instant. The aneroids used in the
+desert explorations are compared with the standard barometer of the
+Meteorological Service immediately before the start and immediately
+after the return of each expedition. At each observation in the desert,
+the date and time, and the temperature of the air, are noted, as well as
+the approximate latitude and longitude of the place. On the return of
+the expedition, the recorded reading is corrected for the error of the
+aneroid on the standard barometer, and the Meteorological Service is
+asked to supply the sea-level pressure at the particular place and time
+of the observation. From the difference of the two pressures and the air
+temperature, the height of the place above sea-level is then worked out
+directly with the aid of Jordan’s table of “Barometrische Höhenstufen.”
+If the place of observation lies considerably above sea-level, a small
+correction is applied to the observed air-temperature to allow for the
+temperature-gradient in the air; for of course it is the mean
+temperature of the air column between the place and sea-level which
+should be employed in the reduction.
+
+It is unlikely that with this procedure either the sea-level pressures
+or those observed at the places can be in error by much more than half a
+millimetre; and consequently, even with some slight uncertainties in the
+air-temperatures, the resulting altitudes may be expected to be accurate
+to within about 15 metres when only a single observation has been made,
+and to within much less than this when, as is usually the case at
+important points, the altitudes have been calculated from a series of
+readings spread over several days. That this degree of accuracy is
+actually attained seems evident on comparison of the levels obtained for
+the same places on different dates. Thus, for instance, at a camp close
+to Bir Terfawi, during a stay of five days in January and two days in
+March 1925, I made in all eighteen aneroid observations. The mean
+altitude found was 244 metres; the average deviation of a single
+observation from this mean was 8 metres, the maximum being 16. On the
+assumption that all sources of error are accidental, the probable error
+of the mean resulting altitude works out at less than 2 metres, which is
+about the same as might be expected in trigonometric levelling to the
+place from the Nile Valley.
+
+The points in the interior of the Libyan Desert of which the levels are
+fairly accurately known now number many hundreds, and with the exception
+of a large blank in the unexplored west, and another between the Nile
+and Merga in the Northern Sudan, they are fairly well distributed over
+the country. It therefore seemed to me to be worth while to make another
+attempt at a contoured map; for the levels now known (though still far
+too few for contours to be drawn with any great precision) might suffice
+for the construction of a map on a comparatively small scale and with
+contours at fairly wide intervals; it was felt that a provisional map of
+this kind, besides affording at least an approximate general view of the
+relief, might help towards the solution of one or more of the problems
+which I have mentioned above. I commenced by sketching in the contours
+at 100-metre intervals on the original sheets of the 1:500,000 map of
+Egypt, a revised edition of which is in preparation. They were then
+reduced to the million scale, and afterwards still further reduced, with
+additions from my own recent observations around Owenat, Gebel Kissu,
+and Merga, from Hassanein Bey’s records of his journey between Jalo and
+El Fasher, from the altitude-data obtained by the Anglo-French Boundary
+Commission in the Sudan,[2] and from the maps of Prince Kemal el Din’s
+expedition of 1926 to Owenat and Sarra Well,[3] to form the map
+following p. 96. The attempt has, I think, fully justified itself, and
+more than fulfilled the hopes that were entertained concerning it.
+
+Probably the feature that will first strike the attention on glancing at
+the contoured map is the series of depressions below sea-level
+stretching westward from near Cairo to Jarabub. The following table
+gives a list of the principal of these depressions, with the approximate
+extent of the areas lying below sea-level, and also (where known) the
+depths below sea-level of their deepest parts. Where the lowest part is
+covered by a lake, the depth shown is that of the water-surface, not
+that of the lake-bottom:
+
+                                 _Approximate area    _Approximate
+  _Depression._                   below sea-level   depth of deepest
+                                  in sq. kiloms._    part below sea
+                                                       in metres._
+
+  Wadi Natrun                             220                23
+
+  Faiyum                                  700                45
+
+  Wadi Rayan                              280                43
+
+  Qattara (including Moghara,          18,000               134
+  Qara, and Tebaghbagh)
+
+  Sittra (including Timata)               300                33
+
+  Bahrein (including Nuemisa)             150                15 (?)
+
+  Watiya                                   70                15
+
+  Areg                                     60                25
+
+  Siwa (including Maasir and              800                17
+  Maraqi)
+
+  Girba                                    20                (?)
+
+  Kheiba                                   40                (?)
+
+  Shiyata                                  20                (?)
+
+  Melfa (including Shebat,                260                18 (?)
+  Exabia, and Gagub)
+
+As will be seen from the figures in the table, by far the largest and
+deepest depression in this series—seven times as large, in fact, as all
+the others put together—is that of Qattara, which has not been shown on
+any previous map, and of which the existence was proved only last year.
+That so deep a depression should exist comparatively near to the
+Mediterranean and to Cairo, and have remained undiscovered until now, is
+striking evidence of how much has still to be learned concerning the
+configuration of the desert surface. The manner in which the discovery
+was made is perhaps worth placing on record. Hearing early in 1917 that
+a military patrol was about to operate in the Qattara region, I lent a
+small aneroid to the officer in charge of the patrol, and asked him to
+take readings with it at various points along his route, with the object
+of getting some idea of the altitudes in that region. It was, of course,
+certain that low-lying country existed along the foot of the Qattara
+escarpment, but no idea was entertained that any great part of it might
+be below the level of the sea. The officer brought back aneroid-readings
+which seemed to indicate that the spring at the foot of the scarp at
+Qattara was about 60 metres below sea-level; but the aneroid itself was
+unfortunately lost during the return journey. The result was too
+surprising for it to be accepted without evidence that the readings of
+the instrument had not been vitiated by accidental rough treatment, and
+consequently I refrained at the time from placing the levels found on
+the maps. But I resolved to confirm them or otherwise on the first
+opportunity.
+
+That opportunity came last year, when it was possible to send a survey-
+party to triangulate westwards from the Wadi Natrun so as to cross the
+place where the aneroid readings had been taken. The work was entrusted
+to Mr. G. F. Walpole, of the Survey of Egypt, who had already
+distinguished himself by successfully carrying out a difficult
+triangulation from the Nile across some 500 kilometres of the Libyan
+Desert _viâ_ Baharia to Siwa. The result of Mr. Walpole’s work was not
+only to confirm the substantial accuracy of the previous estimation of
+the level of the Qattara spring, but to bring to light the existence of
+a vast hollow, thousands of square kilometres of whose floor lie at even
+lower levels, and which at one place descends to a depth of no less than
+134 metres below the level of the sea.[4] This last-mentioned spot, some
+100 kilometres south-south-west of the Qattara spring, is probably the
+lowest-lying point on the land-surface of the African continent.
+
+A scarcely less remarkable feature is the series of depressions
+comprising the oases of Kharga, Dakhla, Abu Mungar, and Farafra; while
+Baharia occupies a more isolated position about midway between Farafra
+and the Faiyum. In contrast with those of the northern series, these
+southern depressions do not penetrate to sea-level. The lowest points in
+Kharga Oasis are probably at or just below the level of the sea, but the
+general level of the Kharga floor is about 70 metres above that datum.
+The lowest point of Farafra is somewhat higher than the general level of
+Kharga; those of Dakhla and Abu Mungar are higher still, while in
+Baharia, though it lies farther to the north, the floor-level averages
+some 130 metres above sea.
+
+A noteworthy thing about the larger depressions is that their northern
+boundaries are formed by steep escarpments, while to the south the slope
+is more gradual. This is the case, for instance, with the Faiyum,
+Qattara, Siwa, Kharga, Dakhla, Abu Mungar, and Farafra. Baharia is
+unique in being entirely surrounded by escarpments, as also in
+containing a large number of hills approximating in height to that of
+the bounding scarps. In the majority of cases the greater steepness of
+the northern boundaries can be correlated with geological structure; the
+northern walls of the Qattara and Siwa depressions, for instance, mark
+the southern limit of certain Miocene strata, while those of Kharga and
+Dakhla coincide with the southern limit of the Eocene limestones.
+
+To the south-west of Dakhla Oasis there stretches a broad tract of
+rising ground, with the Gilf Kebir and the peaks of Arkenu, Owenat, and
+Kissu as conspicuous features. On either side of this tract the general
+level falls, on the one hand towards Kufra and Cyrenaica, and on the
+other hand towards the Nile. The Gilf Kebir is a great broken plateau of
+sandstone, rising very nearly to the 1000-metre contour and stretching
+in a direction a little west of south for over 100 kilometres. I saw the
+south end of this plateau from several points on the way from Terfawi to
+Owenat when travelling with Prince Kemal el Din in 1925, and fixed its
+position by compass-bearings. As seen from the south, the plateau
+appeared to be merely a flat-topped hill; its true form and extent were
+discovered by the Prince in the following year, when he passed along its
+eastern foot on his way from Pottery Hill to Owenat. The highest peak of
+Arkenu, in about lat. 22° 17′, long. 24° 46′, I estimated to rise to
+approximately 1410 metres above sea. The altitude of Gebel Owenat far
+transcends Hassanein Bey’s previous estimate of 1100 metres; a
+trigonometric measurement which I made in 1925 gave its summit as 1907
+metres above sea. The general level of the ground at the foot of Owenat
+I found to average about 600 metres, thus agreeing with Hassanein Bey’s
+figures. Gebel Kissu, though lower than Gebel Owenat, is considerably
+higher than Gebel Arkenu, and being an isolated mountain with a single
+well-marked peak, it forms a very conspicuous landmark; my trigonometric
+determination gave 1726 metres for the altitude of the peak above sea.
+
+To the south-west of Kissu there are a few widely scattered isolated
+hills, but the country in general forms a rolling plain of sand
+extending to the north-east corner of French Equatorial Africa with a
+nearly uniform level of about 700 metres. Beyond the French boundary the
+ground rises towards the Erdi Hills, which attain over 1000 metres.
+
+From the corner of French Equatorial Africa to Merga, a distance of 250
+kilometres, the country consists of alternations of sand-plains and flat
+stony tracts, with a very gradual fall to a level of about 560 metres at
+the edge of the Merga depression.
+
+Between Merga and Owenat the country consists of sandy plains
+alternating with broken stony ground and occasional hills. The general
+level rises from about 570 metres on the northern side of the Merga
+depression to nearly 800 metres on the parallel of 20° 30′, then
+gradually falls to about 600 metres at the foot of Gebel Owenat.
+
+
+1. _Did the Nile, or a Branch of it, ever flow through the Libyan Desert
+to the Mediterranean?_
+
+
+The idea that a “dry river” exists in the Libyan Desert is a very old
+one, and seems to have had its origin in the fact that barren
+depressions in that region are sometimes called by the local Arabs _Bahr
+bela ma_, that is, “sea without water.” But the Arabic word _bahr_,
+which properly means a sea or a lake, as in Bahr Lot (the Dead Sea), is
+also applied in Egypt and the Sudan to a river, as in Bahr el Nil (the
+Nile) and Bahr el Azrak (the Blue Nile).[5]
+
+The earliest depression to be discovered bearing the name of _Bahr bela
+ma_ was the one now known as the Wadi Faregh (the “empty wadi”),
+situated immediately south of the Wadi Natrun. Father Sicard, who
+visited the place in 1712, correctly interpreted the Arabic name to mean
+literally “sea without water,” and having discovered fossil trees there,
+which he imagined to be the petrified remains of masts of ships, he
+inferred that a narrow arm of the sea formerly extended from the
+Mediterranean into the locality.[6] At a later date Sicard’s view that
+the petrified trees were the masts of ships was recognized to be a
+mistaken one; but a greater error was committed by changing the
+translation of _Bahr bela ma_ from “sea without water” into “river
+without water,” as was done by D’Anville in his ‘Mémoires sur l’Egypte,’
+published at Paris in 1766. D’Anville was a great authority in his day,
+and his translation was adopted by subsequent writers for over a hundred
+years. General Andreossi, who commanded Napoleon’s artillery in the
+expedition of 1798 and 1799, made a map of the “Dry River” and the
+“Valley of the Natron Lakes,” in which both depressions are shown open
+at both ends, instead of being closed in as they really are. Andreossi
+considered that the “Bahr bela ma” was the dry bed of an ancient branch
+of the Nile, which left the present Nile Valley somewhere in Middle
+Egypt and entered the sea to the west of Alexandria; he also thought
+that the ancient Lake Moeris was probably formed by the damming of the
+“dry river” near its supposed offtake from the Nile.[7] Later on, after
+Cailliaud and other early nineteenth-century travellers had discovered
+that other depressions existed in widely separated localities and bore
+the same Arabic name of _Bahr bela ma_, it seems to have been hastily
+assumed by the cartographers of the time that all the depressions
+bearing the same name were parts of a single dried-up river-channel.
+Thus, on a large map of Egypt compiled by Colonel Lapin and published at
+Paris in 1856, the “Bahar Belah-mah ou Fleuve sans eau” is stated to
+come from the Congo and is depicted as coursing through the Oasis of
+Dakhla, thence passing to the east of Baharia Oasis and on to the
+Mediterranean. On another large map of Egypt, compiled by Muzzi Bey, the
+then Director-General of Egyptian Posts, and published in Florence in
+1876, a continuous valley is boldly shown leading from the mountains of
+Darfur to the Mediterranean, while cross-valleys, labelled “Old Bed of
+the Nile,” are depicted as connecting the main valley with that of the
+Nile at Korosko and Dongola.
+
+The Rohlfs expedition of 1874 proved that there was no such channel
+running northwards through Dakhla Oasis; that the “Bahr bela ma” which
+had been crossed by Cailliaud between Siwa and Baharia was merely a
+closed-in local depression, and that the continuous empty river-beds
+which were shown on the maps of that period had no real existence.[8]
+
+But though the continuous “Bahr bela ma” channel of the old
+cartographers has been shown to be purely imaginary, and has in
+consequence disappeared from our modern maps, the belief that an old dry
+river-bed _may_ exist _somewhere_ in the Libyan Desert has apparently
+persisted in many minds even to our own day. Only a few years ago I was
+called upon by the Government to discuss a suggestion, made in all
+seriousness by a person of considerable eminence, that the Nile, or a
+branch of it, must at one time have followed a course from somewhere
+near Dongola through some of the Egyptian oases to the Mediterranean;
+and that by tracing out the old channel, and deepening it artificially
+where necessary, a part of the river might be taken off along this path
+and its water utilized to irrigate the desert country on either side of
+it. To the few scientific travellers who have journeyed extensively in
+the Libyan Desert, it will doubtless appear incredible that such a
+suggestion as this could be seriously made. But old traditions
+concerning the geography of little-known regions die hard, and this
+particular one has probably been fostered by the circumstance that on
+most small-scale maps the oasis-depressions have been shown without any
+precise indication of the altitude of the intervening ground, as well as
+by the speculations of geologists as to the existence of a river in the
+region in past geological ages. Blanckenhorn, for instance, published in
+1902 a series of small-scale maps depicting the course of a hypothetical
+river, the “Libyan Ur-Nile,” running northwards through the desert in
+the Eocene and Oligocene periods, and ceasing to exist in the
+Pliocene.[9]
+
+That a river flowed during late Eocene or early Oligocene times in a
+north-easterly direction through what is now the Libyan Desert, and
+entered a sea near what is now the Faiyum, can scarcely be questioned;
+for it is only by the existence of such a river that we can account for
+the assemblage of fossil trees and curious remains of Eocene animals
+(including the ancestors of the elephant) discovered by Mr. Beadnell
+north of the Birket el Qarun and described by the late Dr. Andrews.[10]
+As to the size, length, and exact position of this river of the
+geological past there is very great doubt; but neither its situation nor
+even its existence can be considered material to our present inquiry. No
+one conversant with the known facts bearing on the general geological
+history of Egypt during late Tertiary and Quaternary times could make
+the mistake of thinking that the path of an Eocene river might be
+traceable in the present desert relief. The evidences of such path must
+be purely of a geological nature. In his endeavours to suggest what
+might possibly have been the course of the river on whose banks the
+Eocene animals lived, Mr. Beadnell found in two localities deposits
+which might have been formed along the ancient drainage-line; and in
+both these localities the deposits now occupy the summits of hills.
+There is insufficient evidence of any kind to justify so exact a
+location of the Eocene river as Blanckenhorn’s maps would suggest, and
+none whatever of any relationship, either between the present surface-
+configuration of the desert and Eocene or Oligocene drainage-lines, or
+between those drainage-lines and the River Nile. The appellation “Ur-
+Nile” of Blanckenhorn is thus a misleading one. If we are to look for
+traces of a dried-up river in the surface-forms of the Libyan Desert of
+to-day, it must be a river which, like the streams which eroded the
+great wadis of the Eastern Desert, existed in geologically recent times,
+and which became dry either through the continual deepening of the
+present Nile-channel, with consequent capturing of the former drainage,
+or through climatic changes causing a diminution of rainfall in the
+collecting area.
+
+A study of the contoured map seems to me to be absolutely conclusive on
+the question at issue, for it indicates clearly that at no time within
+the geologically recent period can there have been any such continuous
+channel as has been supposed. Let us try and trace, with the aid of the
+map, the most likely course along which the ancient Libyan Nile-branch,
+if ever one existed, must have flowed. Starting from the Nile in the
+neighbourhood of Dongola, we observe that the lowest possible track is
+through Sheb towards Kharga. But Sheb can only be reached after passing
+over some 350 kilometres of desert lying well above the level of the
+Nile at Dongola, which is about 230 metres above sea. It is, of course,
+quite conceivable that even within geologically recent times the Nile at
+Dongola may have been considerably above its present level; so that we
+may pass over this first difficulty as not being definitely conclusive
+against the hypothesis we are pursuing. From Sheb to the south end of
+the Kharga depression there is a drop of about 160 metres, so that up to
+this point a channel may have been possible. But from here onwards,
+difficulties come in. If we turn northwards, we soon encounter the north
+wall of Kharga, rising to 300 metres above sea-level; so there is no
+continuing that way. Instead, we turn westwards towards Dakhla. But from
+Kharga we have to pass over ground reaching very nearly to the 200-metre
+contour for some 80 kilometres to enter the Dakhla depression, where we
+drop to a level of 119 metres. We have to traverse some 15 kilometres at
+altitudes above 200 metres to reach Abu Mungar, where the level is 117
+metres; then a further 12 kilometres lying above the 200-metre contour
+to get to Farafra, where the level descends to about 90 metres. To get
+out of Farafra, we have again to traverse country lying over 200 metres
+above sea, whether we choose a north-westerly route _viâ_ Sittra to the
+Qattara depression, or a north-westerly one _viâ_ Baharia to the Wadi
+Rayan and the Faiyum. Taking the first of these alternatives as being
+the shorter, we cross some 30 kilometres of ground above the 200-metre
+level, and a further 120 kilometres lying between the 200- and 100-metre
+contours, before reaching the Qattara depression. And once we are in the
+Qattara depression (the lowest point of which is 134 metres below sea),
+there is no possibility of reaching the Mediterranean except by crossing
+a considerable tract where the ground-level is over 100 metres above
+sea-level.
+
+A section of the route described, with a comparison-line passing through
+the present Nile-levels in corresponding latitudes, is shown in the
+figure below:
+
+[Illustration: _Profile from the Nile at Dongola through the depressions
+of Kharga, Dakhla, Abu Mungar, Farafra, and Qattara to the
+Mediterranean. Vertical scale 500 times the horizontal. Total length of
+section about 1700 kilometres; for more than half this distance the
+ground-level is seen to be above a line drawn through the levels of the
+Nile in corresponding latitudes._]
+
+Can any one believe that a course of this kind was ever that of a river?
+To make a through channel would entail deep cutting for more than half
+the total distance of 1700 kilometres, and would consequently involve
+excavating hundreds of millions of cubic metres of rock, much of it of
+considerable hardness. Moreover, it cannot be supposed, in the present
+state of our knowledge of the region, that more favourable lines of
+communication exist and have been overlooked. Many depressions doubtless
+still remain to be discovered; but these, like those already known, will
+surely prove to be merely local basins. So many cross-country journeys
+have now been made in various directions through the desert, that it is
+inconceivable that any continuous channel of the kind we are discussing
+could possibly have escaped notice. Nor can we reasonably suppose that
+recent earth-movements have produced the present severance of the
+various depressions; such movements might perhaps be invoked to account
+for one or two of the separating elevations, but certainly not for all.
+We can therefore be absolutely certain that neither the Nile nor any
+branch of it ever passed through the Libyan Desert to the sea.[11]
+
+Even channels of tributary streams to the Nile, such as abundantly
+survive in the great wadis which enter the Nile valley on its eastern
+side, are markedly absent in the Libyan Desert. The contours on the map
+suggest that possibly one such tributary stream may formerly have
+drained the south-western faces of the great plateau in which the
+depressions of Dakhla and Kharga form bays, and have entered the Nile
+somewhere between Aswan and Halfa; but if such a channel ever did exist,
+its actual traces have long since been obliterated by the smoothing
+action of subsequent denudation, and only the great escarpment of the
+plateau which it half encircled remains to tell the tale.
+
+_Photographs by Prince Kemal el Din, Dr. Ball, and Mr. Walpole_
+
+[Illustration: 1. _Desert exploration with the ordinary car, requiring
+help even on flat ground in sandy tracts_]
+
+[Illustration: 2. _Caterpillar cars carrying heavy loads over a sand-
+dune between Bir Terfawi and Gebel Owenat_]
+
+[Illustration: 3. _The western wall of the Qattara depression 35 km.
+S.S.W. of Qara Oasis. Top of cliff about 3 m. and floor of depression
+133 m. below sea-level_]
+
+[Illustration: 4. _Ain el Hez, Baharia Oasis, irrigated by free flow
+from well_]
+
+[Illustration: 5. _Overlooking the Baharia depression from the pass at
+its southern end: cars bound for Farafra_]
+
+[Illustration: 6. _Northern end of Farafra depression: sandy ravine
+cutting back into chalk plateau_]
+
+
+2. _Origin of the Depressions._
+
+
+It is a marked characteristic of almost all true deserts, that they are
+regions of internal drainage; and the Libyan Desert is no exception to
+the rule. There are a few gullies draining from the plateau to the sea
+along its northern edge, and a few others draining into the Nile Valley
+along its eastern border. But none of these external drainage-lines
+extends for more than an insignificant distance backward into the
+interior of the country. There is, in fact, in the Libyan Desert
+(especially in its north-eastern portion) a marked absence of any
+distinct drainage-lines at all; in their place we find extensive stony
+plains, which either slope gradually down towards the various inland
+depressions, or terminate in an abrupt fall where the depressions are
+cut back into them. Even in the Pluvial epoch, when Europe was glaciated
+and the deep wadis of the Eastern Desert on either side of the Nile were
+being cut out by great streams, there cannot have been any external
+drainage from a large part of the Libyan Desert; for if there had been,
+we should find the Libyan plateau far more intensely dissected than is
+actually the case. The rainfall in this region, even during the Pluvial
+period, must have been far smaller than in the neighbouring tracts of
+the Eastern Desert and Sinai; such rainfall as did occur was drained
+into the series of depressions already described, and there either
+formed lakes or was evaporated. But the action of inflowing drainage
+waters, carrying sand and mud in suspension and salts in solution, is
+not to _deepen_ the basins into which they flow, but rather to _fill
+them up_. Hence we must look to some other cause for the formation of
+the basins themselves. A possible cause is to be found in earth-
+movements—either a local down-folding of the crust, or an upraising by
+faulting of the surrounding tracts. It is not unlikely that such earth-
+movements have to some extent conditioned the formation of the
+depressions; but that the depressions are not simply faulted-down areas
+or subsidences is abundantly clear from an examination of the bounding
+scarps and the floors of the larger oases. Baharia is wholly, and Kharga
+and Dakhla are partially, bounded by high escarpments, in many places
+hundreds of metres high, and the floors of these oases are composed of
+the same rocks as are exposed at the base of the bounding escarpments.
+It is, in fact, obvious that these great hollows are natural
+_excavations_, not subsidences. What was the excavating agency?
+
+To the south-west of the limestone plateau which lies west of the Nile
+and extends northwards from lat. 23° 30′ there is fairly open sandstone
+country, and the oases of Dakhla and Kharga occupy great bays in the
+escarpment which marks the south-west termination of the plateau. It
+seems reasonable to suppose that the Eocene and Cretaceous seas had
+their southern shores somewhere about the latitude of Wadi Halfa, for
+the limestones do not extend far beyond the tropic; and to the south the
+hard limestones of the plateau probably passed gradually into softer
+shallow-water deposits, which would lend themselves easily to
+degradation and to removal by the agency of streams whose traces have
+long since disappeared. We can thus possibly invoke water-action to
+account for the primitive formation of the “bays” in which Kharga and
+Dakhla are situated. But we cannot in this way explain either the
+further deepening of these oases, which converted them into basins, or
+the excavation of the more northerly depressions such as Baharia and
+Qattara.
+
+The problem as to how the excavation of the northern depressions, and
+the deepening of the southern ones, were accomplished is not altogether
+an easy one. There is, of course, no doubt that wind has been the main
+excavating agency. All the depressions occur in areas where soft rocks
+are overlain by hard ones, and once the hard overlying rock was removed
+at any place, the action of the wind on the softer beds would soon
+excavate a hollow. The difficulty is only as to how the hard overlying
+rock was first removed. To some who have studied the question, a
+sufficient explanation is to be found in the general degradation,
+combined with the tectonic disturbances—folds and faults—which are known
+to have affected some of the oasis-areas, as, for instance, that of
+Baharia. Given an anticlinal fold, a general lowering of the surface by
+wind-degradation would eventually result in the exposure of the soft
+underlying beds at the highest point of the fold, and subsequently
+continued wind-erosion would proceed most rapidly on these soft
+underlying rocks. Others, while admitting this explanation as partly
+accounting for the formation of the depressions, consider that other
+factors must have assisted to cause penetration of the hard overlying
+beds, especially in the areas where folding is not very marked.
+
+Of such other factors, I can only suggest what appears to me to be a
+possible one. About twenty years ago I was mapping the Mediterranean
+coastal area in the neighbourhood of Mersa Matruh, some 160 miles to the
+west of Alexandria. In this area, which has about 6 inches of rainfall
+in an average year, there are numerous closed-in basins a few miles long
+and a few hundred yards in width, descending to a depth of 10 metres or
+more in the rather soft limestone which forms the principal rock of the
+locality. These basins, though called wadis on the maps, are more
+usually known as _’ebs_ or “bosoms” by the local Arabs. I was at first
+much puzzled to account for them; but eventually I arrived at what I
+believe to be the true explanation of their formation. The basins, of
+course, collect the surrounding rainfall in the winter, and the drainage
+carries with it into the hollows a certain amount of fine loam and sand
+from the disintegration of the surrounding surfaces. After the rains,
+the bottoms of the basins become covered with vegetation, while the
+surrounding rocky surfaces are left bare. In the hot dry summer,
+however, the vegetation perishes, and much of the loam is removed by the
+wind. We have only to assume (what seems indeed likely) that the
+rootlets of the vegetation disintegrate the limestone floors of the
+hollows to form fresh soil, and that more of this is removed each year
+by the wind than is brought into the hollows by the drainage, to find an
+adequate cause for a slow but progressive deepening of the basins. The
+primitive hollow may well be a very slight depression of the surface,
+such as might arise from greater local softness of the rock and
+consequent more rapid weathering. Once vegetation has commenced to grow
+in a slight hollow of this kind, the action would lead to a slow yet
+steadily progressive deepening year by year, and sand-erosion would tend
+towards an elongation of the depression in the direction of the
+prevailing wind. Can such a process as this, continued through vast ages
+during which the climate was somewhat less dry than at present, have
+been active in the formation of Baharia and similar depressions?
+
+
+3. _The Possibility of Utilization of Depressions in the Libyan Desert
+for Irrigation, Drainage, or Hydraulic Power._
+
+
+The depression known as the Wadi Rayan, discovered by Mr. Cope
+Whitehouse about 1883, situated some 40 kilometres west of the Nile
+Valley and about 100 kilometres south of the latitude of Cairo, has been
+so frequently described and considered in regard to its utilization as a
+reservoir or as a flood-protection for Lower Egypt, that I need say
+little about it beyond a reference to the principal published works
+dealing with it.[12] There is no other depression in the Libyan Desert
+which has received so much attention from the economic standpoint. That
+the Wadi Rayan has not already been utilized as a reservoir has depended
+on a variety of considerations, of which one of the principal has been
+the doubts always entertained as to whether there might not be an
+underground leakage from it, resulting not only in a loss of part of the
+stored water, but also in damage to the adjacent flourishing province of
+the Faiyum. These doubts will not be lessened by the converging lines of
+evidence which I shall bring forward in dealing with the artesian water-
+supplies, all leading to the conclusion that there is an underground
+water-connection between the various depressions of the Egyptian Libyan
+Desert. Of late years, however, it has been proposed to make use of the
+Wadi Rayan, not as a reservoir, but as a sump for disposing of the
+drainage water of Upper Egypt by evaporation; and to this, in
+consequence of the lower level at which the water would stand in the
+depression, there is far less objection from the point of view of
+possible leakage or damage to the Faiyum.
+
+The Wadi Natrun, which, though considerably less deep, is not greatly
+less in extent than the Wadi Rayan, has received some consideration as a
+possible sump for disposing of some of the drainage water of the Behera
+Province of the Delta. So long ago as 1895 the Public Works Department
+studied the possibility of draining Lake Mariut into the Wadi Natrun,
+and thus doing away with the increasing cost of keeping down the level
+of the lake by pumping water from it into the sea.[13] The Wadi Natrun
+has an area, even at the low contour of 10 metres below sea-level, of
+nearly 100 square kilometres, and the average rate of evaporation from
+an open water-surface within it could not well be much less than 3 mm.
+per day; the wadi could therefore dispose annually of at least 100
+million cubic metres of drainage water by evaporation alone, and
+probably of a considerable additional quantity by seepage. But the high
+cost of cutting a drainage channel from Lake Mariut to the wadi, and the
+heavy maintenance charges which would be incurred in keeping the drain
+open, were formidable objections to the scheme. From a reconnaissance
+carried out in 1896, Mr. Verschoyle found that the length of the
+proposed drain would have to be almost 80 kilometres, and for the
+greater part of the way it would have to be a cutting 20 metres in depth
+through desert and ridges of drifting sand; he remarked that if it were
+an easy matter to make the connection, it would be no easy matter to
+maintain it; and he concluded that the scheme was an impracticable
+one.[14] A more recent proposal has been to construct a partly tunnelled
+channel instead of an open drain; but this has likewise been found to be
+impracticable, as involving too heavy an outlay for the benefits which
+would result.[15]
+
+[Illustration: 7. _Qasr Farafra, seen from the north over plain of
+powdery chalk forming floor of depression_]
+
+[Illustration: 8. _Escarpment and “Hattia” of Abu Mungar from the
+south_]
+
+[Illustration: 9. _The “Hattia” of Abu Mungar from the east: well on
+earth-mound on right_]
+
+[Illustration: 10. _The principal well at Qasr Farafra_]
+
+[Illustration: 11. _A street in Qasr Farafra, with irrigation canal_]
+
+When the great extent and depth of the Qattara depression were
+established by Mr. Walpole’s explorations of last year, hopes were
+immediately entertained that the depression might be made to serve some
+useful economic purpose. Any idea of its being of use as a Nile
+reservoir was of course at once cut out, not only by reason of its
+position and distance from the Nile, but also by its immense size; for
+even if we could turn the whole Nile flood into it, some twenty years or
+more would be occupied in filling it to a sufficiently high level,[16]
+and the loss by evaporation from so large an area would be enormous. As
+a receptacle for drainage water from the Delta it appeared equally
+impossible of consideration, because of the great length and depth of
+the channels which would have to be cut to reach it. A more reasonable
+prospect seemed to be that of admitting sea-water from the Mediterranean
+into it by means of a navigable canal from the Arabs Gulf; this would
+only have to be about 56 kilometres in length to reach the nearest point
+of the depression; and once the canal was made and the depression
+filled, ships might sail almost to Siwa. Other advantages which might
+accrue from the formation of this inland sea were an increased humidity
+of the climate along the Mediterranean Littoral of Egypt, leading to
+increased rain-crops in that region, the establishment of a valuable
+fishery, and a possible increase in the water-supply of the oases by the
+causing of a slight rise in the static water-level there. Another idea,
+which rapidly followed the first one, was to utilize the depression as a
+source of electrical power for driving pumps by which the drainage of
+the northern parts of the Nile Delta might be improved. At first sight
+this latter prospect looked a very attractive one. The salt-marsh which
+covers much of the floor of the depression appeared suggestive of a
+former sea-connection, and if we could trace out this old connection,
+the cutting of a canal along it might not, it was thought, be a very
+expensive matter. Moreover, it was apparent that evaporation from an
+inland sea or lake of so large an area would keep pace with quite a
+large influx from the Mediterranean, so that if the influx were
+restricted to such a quantity as would permit of the lake-surface being
+maintained at a level considerably below that of the Mediterranean,
+power could be generated continuously. Suppose, for instance, that we
+maintained a permanent water-level in the lake of 50 metres below the
+sea; the estimated area of the lake at this level being 9000 square
+kilometres and the mean evaporation assumed to be 4 millimetres a day,
+an influx of no less than 36 million cubic metres of water per day could
+be passed into the lake without altering its level, and this with a fall
+of 50 metres would suffice theoretically for the continuous generation
+of over 270,000 horse-power. Of course the lake would gradually get more
+and more saline, but the power might continue to be maintained for very
+many years before the lake became as rich in salt as the Dead Sea.
+
+But alas! an investigation of the northern borders of the depression
+showed that the hoped-for traces of a former connection with the
+Mediterranean Sea do not exist. The depression is entirely shut in from
+the north, either by great cliffs, or by ground lying so high that the
+cutting of a canal to the sea is utterly impracticable. Of the 56½
+kilometres from the sea-level contour of the depression to the coast
+along a line running 12° east of north from Moghara Lake (which is the
+easiest line hitherto found for the cutting of a canal), only 16
+kilometres are at less than 50 metres above sea; 31 kilometres lie at
+altitudes between 50 and 100 metres; and 9½ kilometres are above the
+100-metre level. An open channel being thus put out of the question, it
+was next natural to inquire whether a tunnel, or a channel partly open
+and partly tunnelled, could be excavated to serve for the conveyance of
+sea-water into the depression. But even this, though perhaps not
+impracticable, would be a very costly undertaking. The conveyance of the
+requisite quantity of water, even at a relatively high velocity (which
+of course implies a considerable slope), would necessitate a tunnel or
+tunnels of very large dimensions. To convey 36 million cubic metres a
+day at a velocity of 5 kilometres per hour would require a sectional
+area of 300 square metres—_i.e._ if two tunnels were made, each would
+have to be 14 metres in diameter. The cost of boring and lining such
+tunnels would certainly be very great, and it is doubtful whether the
+value of the power generated could justify the capital expenditure
+involved in the excavation and other works which would be necessary.
+Until more is known of the nature of the strata through which the
+aqueduct would have to pass, and as to the length of tunnelling and
+amount of open cutting which would be required, it is impossible to form
+even an approximate estimate of the expense which the undertaking would
+involve. All that can be said at present is that the utilization of the
+Qattara Depression offers a possible means of obtaining power on a large
+scale for the drainage of the low-lying lands of the Delta, with at the
+same time a prospect of improving in some measure the agricultural
+resources of the Mediterranean Littoral; much further investigation will
+have to be accomplished before any judgment can be formed as to whether
+such a project would be an economically sound one.
+
+In formulating any scheme for improving the drainage of the Delta, it is
+of course important to consider, not only the manner in which the
+drainage water could be ultimately disposed of, but also the
+modifications which would have to be made in the existing drains and
+irrigation canals—modifications which would need to be carried out
+without serious interruption to existing agriculture. A scheme which
+otherwise appeared attractive might easily prove to be impracticable by
+reason of the heavy expenses and inconveniences of the subsidiary works
+which would be required to make it effective.
+
+[Illustration: THE LIBYAN DESERT Showing Surface-Relief, Contours of
+Static Underground Water-levels, Distribution of Sand-dunes, and Routes
+of Principal Exploratory Expeditions in the West and South by Dr. JOHN
+BALL
+
+THE GEOGRAPHICAL JOURNAL JULY 1927]
+
+
+4. _The Artesian Water Supplies._
+
+
+The origin of the artesian water supplies of the Egyptian oases of
+Baharia, Kharga, Dakhla, and Farafra (Siwa seems hitherto generally to
+have been left out of consideration) has been a much-discussed question.
+Some geologists, myself amongst them, have always regarded the water as
+being derived from rainfall in the western Sudan, flowing underground in
+permeable beds towards the Mediterranean. Others have held the view that
+the oasis waters are merely Nile water which has penetrated more or less
+laterally into the adjoining deserts. The arguments that have been urged
+in support of the former view are, firstly, the high temperature of the
+water in many of the oasis wells; and, secondly, that the levels of the
+springs and wells are often much higher than those of the Nile in the
+same latitudes. To these arguments it has been justly replied that
+neither of them is conclusive; the high temperature of the outflowing
+water merely testifies that it has descended to considerable depths at
+some part of its underground path not very remote from the point of
+outflow, but really tells us nothing as to its place of origin; and the
+high level of the springs in Baharia, for instance, as compared with
+that of the Nile in the same latitude, might be accounted for by the
+seepage from the Nile taking place fairly high up in the river’s course.
+There the question remained until 1925, when I was able to visit and
+determine the positions and levels of a number of water-sources farther
+in the interior than any of those on which the “Nile” argument was
+based. Amongst other level-determinations, I ascertained that the Sheb
+well is 228 metres above sea-level, and that Merga Lake, lying far to
+the south-west (in lat. 19° 3′, long. 26° 18′), is at an altitude of no
+less than 509 metres above the sea. Shortly before my tour in the Sudan,
+Hassanein Bey had confirmed Rohlfs’ level of 400 metres for the Kufra
+water-sources, and I had found that of Abu Mungar (north-west of Dakhla
+Oasis) to be 117 metres. At all these places the water-supplies are
+derived from underground sources in the same rock—namely, the Nubian
+sandstone, which covers such vast areas in the Sudan and Egypt.
+
+I had thus four well-determined natural water-levels at the corners of a
+great quadrilateral whose sides averaged over 500 kilometres in length,
+and embraced more than 20 square degrees of the Earth’s surface. Now
+just as in solid geometry the levels of any three points determine the
+inclination of an oblique plane to the horizontal, so on the Earth any
+three levels will determine the inclination of a surface to the geoid
+(of course assuming both geoid and surface to have the same curvature).
+And on making the calculation, I found that I obtained practically the
+same degree and direction of inclination for the natural water-surface,
+whichever three of the four known points I utilized for the calculation.
+In other words, I found that if I took, say, the levels of Kufra, Abu
+Mungar, and Sheb, and deduced from them the inclination of the water-
+surface to the horizontal, I could _calculate_ the level of Merga pretty
+exactly. Extending the trial, I found that I could do the same with a
+fairly close approximation for the other wells in the Sheb
+neighbourhood, and also for wells in the oases of Dakhla and Kharga. The
+conclusion seemed irresistible that all the wells considered were fed
+from a continuous sheet of underground water; and it was evident that
+this water did not come from the Nile, firstly because of the high level
+of Merga, which is above that of the swamps of the Bahr el Ghazal and
+other western feeders of the White Nile, and secondly because of the
+direction of the downward slope of the underground static water-surface,
+which is from the south-west, instead of from the south as it would have
+been had the water been derived from the Nile in the Bahr el Ghazal
+region. The true source of the water must be somewhere more or less
+nearly on a line drawn south-west from Dakhla, for this is the direction
+of upward slope of the underground static water-surface; and if such a
+line be drawn on a map of Africa, it will be found to lead towards the
+Erdi and Ennedi region, on the borders of the Chad basin. It is in the
+highlands of Eastern Erdi and Ennedi, therefore, that we must look for
+the source of the artesian water of the Egyptian oases. What is known of
+this region from the recent explorations of Colonel Tilho lends good
+support to our conclusion.[17] It is a bare and rugged sandstone
+country, where, in spite of a rainfall which is by no means negligible,
+permanent water-sources are scanty, and where, in consequence, there
+must be a considerable absorption of moisture by the rocks; and it lies
+at so high an altitude as to give sufficient “head” for the absorbed
+water to percolate through the porous sandstones and thus to reach
+Egypt.
+
+Being convinced that I had at last arrived at the true origin of the
+artesian water, I next began to entertain the idea of attempting to make
+a map which would show the contours of the underground water-sheet, and
+from which, in conjunction with the contour-map of the surface which I
+had already prepared, I might be able to predict the depth of boring
+required to tap the underground water at any point in the desert. But a
+little consideration showed that this idea was an impracticable one, by
+reason of our ignorance of the underground geological structure over the
+greater part of the desert. The underground water would naturally pass
+along permeable sandstone beds, often confined between impermeable clays
+above and below. And although the general structure of the Libyan Desert
+is doubtless one of simplicity as compared with that of other parts of
+Egypt, yet we know, from observations in the oases and in the Owenat
+region, that the beds are in some places folded and faulted, and that in
+others they have been uplifted and entirely removed by denudation, with
+the exposure of large areas of the underlying ancient crystalline rocks.
+The only parts of the desert for which the boring-depths could safely be
+predicted would be certain small areas within the oases, where wells
+have been sunk in sufficient numbers to give us definite information as
+to the local underground structure; and underground-water maps of these
+small areas, though they might usefully systematize our knowledge
+concerning them, would not be of any use for predictions at points
+situated elsewhere in the deserts.
+
+But while it was thus impracticable to prepare maps showing everywhere
+the depth at which underground water actually exists, I conceived that
+it might be quite possible to prepare maps showing _static water-
+levels_; that is, the levels to which the water would anywhere rise
+hydrostatically when once it _was_ tapped by borings. For the slope of
+the static water-surface between known points will be largely
+independent of the underground structure of the intervening country.
+Apart from any physical changes which may still be going on in the
+underground rocks themselves through geological agencies—changes which,
+if taking place at all, must be so slow as to be negligible except in
+the course of centuries—the only factors which can affect the slope of
+the static water-surface, once it has been established, are variations
+either in the rate of supply of water to the beds, or in the rate at
+which it is removed from them. As regards variations in the rate of
+supply, it is obvious that variations in the annual rainfall of the Erdi
+and Ennedi region must cause very considerable variations from year to
+year in the amount of water received by the underground beds. But the
+resistance of friction to the flow of water through the pores of the
+sandstones is so great, that the annual oscillations of pressure must be
+rapidly damped out as the distance from the place of influx increases;
+consequently the levels of the water in the wells of the Egyptian oases
+(and even, so far as is known, that of the lake at Merga) show little or
+no annual variation. And with regard to variations in the rate of
+removal of water from the beds (by outflow to the Nile, or to the sea,
+or into lakes wherein it evaporates, or by the exploitation of wells and
+springs for irrigation purposes), these changes, though possibly in some
+cases they may be progressive, and in restricted localities very
+sensible, can exercise but little influence from year to year on the
+general distribution of water-pressure within the underground strata. We
+may therefore conclude that the gradient of the static water-surface
+will everywhere have assumed practically a steady state. Unlike the
+actual water bearing beds themselves, which may be much folded, the
+static water-surface will in general have simple gentle slopes
+everywhere in the open desert. In the oases, of course, where numbers of
+wells yielding large outputs have been bored in proximity to each other,
+the static water surface will be wrinkled; but over the vast bulk of the
+desert the contours may be expected to be smooth curves. The
+diagrammatic section below will, I think, make clear this point about
+the general non-dependence of the shape of the static water-surface on
+the geological structure:
+
+[Illustration: _Diagrammatic section showing that the static water-level
+is largely independent of the underground structure_]
+
+In the diagram, FEKHG represents a water-conveying stratum, folded
+throughout its course and faulted at HK, but having a general downward
+slope from F to G. A and B represent points at which the water just
+rises to the ground-level, either through natural fissures or in
+artificial borings. The straight line drawn through A and B represents
+very approximately the static level at any point between A and B, that
+is, the level to which the water would rise in bores carried down to the
+water-bearing bed. A boring at C, for instance, would have to go down to
+E to tap the water, but once the bed was tapped the water would rise in
+the bore as far as D. At the fault HK, the pressure of the water at K
+will cause it to rise through the crushed rock at the fault-plane and
+re-enter the porous stratum at H. If there is a considerable outflow at
+B, and the fault-plane is a very narrow fissure, we may expect some drop
+in the line AB over the fault, by reason of the extra frictional
+absorption of head at this place. But unless the thickness and degree of
+permeability of the fault-rock are markedly different from those of the
+sandstone bed itself, the drop of pressure will not greatly disturb the
+general slope AB. In any case it is apparent that the level of the
+static water-surface at any place between A and B is capable of being
+estimated with a far closer degree of approximation than is the level of
+the water-sheet itself. We may therefore justifiably assume a uniform
+gradient for the static level between points at which that level is
+known, disregarding folds in the strata; and though we cannot entirely
+allow in detail for unknown faults and variations in permeability, it
+must be borne in mind that the _total_ effect of all the unknown factors
+between any two known points is already automatically allowed for in our
+data. It is only the variations from uniformity, due to the unknown
+distribution of the faults and of the departures from the average
+permeability, which can affect us; and these variations and departures
+are probably but small in most of the great unexplored areas of the
+south-west of Egypt, where the geological structure, from all we know of
+it, appears generally to be remarkably uniform.
+
+The first requisite for the construction of a map showing the contours
+of the static water-surface was, of course, a sufficiency of well-
+determined positions of points where the static water-level of the
+artesian supply was fairly exactly known. Such points are the springs
+and wells of the various oases, the surfaces of lakes occupying
+depressions and presumably fed by underground supplies, and any places
+on the Nile where the river taps artesian beds.
+
+In regard to the wells and springs, it was obvious that only those known
+to derive their supplies from artesian sources could be utilized as
+giving points on the static water-surface. This consideration cut out
+from the discussion the springs of Owenat and Arkenu, which are known to
+be fed by local rainfall, and also the small water-sources of Kurkur,
+Dungul, Nakheil, and Ain Amur, which occur in situations where
+percolation from occasional local rainfall seems to be the only possible
+source of supply. And for reasons which will appear presently, none of
+the wells and springs situated to the north of the Siwa-Qattara-Faiyum
+chain of depressions could be considered as entering into the problem.
+With these exceptions, every water-source situated within the area of
+the Libyan Desert covered by the map, and whose level was known, was
+utilized; but in the greater oases and in the Wadi Natrun the wells and
+springs are so numerous and so close together that in these localities
+it was necessary to select one or two wells as representatives of a
+group. I had no hesitation in including the wells of Siwa and the Wadi
+Natrun, because the temperature of some of the wells and springs of
+Siwa, and the quantity of the output of water at both places, seem to me
+to afford conclusive evidence of the artesian character of their
+supply.[18] The wells of the little oasis of Lageita, to the east of the
+Nile near Qena, were included, for although they are not in the Libyan
+Desert, they most probably derive their supplies from the same
+underground flow which feeds the western oases. I have included only one
+well in which the water does not rise nearly to the ground-level. That
+well is one which was bored by the British Army during the Great War, at
+a place called B6, some 40 kilometres to the east of Baharia Oasis. The
+level of the ground at this point is 112 metres above sea-level, and as
+the water was stationary in the bore at 78 metres below the ground, the
+static level here is 34 metres above sea.[19] The well is said to have
+yielded some 800 gallons per hour without the water-level in the bore
+being sensibly changed.
+
+The level of the well at Sarra has recently been determined by Prince
+Kemal el Din; but I have not included it in my data, because he informs
+me that the water-level fluctuates by 20 metres or more in different
+years, while the level of the artesian water of the Egyptian oases and
+Merga is very nearly constant. The inference I draw from the great
+fluctuations in the water-level at the Sarra well is that it is
+dependent on percolation from a more or less local rainfall rather than
+on the same flow which feeds the Egyptian oases.
+
+
+5. _Permanence of Lakes._
+
+
+In regard to lakes and salt-marshes, the permanence of those occupying
+the depressions of Areg, Bahrein, Sittra, and Qattara can only, I think,
+be adequately explained by regarding them as fed, at least in part, by
+underground supplies coming into them from the south. The total area of
+the lakes of Bahrein, Nuemisa, Sittra, and Moghara is nearly 20 square
+kilometres, and that of the salt-marshes (_sabakha_) is not less than
+5000 square kilometres. The depressions are situated in a region which
+is nearly rainless; in Siwa the mean annual rainfall is only about a
+quarter of an inch, and that in the depressions farther south, such as
+Bahrein and Sittra, is doubtless even smaller. The mean daily
+evaporation from the lake-surfaces cannot well be less than some 4 mm.,
+which would mean a lowering of the lake-levels by evaporation of 1½
+metres each year unless there was some inflow to make up for the loss.
+And though the rate of evaporation from the salt-marshes, area for area,
+is doubtless very much smaller than that of the lakes, the 250-fold
+greater extent of the marshes makes it certain that the total quantity
+of water annually evaporated from them must far exceed that from the
+lakes.
+
+It appears unlikely that the loss by evaporation in the lakes and
+marshes can be entirely made up from local rainfall and by seepages from
+the northern slopes. The rocks forming the surface of the great Miocene
+plateau, 200 metres high, which separates the depressions from the sea,
+are chiefly limestones and clays; the beds are nearly horizontal, but
+such slight dips as exist are believed to be towards the sea. The
+average annual rainfall on the coastal portion of the plateau is about 6
+inches; but it falls off rapidly inland, till it is only about a quarter
+of an inch near Siwa. The heaviest rainfall on the plateau thus occurs
+along a strip parallel to the coast, where it is largely drained off
+towards the sea by the gullies which indent the plateau-edge. Of that
+which falls on the plateau-surface farther inland comparatively little
+is absorbed, owing to the generally non-porous nature of the uppermost
+rocks; after a heavy shower, water lies on the surface in shallow pools
+for a few days and is soon evaporated. So impervious to water is the
+limestone in this region, that the Romans excavated chambers in it to
+form reservoirs, of which many hundreds still exist. Nor can we think
+that much surface drainage-water from the country to the south ever
+finds its way into the depressions; for there is an almost complete
+absence of drainage-lines entering them. At the feet of the northern
+scarps of the Qattara depression, and along the north-eastern shores of
+the lakes in the Wadi Natrun, there are, indeed, small springs which
+show that some of the rain falling on the plateau does actually
+penetrate the rocks and escape by seepage into the depressions. But the
+amount of this seepage appears to be insignificant compared with the
+volume of water which must annually disappear from the lakes and marshes
+by evaporation. A further consideration bearing on this point is that
+whatever may have been the agency by which the depression of Siwa was
+formed, that same agency almost certainly operated to produce the other
+depressions of the northern chain; and it seems most unlikely that a
+connection should have been opened up with the underground water-bearing
+beds in Siwa, and not also in the larger and much deeper depression of
+Qattara.
+
+It would be a difficult matter to estimate the relative proportion of
+the water entering the depressions by underground flow from the south,
+to that contributed by local rainfall and seepage from the northern
+slopes. But that is not necessary for our immediate purpose. It is
+sufficient to show that there must be _some_ influx into the depressions
+from the same source as that which supplies the wells of the greater
+oases, to establish the existence of that underground water-connection
+which is all that we need to justify us in regarding the levels of the
+lakes and salt-marshes as furnishing us with points on the static water-
+surface; and from the considerations mentioned above it seems to me
+certain that some influx of underground water really does take place.
+
+I have also thought it justifiable to include the Birket el Qarun in my
+collection of static water-level data, because although that lake was
+probably first formed by an overflow of the Nile into the Faiyum, and is
+even now being fed by Nile water through the Faiyum drains at the rate
+of some 350 million tons a year, there is a certain amount of evidence
+suggesting that it has some underground water-connection with the
+Qattara depression. That evidence, to which attention was first drawn by
+Professor Schweinfurth,[20] consists in the relatively low salinity (1·3
+per cent.) of the lake, notwithstanding the long period through which it
+has been subject to evaporation and the fact of its having shrunk to
+dimensions very much smaller than it possessed in ancient times. Unless
+there has been a large underground efflux of salt water from the lake,
+it appears impossible to account for its present degree of freshness. In
+Professor Schweinfurth’s day, of course, the existence of the Qattara
+depression was unknown, and it was puzzling to suggest where the salt
+water had gone to.[21] An underground leakage from the Birket el Qarun
+into the Qattara depression is quite conceivable, for although the two
+places are separated by some 200 kilometres, there is a very
+considerable fall between them. Thus the salt in the marshes of the
+Qattara depression may possibly have come in part from the Birket el
+Qarun. The present rate of discharge of the Faiyum drains into the lake
+is, however, just sufficient to make up for an average daily evaporation
+from the lake-surface of a little over 4 mm., which is about the rate we
+might expect; and although the level of the lake-surface has fallen some
+5 metres since observations of it were first made in 1886, it is now
+nearly stationary; hence it does not appear likely that there is much
+underground leakage at present. If the former leakage from the lake took
+place by lateral flow into porous strata near its surface, of course the
+leakage may have been arrested by the lowering of the lake-level
+uncovering the porous beds into which it took place; but I think a more
+likely explanation is that the leakage occurred at or near the bed of
+the lake, and has gradually been reduced by the continued deposition of
+Nile mud on the lake-bottom, and by the diminution of head due to the
+fall in the water-level.
+
+In regard to the tapping of the artesian waters by the Nile, there is
+only one locality in which this is known to take place; but the quantity
+of underground water which is there withdrawn by the river is probably
+very considerable. When I was surveying the Nile Valley between Aswan
+and Korosko in December 1898, I observed that in the vicinity of the
+temple of Dakka (about 105 kilometres south of Aswan) the lands on the
+west bank of the river were being irrigated with warm water, drawn by
+“sakias” (water-raising machines) from pits sunk in the alluvial flat
+which extends between the river and the edge of the sandstone desert.
+The length of the tract over which the warm water was being withdrawn
+for irrigation was found to be about 16 kilometres, stretching from 2
+kilometres north of Dakka temple southwards to the temple of Maharraga;
+and the width of the alluvial tract at Dakka, where it is widest, was
+about 1300 metres. Some of the water-pits were more than a kilometre
+from the river. Levelling from the Nile (the surface of which was then
+about 99 metres above sea) across the cultivation to one of the sakia-
+pits 750 metres west of the river, I found the level of the ground at
+the sakia-pit to be 7·9 metres above that of the Nile, and the water-
+surface in the pit to be 8·4 metres below the ground-level; there was
+1·2 metres depth of water in the pit. The temperature of the water in
+the pit I found to be 83° F., while that of the Nile was 60° F. and that
+of the air was 67° F. The headman of Dakka told me that the exploitation
+of this warm underground water had begun about 1887; they dig out the
+sandy mud, and then see the water oozing rapidly into the pit out of the
+sandstone below. On crossing to the east bank of the river, I found that
+there also the warm water was being similarly raised for irrigation,
+though to a smaller extent, because on that side the sandstone desert
+approaches more closely to the river and there is much less cultivable
+land. The exploitation of the water on the east side of the Nile
+extended only over a distance of about 5 kilometres along the bank, with
+a maximum width of alluvial plain of 600 metres, just at the place where
+the great Wadi Alagi debouches into the Nile Valley. As the sandstone
+bed from which the warm water issues is less than 2 metres below the
+level of the water-surface of the Nile, and the water occurs on both
+sides of the river, it is certain that the water-bearing bed is cut
+through by the Nile channel itself; the seepage into the river along the
+stretch of 16 kilometres must therefore be very considerable. It seems
+evident that the water is not derived from the bed of the Wadi Alagi,
+great drainage-channel though that wadi is; for we could not then
+account for the temperature of the water, nor for its appearing to a
+larger extent on the west bank than on the east, with the river in
+between. Moreover, the water appeared to be much more free from salts
+than we should expect it to be if it were merely drainage from the Wadi
+Alagi. It strongly resembles, in fact, both in temperature and
+character, the artesian water of the greater oases, and there can hardly
+be the smallest doubt that at Dakka the Nile is not only continually
+abstracting artesian water from the same underground water-sheet that
+feeds the oases, but is abstracting it in far larger quantities than
+those yielded by all the oasis wells and springs put together.[22] It is
+certainly remarkable that the place where considerable supplies of warm
+underground water enter the Nile should coincide with the embouchure of
+what is perhaps the greatest drainage channel of the Eastern Desert of
+Egypt; but I think it is likely that the explanation of the coincidence
+may be a tectonic one; the water-bearing beds may have been brought up
+by a local fold in the strata, and the same fold may in some way have
+conditioned the formation of the primitive drainage-line which was
+ultimately to become the Wadi Alagi.
+
+Having now indicated briefly the grounds for their acceptance, I give
+below a table showing the various points which I have adopted as
+furnishing data for constructing the contours of the static water-
+surface underlying the Libyan Desert, together with the altitudes of the
+points above or below sea, and the sources of these level-data. The
+levels are doubtless in some cases slightly inaccurate; but a few metres
+of error are immaterial to the object in view, and it is believed that
+even those levels which rest on barometric determinations are
+sufficiently accurate for our purpose.
+
+          LIST OF ADOPTED POINTS ON THE STATIC WATER-SURFACE
+
+         _Place._               _Level        _Determined by._
+                               (metres)._
+
+  Wadi Natrun, surface of         − 23    Ball, Trigonometric
+  lakes                                   levelling, 1914.
+
+  Birket el Qarun, surface of     − 45    Survey of Egypt, 1926.
+  lake                                    Based on spirit levelling
+                                          from Alexandria.
+
+  Moghara, surface of lake        − 23    Walpole, Trigonometric
+                                          levelling, 1924.
+
+  Qattara Depression, various   − 80[23]    „   „   „
+  points on salt-marsh, the
+  lowest being
+
+  Sittra, surface of lake         − 16      „   „   „
+
+  Areg, surface of lake           − 25      „   „   „
+
+  Siwa                            − 17      „   „   „
+
+  Jaghbub                         + 32    Hassanein, Barometric
+                                          observations, 1923.
+
+  Jalo                            + 61      „   „   „
+
+  Bir Butaffal                    + 98      „   „   „
+
+  El Harrash                     + 310      „   „   „
+
+  Awadel (Kufra Oasis)           + 434      „   „   „
+
+  Ezeila (Kufra Oasis)           + 389      „   „   „
+
+  Bawitti (Baharia Oasis)        + 129    Ball, 1917, and Walpole,
+                                          1924. Trigonometric
+                                          levelling.
+
+  El Hez (Baharia Oasis)         + 134      „   „   „
+
+  B6 Well (water surface in)      + 34    Walpole, Trigonometric
+                                          levelling, 1924, and
+                                          military records of depth,
+                                          1916.
+
+  Lageita (Eastern Desert)       + 121    Murray, Trigonometric
+                                          levelling, 1921.
+
+  Farafra                         + 90    Ball, Barometric
+                                          observations, 1924.
+
+  Abu Mungar                     + 117      „   „   „
+
+  Mut (Dakhla Oasis)             + 119      „   „   „
+
+  Kharga (average of numerous     + 70    Beadnell, Spirit levelling,
+  wells)                                  1909.
+
+  Ain Ismail (Kharga Oasis)       + 67    Ball, Barometric
+                                          observations, 1925.
+
+  Bir Murr                       + 156      „   „   „
+
+  Bir Abu Hussein                + 182      „   „   „
+
+  Bir Kassaba                    + 176      „   „   „
+
+  Sheb Well                      + 228      „   „   „
+
+  Safsaf                         + 230      „   „   „
+
+  Bir Terfawi                    + 244      „   „   „
+
+  Merga, surface of lake         + 509      „   „   „
+
+  Dakka, water-surface in         + 99    Ball, Spirit levelling from
+  wells                                   the Nile, 1898.
+
+To prepare a map showing the contours of the static water-surface, I
+took a graticuled sheet and plotted the above-scheduled points on it in
+their ascertained geographical positions, affixing the adopted level to
+each. To get points on the various contours at vertical intervals of 100
+metres, I joined each pair of points on the map by a pencil line, and
+then, by interpolation from the terminal levels, found the points on
+this line where the various contours crossed it, on the assumption of a
+uniform gradient between the terminal points. Many of the lines thus
+drawn of course crossed each other, so that interpolation of the static
+level at the point of their intersection gave two values for the same
+place. But their agreement was wonderfully close, considering the
+fewness and the scattered nature of the datum-points, and this went a
+long way to encourage me in the belief that the hypothesis on which I
+had been working, namely, that of an underground water-connection
+between all the points included in my list, was correct. I found that
+the contours of the static water-surface could be approximately
+represented by a series of smooth curves, as shown (on a reduced scale)
+in the outline map below.
+
+Apart from the general smoothness of the curves, especially in the
+south-west, where it may in part be due to the scantiness of control-
+points, the most striking thing on this outline map is the north-
+eastward projection of the 100-metre static contour, where it runs out
+so as to include Baharia Oasis. The reasons for this projection are
+obviously the efflux of water, on the one hand north-westwards into the
+great Qattara depression, and on the other hand into the Nile at Dakka.
+The indentation of the 400-metre contour near Kufra is likewise
+explained by the withdrawal of water from the wells of that oasis. The
+general parallelism of the curves in the south-western part of the map,
+showing a gradual rise in a south-westerly direction towards the Erdi
+and Ennedi country (which, as I have already stated, is the most
+probable source of the underground water) is strikingly apparent. I have
+not been able to extend the contours far to the east and west of Merga,
+for lack of control-points. It is much to be hoped that some future
+traveller will determine the water-levels at Selima and Lagia, which
+would enable the static contours to be extended into the region between
+Merga and Dongola; provided, of course, that an examination of the
+water-sources at these places proves their supplies to be artesian.
+
+The most effective way of testing any working hypothesis in natural
+science being the prediction of hitherto unobserved facts, I venture to
+forecast that if, as is most likely, the water-sources of Selima and
+Lagia are artesian, their levels when eventually determined will not be
+found to differ very much from 270 and 390 metres above sea
+respectively. These are the approximate levels deduced by prolonging the
+static water-contours of my maps into the localities of these wells,
+assuming the contours to continue as smooth curves.
+
+[Illustration: _Outline map of the Libyan Desert, showing the points
+where the static water-levels are known, and the deduced contours of the
+underground static water-surface, on the hypothesis of a continuous
+hydraulic connection between the points_]
+
+Another interesting prognostication which I think may fairly be deduced
+from the map is that if ever the well at Sarra is considerably deepened,
+a much more abundant water-supply will probably be obtainable. The
+ground-level at Sarra, according to observations made by Prince Kemal el
+Din, is 461 metres above sea, and the water-level in the well varies in
+different years from about 390 to about 410 metres above sea. But an
+examination of the static contours of the map shows that the static
+level of the true artesian water in the neighbourhood of the well is
+probably somewhere about 500 metres above sea, though an exact
+estimation of the static level at that spot is not possible because of
+the lack of data farther west. As already remarked on p. 110, I think
+the present supply at Sarra is derived from more or less local rainfall,
+conveyed by higher-lying permeable strata than those which convey the
+main artesian supplies of Kufra and the Egyptian oases; by deepening the
+well considerably, lower-lying beds might be reached so as to tap the
+main supply, and the water might even be expected to overflow at the
+surface.
+
+Interesting as are the static water-level contours in themselves, they
+become vastly more so when superposed on the ground-contours, as is done
+in the larger map (_G.J._, July, following p. 96). From the two sets of
+contours on that map we can estimate at any point the approximate depth
+of the static water-level below the ground; and this information affords
+new light on some of the most interesting, but hitherto the most
+difficult, of the problems connected with the Libyan Desert.
+
+[Illustration: _Pottery Hill, possible site of “Zerzura,” from the
+south-west_]
+
+[Illustration: _Bir Kassaba, a watering-place on the Darb el Arba’in_]
+
+[Illustration: _The waterless stretch of the Darb el Arba’in between Bir
+Murr and Kharga_]
+
+[Illustration: _Bir Sheb, a well on the Darb el Ar ba’in_]
+
+
+6. _Can the Present Water-supplies of the Mediterranean Littoral be
+supplemented by Artesian Borings?_
+
+
+The present water-supplies of the Egyptian portion of the Mediterranean
+littoral, derived mainly from shallow wells dependent on the local
+rainfall, are neither very abundant nor of very good quality. At one or
+two of the most important settlements along the coast, such as Matruh
+and Sollum, attempts have been made to improve the supplies by sinking
+wells to a considerable depth in situations where it appeared likely
+that the drainage from the inland plateau would be specially abundant.
+But these have met with little success; the yield has been found to be
+very moderate in quantity, and of poor quality owing to dissolved salts.
+The question has often been raised as to whether very deep borings,
+carried down right through the Tertiary strata and into the Nubian
+sandstone, might result in the procuring of an artesian supply of the
+same excellent water as occurs in the oases. Hitherto it has not been
+possible to give a definite answer to this question, and geologists have
+been reluctant to recommend deep borings, which would entail great
+expense, without feeling some assurance that they would be successful.
+The depth to the Nubian sandstone is unknown, but is certainly great;
+and if borings were carried down into the sandstone, it was not known
+whether the water would rise to anything like the ground-level. From our
+new map, however, we obtain a very decisive verdict on the matter. The
+Nubian sandstone, even if reached, would not be found to be charged with
+artesian water under anything like the pressure that it is in the oases;
+leakage into the Qattara and other depressions will have depleted the
+beds of much of the water coming from the south-west, and will have
+lowered the static head to such an extent, that the water left in the
+sandstone will have too little pressure to rise far into the bores. Any
+idea of sinking deep artesian wells along the coast to tap the Nubian
+sandstone can consequently be definitely abandoned. We are driven to the
+conclusion that in any attempt to improve the local water-supplies of
+the littoral settlements, we can count only on local rainfall for our
+primary source, and we must do our best to collect the run-off before it
+has had an opportunity to absorb much salt. The Romans evidently
+understood this when they excavated the large rock-cisterns on the
+plateau, of which there are hundreds. We cannot do better than imitate
+their example, and arrange for the collection and storage of a
+sufficient volume of rainwater as it runs from the rocky surface of the
+plateau. We may do this by restoring to use the old reservoirs; or we
+might possibly achieve our end by damming some of the rocky gullies
+which bring down the run-off from the plateau to the plain. Now that the
+artesian idea is shown to be out of the question, there is justification
+for a thorough investigation as to the best method of collecting and
+conserving the local rainfall.
+
+
+7. _Are the Artesian Water Supplies of the Oases diminishing?_
+
+
+The native cultivators in certain parts of Kharga and Dakhla have for
+some years past found that their wells no longer discharge at so high a
+level as formerly, and in consequence some of their land has gone out of
+cultivation. From this fact, and from the evidence of the former greater
+prosperity of the oases which is afforded by the various ruins of
+temples, forts, and villages, by the large areas of formerly cultivated
+lands, and by numerous sanded-up wells, it has sometimes been inferred
+that the total yield of the oasis-wells is now but a fraction of what it
+formerly was. But, as Mr. Beadnell has pointed out,[24] the remains of
+the past which exist in the oases belong to successive generations, so
+that we cannot fairly draw such a conclusion from them; and the
+reduction or cessation of the discharge of certain wells does not
+necessarily imply any falling-off in the total water-output of the
+oases. Mr. Beadnell’s experiments on flowing wells in Kharga have
+clearly shown how the opening of a new well at a slightly lower level
+will affect the discharge of an old well, even one at a distance of a
+kilometre or more, by lowering the static head in its vicinity.[25] And
+since a large number of wells have been bored in recent years both in
+Kharga and Dakhla, it is most likely that the discharge from these wells
+has caused a falling-off in the yield of older ones situated at slightly
+higher levels. In this connection it will be well to note that it is not
+the mere _existence_ of a new well that affects the static head, but the
+_discharge_ from it. If the new well is securely closed so as to
+discharge nothing, it has then no effect on the static head and
+therefore none on the neighbouring wells. But for this to hold, it is
+important that the well which is closed should be closed throughout its
+entire depth; it is not sufficient merely to close its mouth so that it
+does not discharge any water on to the ground, for there may still be
+rapid leakage somewhere in the bore (unless effectively cased) into
+porous unsaturated underground strata. Owing to the rapid rate at which
+iron pipes are corroded in the wells of the oases, leakage of this kind
+is more likely to happen with an abandoned modern well cased with iron
+piping and plugged near the top, than with old wells which were filled
+up with clay and sand. These factors, the mutual interference of wells
+and the importance of preventing underground leakage, especially from
+abandoned wells, being now thoroughly understood, steps are being taken
+towards ensuring that future sites for new wells shall be judiciously
+selected, and that leakage and waste from abandoned wells shall be as
+far as possible arrested.
+
+The method of carrying out measurements of well-discharges in the oases
+is so inaccurate, and the records of the past output so defective, that
+it is not possible to gather from them whether the total yield of
+artesian water is at present diminishing or not. Mr. Beadnell considers
+it likely, however, that the general average water-pressure in the oases
+has been very much reduced within the historical period, owing to the
+long-continued exploitation of the artesian supplies.[26] The general
+study of the Libyan Desert which I have made in the last few years
+suggests that a gradual reduction in the static water-pressure in the
+oases may possibly have been brought about by other agencies than the
+exploitation of the water in the oases themselves.
+
+The first and most important of these other agencies is the withdrawal
+of artesian water by the Nile in the neighbourhood of Dakka. As
+mentioned on p. 113, it is practically certain that sandstone beds
+carrying artesian water are cut through by the Nile along a distance of
+several kilometres in that locality, and the influx of artesian water
+into the Nile may far transcend in quantity that removed by the wells
+and springs of the oases. The Nile has probably deepened its channel in
+this region by a few metres within historical times, and thus cut
+through a greater section of the water-bearing beds. An increase in the
+sectional area of the beds cut through would naturally mean an increase
+in the quantity of artesian water passing into the Nile, and hence a
+lowering of the static water-surface extending perhaps to the oases.
+
+The second possible other cause operating to diminish the static head of
+the artesian water of the oases is the progressive desiccation of a lake
+which may once have occupied a part of the Qattara depression. As
+mentioned on p. 110, the floor of this great depression, large areas of
+which are 80 metres and more below sea-level, is partly covered by a
+salt-marsh, which is so soft and watery that it can only be crossed at a
+few places. The hundreds of great water-cisterns cut in the limestones
+of the plateau to the north of the depression—cisterns most of which are
+now dry—as well as other ruins along the coast indicative of a
+considerable former population, seem to show that the rainfall in the
+littoral region has within the historical period been greater than it is
+at the present day. When the rainfall in the coastal region was greater,
+there must have been more drainage into the Qattara depression, and what
+is now salt-marsh was thus possibly once a lake of some depth. Assuming,
+as I think is likely, that an underground water-connection exists
+between the marsh occupying the bottom of the depression and the
+artesian water of the oases, it is obvious that any progressive lowering
+of the lake-level consequent on the change of climate must have lowered
+the static water-surface in the country extending southwards towards the
+oases. In the oases themselves the lowering of the static surface would
+of course be much less than at the lake; but it is quite conceivable
+that even in the oases the lowering may have amounted to the few metres
+which would cause some of the older and higher-lying wells to cease to
+flow.
+
+As both the deepening of the Nile channel in Lower Nubia and the
+desiccation of the Qattara depression are probably still slowly
+progressive, it is possible that these causes may to some extent account
+for any slow lowering of the static water-surface in the oases which may
+be still going on.
+
+
+8. _“Lost” Oases—“Zerzura.”_
+
+
+Of all the questions asked by intending travellers in the Libyan Desert,
+none is more frequent than that as to the most likely whereabouts of
+undiscovered oases, and especially as to the possibility of finding the
+mysterious “Zerzura, or Oasis of the Blacks.” Hitherto the only aid
+which I have been able to render to such inquirers has been to acquaint
+them with the various statements which have been made by Arabs at
+different times as to the situation of Zerzura, with the routes which
+have been followed by others (including myself) who have sought in vain
+for it, and with the indications of old tracks which have been
+encountered by these previous travellers. So contradictory have been the
+various Arab statements, and so numerous the vain attempts to find the
+place, that I have at times felt almost convinced that “Zerzura” is a
+myth. But Owenat and Merga were little more than traditions until a year
+or two ago, and I think there is a sufficient possibility of the
+existence of undiscovered springs or oases to encourage a further look-
+out being kept for them, more especially as a consideration of the
+general surface-contours and static water-contours which are now
+available may furnish a new aid in the matter by narrowing down the
+field of search.
+
+As regards Arab traditions concerning Zerzura, the earliest account of
+them which I have been able to trace is that of Sir Gardner Wilkinson in
+his ‘Topography of Thebes and General View of Egypt,’ published in 1835,
+p. 359. Wilkinson’s book is now rather scarce, and his statement
+concerning Zerzura is so short that I quote it in full:
+
+“About five or six days west of the road from el Hez to Farafra is
+another Oasis, called Wadee Zerzoora, about the size of the Oasis Parva,
+abounding in palms, with springs, and some ruins of uncertain date. It
+was discovered about nine years ago (_i.e._ about 1826) by an Arab in
+search of a stray camel, and from the footsteps of men and sheep he
+there met with, they consider it inhabited. Gerbabo, another _Wah_, lies
+six days beyond this to the west, and twelve days from Augela; and
+Tazerbo, which is still farther to the west, forms part of the same
+Oasis; and they suppose that Wadee Zerzoora also communicates with it.
+The inhabitants are blacks, and many of them have been carried off at
+different times by the Moghrebins for slaves; though the “Vallies of the
+Blacks,” a series of similar Oases, lie still farther to the west.”
+
+In footnotes he adds:
+
+“It is supposed that the blacks, who invaded Farafreh some years ago,
+and kidnapped a great number of the inhabitants, were from this Oasis.
+
+“By another account Zerzoora is only two or three days due west from
+Dakhleh, beyond which is another _wadee_; then a second abounding in
+cattle; then Gebabo and Tazerbo; and beyond these is Wadee Rebeeana;
+Gebabo is inhabited by two tribes of blacks, the Simertayn and
+Ergezayn.”
+
+Particular interest attaches to Wilkinson’s account of the tradition,
+not only because of its being the earliest, and therefore less likely to
+be coloured by imagination than later versions, but also because of the
+remarkable fact that although some of the other places named in the
+above extract were unknown to European geographers at the time, they
+have all since been discovered; “Gebabo” and “Tazerbo” by Rohlfs, and
+“Rebeeana” by Mrs. Forbes and Hassanein Bey.
+
+The weak point in the account is the loose Arab way of stating
+directions. Gebabo (Kufra) is not _west_ of the Baharia-Farafra road,
+but south-west. If we amend the bearing in the first account
+accordingly, and take Zerzura at about midway between the Baharia-
+Farafra road and Kufra, it must lie near the intersection of the
+parallel of 26° with the meridian of 26°, or some 200 kilometres east of
+the Mehemsa Hattia, and some 150 kilometres north-west of Rohlfs’
+“Regenfeld” camp. In the second account given by Wilkinson, the words
+“due west” suggest that the bearing is more certain. Two or three days
+(say 120 kilometres) due west of Dakhla would put Zerzura in about
+latitude 25½°, longitude 27½°, or about 30 kilometres north of Rohlfs’
+“Regenfeld.” There is thus a difference of more than 100 kilometres
+between the two positions indicated by the accounts given to Wilkinson.
+
+Rohlfs evidently knew of the traditions regarding Zerzura before making
+his attempt to reach Kufra from Dakhla in 1874; and since such an
+intermediate oasis, if it really existed, would be an immense aid to him
+in attaining his objective, he made careful inquiries concerning it
+before leaving Dakhla. But he found that though every one in Dakhla knew
+the names of Zerzura and Kufra, no one could tell him where Zerzura was,
+nor how far it lay from Dakhla.[27] Evidently Rohlfs placed little faith
+in its existence, for during his journey he gave the name “Zerzura” to a
+locality, about 120 kilometres west-south-west of Dakhla, where he found
+nothing but very sparsely scattered vegetation.[28] Ascherson, who was
+with Rohlfs in Dakhla, was however informed by Hassan Effendi, one of
+the principal inhabitants of Mut, that about a hundred years before
+(_i.e._ about 1770) there had been frequent raids on Dakhla by Arabs
+from the south-west; and that in order to stop these raids the Mameluke
+Government of the time installed a military colony, called the Surbaghi,
+in the village of Qalamun. These Surbaghi went out and destroyed all the
+wells for seven or eight days’ distance along the road by which the
+raiders had come. This road, which up to that time had served as a trade
+route from Darfur, before the road through Kharga was opened, was in
+consequence forsaken. The road was said still to exist, and to lead into
+the desert from Mut. At a day-and-a-half’s journey from Mut there were
+said to be two “pillars,” half an hour’s distance apart, which Hassan
+Effendi’s guard likened to minarets. In the neighbourhood there were
+said to be large stone-quarries, and about thirty years previously an
+iron instrument had been found; the instrument was still in Hassan
+Effendi’s possession, and some wonderful stories had grown up as to the
+manner of its use.[29]
+
+Ascherson himself believed this information of Hassan Effendi’s to be
+substantially correct, but he thought the “pillars” might be merely
+pinnacle-shaped natural rocks. I am likewise inclined to believe it
+trustworthy, but I think the “wells” may have been merely water-
+dumps—that is, collections of jars of water. My reason for this view is
+twofold. In the first place, the ground-levels in that part of the
+desert are so far above the static water-level, that if wells were bored
+they would have to be very deep, and the water would not rise anywhere
+near the surface (there can, of course, be here no question of local
+rainfall as a source for the water in the “wells”); and in the second
+place, it seems very probable that the accumulation of large broken
+earthenware jars which I discovered in 1916 at the place I named
+“Pottery Hill” (latitude 24° 26′ 27″, longitude 27° 38′ 54″) is one of
+the dumps in question.[30]
+
+The importance of the above interpretation of Hassan Effendi’s story, if
+accepted, as I think it must be, lies in the proof which it furnishes
+that there cannot possibly be an undiscovered oasis anywhere near a
+point seven or eight days from Dakhla in the direction of Kufra. A
+consideration of the static water-levels and the contours of the ground
+would lead us to this conclusion if the views I have advanced on the
+underground-water question are correct; but the fact that the raiders
+would never have made such a large water-dump if water could have been
+readily got from a well in the vicinity makes the conclusion almost
+certain independently of my hypothesis, and thus incidentally tends to
+support the latter. Is it possible that “Zerzura,” which I am told
+signifies in Arabic a starling (but is commonly applied to any small
+bird), is here a corruption of some other name derived from “zeer,” a
+water-jar, and that instead of “the _oasis_ of the blacks,” Zerzura was
+really “the _water-depôt_ of the blacks”?
+
+Schweinfurth has recorded[31] a story which was told him in Kharga
+Oasis, that in 1872 some Arabs of a Darfur caravan, who had missed the
+road, found a small oasis about 1½ days’ journey to the west of Beris.
+The oasis was said to contain pools full of wild geese, date-palms, and
+a temple. People who went out of Beris to find the place returned
+unsuccessful. I am, however, rather inclined to think that the story has
+some basis of fact, because in 1898 I myself found springs and traces of
+ancient conduits in the dunes north-west of Beris, and there may be
+other water-sources beyond the point which I reached.[32]
+
+A writer in the ninth edition of Murray’s ‘Guide to Egypt,’ published in
+1896, gives the following different statements of Arabs as to the
+position of Zerzura:
+
+(1) Some days south of the Dakhla Oasis.
+
+(2) Five days west of Farafra Oasis.
+
+(3) Three days west of Dakhla Oasis.
+
+(4) Two or three days west of Selima Oasis.
+
+The first of these statements would correspond sufficiently well with
+Bir Terfawi, which is about 280 kilometres due south of Dakhla Oasis;
+the second would place Zerzura about in latitude 27°, longitude 26°; the
+third corresponds with the second account given to Wilkinson; while the
+fourth would place it far to the south-west, in latitude 21½°, longitude
+28°, or rather less than halfway between Bir Terfawi and Merga.
+
+Mr. Harding King, who in his journeys in the Libyan Desert in 1909 and
+1911 devoted much attention to the collecting of native information
+regarding its geography, thinks that “Zerzura” may possibly be only a
+generic name applied to any mythical or undiscovered oasis.[33] He heard
+the name applied to the following localities:
+
+(1) Rohlfs’ “Sersura.”
+
+(2) The “Egyptian Oasis,” said to have been seen by an Arab from the top
+of a high black hill lying in the dune-belt ten long days by ordinary
+caravan from Kharga. Another Arab is said to have seen what is possibly
+the same place eight days somewhere to the south of Dakhla. Both these
+accounts agree that the place is a large oasis lying at the foot of a
+scarp and containing olive trees.
+
+(3) A stone temple eighteen hours’ journey west of Gedida in Dakhla
+Oasis.
+
+The information given to Mr. Harding King as to the position of the
+“Egyptian Oasis” would place it about in latitude 23°, longitude 28½°,
+or only some 50 kilometres north-west of Bir Terfawi; in fact,
+considering the vagueness of the information, it might correspond fairly
+well with Terfawi itself, except that there are no olive trees at
+Terfawi, nor is that place overlooked by any escarpment or hill. The
+“high black hill in the dune-belt” may just possibly be one of those I
+mapped near the farthest point I reached with Moore in 1916, in about
+latitude 24°, longitude 26°, though this would imply a direction south-
+west of Dakhla, not south. It is not likely to be Gebel Kamil, as that
+mountain lies much nearer to Merga than to Dakhla.
+
+The “stone temple,” according to the information given to Mr. Harding
+King, would be about 80 kilometres west of Dakhla Oasis, in about
+latitude 25½°, longitude 28°, or not very far from the second of the
+positions indicated by Wilkinson for Zerzura. But I think it is likely
+that the statement really refers to the “Deir el Hagar,” a well-known
+temple ruin much nearer to Dakhla.
+
+So much for the various statements as to whereabouts Zerzura _may be_.
+Let us now inquire where it is _not_. On the map I have indicated the
+principal routes followed by explorers of the southern and western parts
+of the Libyan Desert during the last fifty years. We may be tolerably
+certain that Zerzura is not on, or very close to, any of these routes;
+for although a depression within a kilometre or so of one’s track may
+easily be passed without notice, the existence of a large oasis, such as
+most of the traditions make Zerzura out to be, would almost certainly be
+betrayed to an explorer by animal-tracks leading to it from considerable
+distances, except possibly in places where the ground was very sandy.
+
+[Illustration: _Pottery Hill from the south: at foot, petrol and water
+supplies of Prince Kemal el Din’s Expedition of 1923_]
+
+[Illustration: _Jars at northern foot of Pottery Hill found by Prince
+Kemal el Din in 1923_]
+
+[Illustration: _Jars, worn away by sand-laden winds, found by Dr. Ball
+in 1917 at southern foot of Pottery Hill_]
+
+There is another method now available to us by which we may narrow down
+the search for Zerzura or other “lost” oases; and that is, by a
+consideration of the general contours of the country and those of the
+static water-surface. Every oasis in the Libyan Desert must owe its
+existence to one or other of two conditions: either it must depend on
+springs fed by local rainfall, in which case, like the oases of Owenat
+and Arkenu, it probably lies near to mountains of considerable height;
+or else it must depend on underground supplies, and must therefore lie
+in a depression wherein the ground-level and static water-level are
+practically coincident, as in the cases of Kharga, Dakhla, Farafra,
+Baharia, and Siwa. It is highly doubtful whether any mountain masses at
+all approaching the altitude of Arkenu and Owenat can remain
+undiscovered in any of the various areas in which Zerzura has been
+traditionally placed. Zerzura is immensely more likely to be in a
+depression, and indeed Wilkinson’s name _Wadi_ Zerzura almost
+conclusively points to that view. The depression is more likely to be
+shallow than very deep, for it presumably lies in the sandstone country
+of the south-west, and all the known depressions of great depth are
+confined to the limestone country of the north-east. It is quite easy to
+trace out on our new map the areas wherein the static water-surface
+would be reached by a depression of say 50 or 100 metres below the
+general ground-level as indicated by the contours. We must, however,
+remember that our ground-contours are liable to be considerably in error
+in areas where observations have been few—that is, in precisely those
+areas where the depression, if it exists, is most likely to be found. So
+we must allow a good margin for our depth, and I have chosen 100 metres
+on this account. On the map I have drawn the “locus” of points in the
+southern part of the Libyan Desert where the static water-surface is 100
+metres lower than the general ground-surface[34]; and I have edged with
+red the only areas in the west and south in which the two surfaces are
+within 100 metres of each other—that is to say, the areas within which
+comparatively shallow depressions with underground springs _must_ lie,
+if our contours of the two surfaces are drawn with even approximate
+correctness. It will be seen from the map that this restricts the search
+very considerably. In fact, if Zerzura is within the frontiers of Egypt,
+it lies in all probability either to the west of longitude 26° 20′ and
+north of latitude 26°, or to the east of longitude 27° and south of
+latitude 23° 30′. We may dismiss from our field of search all the broad
+tract of rising ground which extends from near the Dakhla escarpment
+south-westwards towards Owenat; for unless Zerzura is in a depression of
+great depth, it cannot possibly lie within this tract. This
+consideration shows that most of the previous rough estimations of the
+possible position of Zerzura must have been erroneous; and indeed it is
+remarkable how few of the various explorers’ tracks traverse the only
+two areas within which Zerzura, if it exists, almost certainly lies. The
+northern area has only been crossed by the Rohlfs expedition of 1874,
+and that near its eastern edge; while the western part of the southern
+area has only been crossed by Prince Kemal el Din’s expedition of 1925.
+Of all the Arab traditions, only those which would place Zerzura far to
+the west of Farafra, or far to the south-south-west of Dakhla, can now
+have any high degree of credibility. It can hardly, I think, be doubted
+that the various traditions refer to more than one place; and in view of
+the almost totally unexplored state of the only two areas in which our
+new contour-maps indicate the possibility of unknown oases existing, it
+is quite conceivable that at least one oasis may lie within each of
+them. In the northern area, Siwa would probably make the best starting-
+point from which to conduct a search, as it is easily reached by motor-
+car from Alexandria or Cairo, and exploratory journeys southwards from
+Siwa would mostly lie along the direction of the dune axes. In the
+southern area, Terfawi or Sheb would form the best base for exploration;
+at each of these places there is a good water-supply. I am inclined to
+think that the southern area is more likely to yield results than the
+northern one. Water was found to exist at a point some 15 kilometres
+west of Terfawi, and though no other source was observed on the way to
+Owenat, it was impossible to see very far on either side of the track,
+so that such a source might have been passed within a few miles without
+notice. Terfawi itself is very difficult to find, being inconspicuous
+even when one is fairly close to it; and as it is in a sandy area,
+tracks leading to it are soon obliterated. Another factor which favours
+this southern area is that, the country being all Nubian sandstone, the
+depth from the surface to the actual water-bearing beds (as
+distinguished from the depth to the static water-surface) is certain to
+be much less than in the northern area, where Cretaceous strata may
+overlie the sandstone; hence there is more likelihood of the existence
+of natural springs in the south than in the north.
+
+
+9. _Can Travel in the Great Tracts of the Libyan Desert which are now
+Waterless be facilitated by the Sinking of New Wells?_
+
+
+It has sometimes been asked whether tracts which it is now difficult or
+impossible to cross by camel, owing to the non-existence of wells or
+natural springs within them, might possibly be opened up to transport by
+the sinking of new wells along a proposed route, as has in fact been
+done by the Senussi in the case of the well at Sarra, between Wanianga
+and Kufra. Hitherto there have been no data from which to form an
+opinion on this question; but a study of the new map will enable at
+least a qualified answer to be given.
+
+To be a practicable proposition, a new well must fulfil two conditions:
+the first, that the boring must not be required to descend to a very
+great depth; and the second, that the water when struck must rise in the
+bore to within a reasonable distance, say 20 to 30 metres, of the
+ground-level at the place. The first of these conditions will be
+satisfied if the geological horizon of the water-bearing bed is
+comparatively near the surface; the second, if the level of the static
+water-surface in the locality is within 20 or 30 metres of the ground-
+level.
+
+As regards the first of these conditions, we know that the water-bearing
+beds underlying the Libyan Desert are situated within the geological
+formation called the Nubian sandstone. We may therefore eliminate from
+our consideration all tracts in which this formation is overlain by any
+great thickness of younger rocks, for all these rocks will have to be
+passed through in order to tap the water-bearing strata.
+
+Concerning the second condition, our map at once informs us that the
+areas in which the static water-surface is within 20 or 30 metres’ depth
+below the ground-level are decidedly limited. Even if we suppose that
+there may be unknown depressions extending to a depth of 70 metres below
+the general level of the country indicated by the contour-lines, we see
+that the localities in which the second condition would be fulfilled are
+confined to the tracts edged with red on the map. Outside these tracts,
+not only have we no chance of discovering oasis-depressions, but we also
+have no prospect of being able to bore wells in which water would rise
+to within a reasonable distance of the ground-level. Thus the broad
+tract of rising ground which extends south-westwards from Dakhla to
+Owenat must always remain a waterless waste.
+
+In the Egyptian portion of the Libyan Desert there are, as will be seen
+from the map, only two areas in the west and south in which new wells
+might successfully be bored: one extending for some 300 kilometres or so
+to the south of Siwa Oasis in the neighbourhood of the western frontier,
+the other extending for some 400 kilometres westward from the Nile in
+the neighbourhood of the southern boundary. Fortunately, however, these
+areas are so situated that wells sunk within them might be of
+considerable use in opening up the outermost parts of Egypt to
+exploration. A well near the western frontier about on the parallel of
+26°, for instance, might make it just possible for cars or camels to
+reach Owenat directly from Siwa, since it would divide the present
+waterless stretch of 830 kilometres into two stretches of about half
+that length; while one or two wells near the southern frontier between
+the meridians of 27° and 28° would render Owenat fairly accessible to
+camels coming from the Nile _viâ_ Terfawi or Sheb, and might be of great
+use to explorers or geologists desirous of making a detailed
+investigation of the Owenat and Arkenu region. From the little I saw of
+Gebel Owenat during my visit to it in 1925, its geology must be of
+extreme interest. The south-western part of the mountain appears to be
+entirely composed of granite and other crystalline rocks, while the
+eastern part presents huge cliffs of sandstone, with crystalline rocks
+showing only at the foot; there is evidently a great fault traversing
+the mountain mass, with a downthrow to the east; and the thickness of
+Nubian sandstone exposed on the eastern precipices is greater than that
+at any other place I have seen.[35] Gebel Kissu probably resembles the
+south-western part of Owenat in being entirely composed of crystalline
+rocks.[36] East of Gebel Owenat there are many igneous hills, some of
+which exhibit bands of a dark brown colour. These bands, though probably
+mostly igneous dykes, may possibly in some cases be the gozzany outcrops
+of veins containing metallic minerals; I had no opportunity of examining
+them at close quarters, but I saw enough to make me long for facilities
+to undertake such an examination.
+
+It may be remarked that a well in the northern area would probably have
+to be very deep, owing to the comparatively high geological horizon of
+the surface rocks there; but in the southern area, where the Nubian
+sandstone forms the surface rock, no great depth of boring would be
+likely to be required in order to tap the water-bearing beds.
+
+Apart from the question of new wells in the more remote parts of the
+Libyan Desert, a study of the map gives us some hints which may be of
+value should it ever be desired to sink wells in places nearer to the
+oases and to the well-known tracks. There appears, for instance, to be
+no reason why wells should not be successfully bored at intervals along
+the Darb el Arbain between the south end of Kharga and Lagia, or on the
+south side of the Qattara and other depressions of the northern part of
+the desert. In the southern area, the best sites to select for wells
+will be depressions wherein the ground-level approximates most closely
+to the static water-level; and readings of an aneroid barometer,
+compared with corresponding readings at a place of known altitude, such
+as Sheb or Terfawi, would enable the most favourable sites to be
+determined. Observations of the geological structure will also be of
+importance; for anticlinal folds, by bringing the water-bearing strata
+nearer to the surface, would diminish the depth to which borings would
+have to be carried to tap the beds; while faulting might likewise
+introduce favourable conditions by producing cracks and fissures along
+which the water could rise. It may be remarked that tamarisk-bushes are
+generally a sign that water exists at no great depth. In regard to wells
+in or near the northern depressions, it is obvious that sites on the
+south side of the lakes and salt-marshes should be selected; for not
+only is the artesian static level higher in the south, but the water is
+less likely to be contaminated by salts derived from the lake and
+marshes.
+
+
+10. _The “Tortoise Marshes” of Ptolemy._
+
+
+The passage in Ptolemy’s ‘Geographia’ (lib. IV. cap. 6, sect. 4) in
+which the position of the “Tortoise Marshes” is given may be translated
+as follows[37]:
+
+  There are two great rivers running into the Mediterranean; one of them
+  is the Gir, joining Mount Usargala with the Garamantic narrows, from
+  which, changing its course, the river is located in long. 42°, lat.
+  16°, and makes the Tortoise Marshes (Chelonitides Paludes), whose
+  position is long. 49°, lat. 20°.
+
+The information on which Ptolemy based this statement, at least as
+regards the river _Gir_, must have been very fragmentary, if not,
+indeed, grossly erroneous; for there is no river flowing to the
+Mediterranean anywhere near the positions he gives. But there can be
+little doubt that his names _Gir_ and _Chelonitides Paludes_ refer to
+real places, and there has been much speculation among geographers as to
+their identification.
+
+Thus, for instance, Knoetel[38] suggested that the _Gir_ may have been
+the Wadi Djedi, to the south of Biskra, and the Tortoise Marshes the
+modern Lake Melghir (lat. 34°, long. 6°)[39] Dr. William Smith[40]
+thought that the _Gir_ was really a branch of the Niger, and the marshes
+the modern Lake Fittri (lat. 13°, long. 18°). On the map of Africa on
+the 2-million scale published by the Geographical Service of the French
+Army in 1899 the Tortoise Marshes are depicted as lying in about
+latitude 19° 20′, longitude 27°, with a note stating that this
+delineation is taken from an earlier map of the Nile Basin by Miani, who
+regarded the marshes as being connected with the “dry river” which was
+then supposed to run northwards through Dakhla Oasis. Colonel Tilho has
+lately suggested[41] the lowlands to the north-east of Lake Chad (lat.
+18°, long. 17°) as a possible site for the marshes; while still more
+recently Mr. Harding King[42] has thought that they might perhaps be
+identified with the salt lake of Merga (lat. 19° 3′, long. 26° 19′).
+
+Of the various localities which have been suggested as possible sites
+for the Tortoise Marshes, the only one which I have visited is Merga. I
+found the salt lake at that place to be very small, covering only some
+10 acres; it lies at an altitude of 509 metres above sea-level, and
+though it is situated in a rather wide shallow depression, I saw no
+traces of any extensive salt-marshes around the lake, while the
+configuration of the surrounding country appeared to me to be such that
+the depression cannot possibly have formed part of a continuous
+drainage-channel.
+
+I do not know whether it has hitherto been suggested that Kufra Oasis
+may be the site of the Tortoise Marshes; but on correcting Ptolemy’s
+figures for the errors in his adopted position for Alexandria and in the
+length which he assumed for a degree of latitude, I find Kufra is in
+very much closer agreement with them than any of the places named above.
+In Ptolemy’s day, even the latitudes of but few places had been
+astronomically observed (Alexandria, where he himself resided, was
+supposed by Ptolemy to be in latitude 31°, instead of its true 32° 12′),
+and as the only method at that time known for astronomically determining
+differences of longitude was by the observation of eclipses, the number
+of observed longitudes was smaller still. The process by which Ptolemy
+deduced his position for the Tortoise Marshes and other places in the
+interior of Libya was most probably that of first estimating their
+distances south and west of Alexandria from travellers’ itineraries,
+then converting these distances into degrees of latitude and longitude,
+and finally subtracting the differences thus found from the latitude and
+longitude of Alexandria. But Ptolemy made the great mistake of assuming
+the length of a degree of latitude (or of equatorial longitude) to be
+500 stades, instead of the 700 stades which it really is. Thus all his
+dead-reckonings resulted in differences of latitude and longitude which
+were too great in the proportion of seven to five. If we correct
+Ptolemy’s position for Alexandria, and his dead-reckoning for the
+erroneous assumption which he made regarding the size of the Earth, as
+follows:
+
+                                            _Latitude._     _Longitude._
+
+  Ptolemy’s position for Alexandria             31°           60° 30′
+
+  Ptolemy’s position for the Tortoise
+  Marshes                                       20°           49°
+                                                -------       -------
+  Ptolemy’s difference of lat. and long.        11°           11° 30′
+
+  These differences reduced in the
+  proportion of 5 to 7 become, in true
+  degrees and minutes                            7° 52′        8° 12′
+
+  The true position of Alexandria is            32° 12′       29° 53′
+                                                -------       -------
+  Whence the corrected position for the
+  marshes in our modern system becomes          24° 20′       21° 41′
+
+we get lat. 24° 20′ and long. 21° 41′ for the Tortoise Marshes.
+Comparing this position with that of Kufra, we find that the latitude
+agrees very closely with the 24° 14′ which Hassanein observed at Taj,
+the principal village of that oasis; while the longitude, though more
+than a degree and a half west of that of the principal village, is
+almost exactly correct for Taiserbo, the north-western oasis of the
+Kufra group.
+
+Besides this remarkably close agreement as regards position, Kufra
+presents several natural features which would tend to support the view
+that it may be the locality which Ptolemy meant by the “Tortoise
+Marshes.” Not only is Kufra an extensive tract of relatively low-lying
+ground with numerous lakes and salt-marshes of very considerable size,
+but it has distinctly the form of a valley. Hassanein Bey repeatedly
+refers to it as a valley in his description[43]; Mrs. Forbes also speaks
+of the “Wadi of Kufra,” and mentions that as a result of a ride westward
+from Taj the “wadi” was found to have no definite ending to the
+west.[44] What more natural, therefore, than that some ancient traveller
+should have imagined that Kufra was a series of marshes formed by a
+river coming from the south-west? And is it not possible that Ptolemy,
+in endeavouring to piece together the scraps of information he could
+get, may have mistakenly inferred that this river ultimately reached the
+Mediterranean, and also have confused the account of it with those of
+other streams further south, which may have been branches of the Niger?
+
+
+11. _The Sand-Dunes._
+
+
+The sand-dunes of the Libyan Desert have been the subject of careful
+studies by Mr. Beadnell and Mr. Harding King, and practically all that
+was known concerning them up to the outbreak of the Great War is
+contained in the excellent papers by them which were read and discussed
+at meetings of the Royal Geographical Society in 1910, 1916, and
+1918.[45]
+
+With the commencement of the Senussi campaign in Egypt in 1915, the
+sand-dunes sprang into new and unexpected importance, from the fact that
+they formed one of the principal hindrances to the free movement of
+troops, and more especially of military motor transport, across the
+desert. Every line of dunes of any considerable extent had now to be
+carefully mapped and examined for possible car-passages through it; the
+result was to add greatly to our knowledge of the distribution and
+extent of these remarkable features of the Libyan Desert, especially in
+the northern parts, where many long lines of dunes were found of which
+the existence was previously unsuspected by geographers, though they are
+familiar landmarks to the Bedouin of the region, and all except the
+smallest bear Arabic names. After the cessation of hostilities, interest
+in the distribution of dunes was maintained, because of the increasing
+use of motor-cars in place of camels for transport across the desert;
+and this circumstance has recently led to light being thrown on the
+distribution of the dunes in those southern regions which lay outside
+the field of operations during the war.
+
+It is rather curious to note that although the sand-dunes form the
+greatest obstacle to motor-cars in the desert, yet it is chiefly by
+means of motor-cars that the true extent and distribution of the sand-
+dunes has been ascertained. Dunes are the most difficult of all desert
+features to map properly by ordinary reconnaissance methods with camel
+transport. Their smooth outlines provide no points on which
+intersections can be made, and no survey marks put on them will remain
+in place for more than a few hours, or at most a few days; they occur
+mostly in nearly level country, where it is impossible to find a station
+whence they can be overlooked; the absence of shadows on them renders it
+impossible to say whether one is looking at a single line of dunes, or
+at several lines, miles apart, one behind the other in echelon. The only
+sure way of mapping dunes is to traverse their sides along their entire
+length, and this is impracticable with camels owing to the enormous
+distances which would have to be covered without water. But with motor-
+cars one can run alongside them at 40 kilometres an hour instead of the
+camel’s 4, and their distribution can thus be rapidly and easily
+ascertained.
+
+[Illustration: _Prince Kemal el Din’s expedition of 1925 leaving Bir
+Terfawi for the waterless journey of 250 miles to Owenat_]
+
+[Illustration: _The well at Bir Terfawi in the palm clump_]
+
+[Illustration: _Jebel Owenat from the south: the triple peak (5635 feet)
+in centre_]
+
+[Illustration: _The lake at Merga located by Prince Kemal el Din’s
+expedition of 1925_]
+
+On the map I have shown the distribution of sand-dunes as far as it is
+known at the present day. A comparison of this latest map with that
+given by Mr. Beadnell in his paper of 1910 will show how large is the
+number of dunes discovered and mapped in more recent years; and it will
+be noticed how pronounced is the constancy of the general direction of
+all the lines in the Egyptian portion of the Libyan Desert. Though no
+new lines of dunes comparable in extent with the great Abu Moharik belt
+have been added to the map, some of the newly discovered lines are of
+very considerable length, and they show an even more remarkable ratio of
+length to breadth than the Abu Moharik belt; the Ramak dunes, for
+instance, extend from near Moghara south-south-eastwards in a straight
+line for more than 100 kilometres, with only a single small break, and
+their width nowhere exceeds 1 kilometre. Moreover, the linear
+arrangement in a direction from about 20° west of north to about 20°
+east of south is even more pronounced in the newly discovered northern
+dunes than it is in those previously known farther south. In the “great
+sand sea” of Rohlfs, to the west of Dakhla and Farafra, the same
+directional arrangement of the individual lines of dunes is very
+noticeable, at least in the part near “Regenfeld” where I have
+penetrated it; and I found the same general direction to hold for most
+of the lines of dunes which I crossed on the way from Terfawi to Owenat.
+During the war I received a number of reports of dunes extending nearly
+east and west, often in curved lines; but investigations on the spot
+showed practically all these reports to be erroneous, the commonest
+mistake having been that of sighting different lines of dunes from
+different places under the impression that they were a single one.
+
+The general south-south-easterly direction which is so marked a feature
+of the lines of dunes in the Egyptian portion of the Libyan Desert is
+not, however, preserved in the extreme south-west of the country. The
+line of dunes which crosses the Egypt-Sudan frontier in longitude 25°
+40′ has a direction of about 30° _west_ of south, and the dunes which
+extend from near Gebel Arkenu to the west of Gebel Owenat run about 38°
+west of south. From a point about 80 kilometres to the south-west of
+Gebel Owenat, the dune-lines make a still further distinct bend to the
+west, changing their direction by some 20° to about 58° west of south,
+and this latter direction is maintained by the dunes in the
+neighbourhood of Sarra Well.[46] According to the maps of Rohlfs and
+Mrs. Forbes, the dunes in the sandy tract to the north-west of Kufra
+follow approximately the same bearing as those near Sarra. The
+suggestion has been made that this swinging round of the direction of
+the dunes in the south-western part of the Libyan Desert may be due to a
+deflection of the prevailing wind by the mountain-masses. It is almost
+certainly caused by differences in the prevailing winds—in fact, the
+dune-lines probably afford a very exact index to the general wind-
+direction in the areas where they occur—but I am inclined to think that
+the regional distribution of atmospheric pressure, rather than local
+deflection by mountain masses, is the cause of the differences of wind-
+direction. The dunes, especially those near Sarra and Kufra, extend into
+localities which seem to me to be too distant from the mountains for the
+influence of the latter on the prevailing winds to afford a satisfactory
+explanation.[47]
+
+In certain areas, especially in the south, as for instance on the Arbain
+road from Kharga to Sheb, around Pottery Hill, and in the region between
+Owenat and Erdi, there are sand-covered tracts which can hardly be
+mapped as dunes, because there are no very marked crests. These tracts
+are easily crossed by camels, but give much trouble with cars, and would
+probably be classed as dunes by car-drivers; but they are really only
+vast undulating fields of drifted sand. On the old Darb el Arbain slave
+road between Kharga and Sheb, one travels for more than 100 miles over
+drifts of this kind. No footprint of camel marks the track, footprints
+being obliterated in an hour or two. But one is never in doubt about the
+way, for it is indicated by the skeletons of countless camels which have
+perished on the weary march.
+
+As regards the relation between dunes and the relief of the ground,
+there is a gradual rise in the general level of the country from north
+to south of about 1 in 1000, so that the southern ends of most of the
+Egyptian lines of dunes lie at higher altitudes than the northern ends;
+but I am fairly certain that this slight general inclination has nothing
+to do with dune-formation. The straightest and cleanest-cut lines of
+dunes are found in the flattest and most open parts of the desert, over
+which the wind can sweep without obstruction. When a line of dunes
+encounters in its southward progression a sudden fall in the ground-
+level, as for instance at the north end of Kharga Oasis and to the east
+of Pottery Hill, it usually breaks up; sometimes, as in Kharga, to
+resume its course as dunes in more open formation, and at other times,
+as near Pottery Hill, to spread out into a rolling plain of drifted
+sand. It is not uncommon for the south end of a line of dunes to split
+up, even on level ground, and the same can be seen, though less often,
+at the northern ends; but this is probably merely because the dunes are
+lower near the ends of the lines, and the width of the individual dunes
+is consequently less.
+
+The great lines of dunes are probably extending southwards at a very
+slow rate, under the action of the prevailing wind. In this connection
+it is interesting to note that Arabs always speak of the south end of a
+line of dunes as its head, and of the north end as its tail; just the
+opposite of what one would instinctively call them from merely looking
+at the map, but correct if the dunes are known to have a southward
+progression. Mr. Beadnell found the average rate of progression of
+isolated small dunes in Kharga to be 15 metres per annum; but he remarks
+that large dunes move more slowly than small ones, owing to the greater
+mass of material to be transported. It is interesting to make a rough
+guess at the antiquity of the dunes from the length of certain of the
+longer lines and an estimated rate of progression. The Abu Moharik belt,
+for instance, has a total length of about 350 kilometres; hence, if its
+tail has remained stationary and its head has advanced southwards at an
+average rate of 10 metres a year, some 35,000 years must have been
+occupied in its formation.
+
+As regards the height of the dunes, the only one I have carefully
+measured is that close west of Rohlfs’ camp at Regenfeld, which I found
+to be exactly 30 metres. Some of the dunes in the Abu Moharik and Ramak
+belts are considerably higher; but I doubt if any of the Libyan dunes
+rise much over 50 metres above ground-level, except in the “great sand
+sea” which commences some 14 kilometres to the north-west of Regenfeld,
+where Jordan estimated them to attain 100 metres or more.[48] The top of
+a high dune to the south of Melfa Oasis was found by triangulation to be
+172 metres above sea; the level of the ground at its foot is unknown,
+but is probably not very much above the sea-level, so that Jordan’s
+estimate of the height of some of the dunes in the “sand sea” is
+probably by no means an exaggerated one.
+
+Much has been written about eddies in connection with dune-formation.
+The only observations I have made in this connection have been when
+endeavouring to place temporary marks on dunes to form survey-points;
+and the observations seem to show that if eddies are artificially
+created at the top of a dune, a rapid lowering of the dune-crest
+results. At first I used to thrust a walking-stick or ranging-rod some 2
+or 3 feet into the dune-crest. The stick or rod, though quite firmly
+fixed at first, was always found after a few hours lying halfway down
+the dune-slope. The same thing happened with tripods firmly pressed down
+into the crest. When I was carrying out a little triangulation to
+determine the position of Rohlfs’ cairn at Regenfeld in 1924, I had
+occasion to place a mark on a high dune, and I used an empty 4-gallon
+petrol tin filled with sand, bedding it well into the dune; but the next
+day it had disappeared from the crest, and was found, as the other
+things had been, halfway down the slope. These observations remind me of
+Mr. Barclay’s description of the method of dealing with sand-
+accumulations in the Peruvian Desert,[49] where, as soon as dunes appear
+and threaten to obstruct a railway, the local inhabitants turn out and
+scatter pebbles and stones on the dunes, and very soon the dunes are
+gone, having been carried away owing to eddies produced by the wind
+around the stones. This device for removing incipient dunes might, I
+think, be tried along the Kharga Railway, where the screens put up at
+Mr. Harding King’s suggestion have not been entirely successful in
+preventing the encroachment of sand on the line. I am the more disposed
+to think an experiment of this kind might succeed, because I have
+noticed that at the gap in the Ramak dunes (discovered by Col. Partridge
+and known during the war as “Partridge Gap”) there are great lumps of
+silicified wood, which may have led to the gap being formed.
+
+There are two other physical characteristics of dunes which I do not
+remember having seen mentioned. One of these is their low thermal
+conductivity. On a hot day, one has only to thrust one’s hand a few
+inches into a dune to encounter cool sand, while on a bitterly cold day
+one can warm one’s hands by the same action; thus while the diurnal
+variation of temperature of the surface of a dune is frequently very
+great, it is almost insensible at quite a shallow depth in the interior.
+It follows that diurnal expansion and contraction of the grains
+composing the dune must be confined to the superficial layers, and the
+movements thereby caused may materially assist in consolidating the
+dune, by causing a “packing” of the grains. The other characteristic I
+have noticed is that the surfaces of the dunes always appear to be
+harder underfoot in the cool of the early morning than in the heat of
+the day. This may, of course, be an illusion owing to one’s being less
+sensitive to fatigue in the early morning, when one’s body has been
+refreshed by sleep, and when the air-temperature is comfortable; but I
+have often fancied there is more in it than that, and it may easily be
+that the “packing” which has taken place owing to the considerable fall
+of temperature in the night, to say nothing of the action of dew, may
+have caused a real compacting of the surface layers.
+
+In his paper of 1910, Mr. Beadnell considered that the sand composing
+the dunes must be derived from arenaceous formations to the north, and
+he made special mention of the need for a careful survey of the region
+in which the dunes originate. A reference to the map will show how
+correct was Mr. Beadnell’s surmise, at least in the case of the Egyptian
+dunes which he had studied. The dunes originate in the great depressions
+which stretch from Siwa to the Wadi Natrun, where, as Mr. Beadnell
+inferred, the supply of arenaceous material from the loosely compacted
+Miocene and Oligocene beds is almost inexhaustible. We have here one of
+the clearest possible examples of the powerful action of the wind as an
+excavating and transporting agent. The Qattara depression has been
+excavated, largely by the wind, to a depth of over 130 metres below sea-
+level, and the excavated material has been carried southwards, some of
+it for nearly 1000 kilometres in distance and more than 500 metres in
+height, to form the dunes. We see not only the excavated hollow, but
+also the transported and piled-up arenaceous material removed from it;
+and the total quantity of rock thus removed and re-deposited must amount
+to hundreds of cubic kilometres.[50]
+
+It is noticeable, however, that to the north of the “great sand sea” of
+Rohlfs, which extends for an unknown distance westward, the known
+depressions are much shallower; and yet the total quantity of sand in
+this region is probably greater than that of all the more easterly dunes
+put together. The country between Jarabub and Jalo is however almost
+unexplored, though it is known to be very sandy and to be the theatre of
+violent sandstorms. Whether similar deep depressions to that of Qattara
+have been excavated along, or to the north of, the Jarabub-Jalo road, or
+whether the area of loosely compacted arenaceous rocks exposed there is
+so enormous as to have yielded such a quantity of sand without any very
+deep depressions being formed, is a point that can only be cleared up by
+further exploration in Cyrenaica.
+
+We now come to the most puzzling of all the questions connected with the
+sand-dunes, namely, why do they persist as sharply defined long narrow
+lines in certain places, while the ground elsewhere is almost absolutely
+free from sand? As Mr. Harding King puts it, the dunes appear to have a
+curious power of collecting all the sand in the neighbourhood.[51] The
+sheltering effect of ripples in the dunes, and a “shepherding” effect of
+winds a point or two off the normal, have been suggested as possibly
+affording some explanation of this apparent collecting-power,[52] but
+neither of these suggestions appears to be adequate. I think a more
+direct explanation is possible; namely, that the dunes really _have_ the
+power of attracting sand from the air, and that this power is due to the
+well-known law of attraction between a conducting surface at zero
+potential and an electrically charged body. That air-borne sand-grains
+in the neighbourhood of a dune may carry electric charges was shown by
+Mr. Harding King’s experiments, recorded in his paper of 1916, and I
+have since found sand-grains to carry electric charges of high voltage
+during sandstorms in Cairo. As to how the grains become charged with
+electricity, it can hardly be due to friction against each other, for
+the sand is remarkably uniform in composition, consisting generally of
+rounded grains of almost pure silica coated with a thin translucent film
+of ferric oxide,[53] and for the production of frictional electricity it
+is generally regarded as necessary that the two bodies which are rubbed
+together should be of dissimilar materials;[54] nor is it likely that
+they are electrified by friction with the air, for they travel with it.
+But if the particles are carried into an upper air-stratum, they will
+gradually acquire its potential by conduction; and the ordinary
+potential gradient in the air in Egypt is over 100 volts per metre of
+height.[55] Hence a sand-particle carried along for some time in the air
+at a height of only 20 metres above the ground may become charged to 7
+electro-static units of potential, or 2100 volts; and on approaching a
+dune rising to about that height it will be attracted to the dune, which
+being in connection with the earth is at zero potential. The attraction
+will be very small unless the sand-grain approaches very closely to the
+surface of the dune; from some calculations I have made, a spherical
+grain of silica half a millimetre in diameter charged to a potential of
+2100 volts will not be attracted with a force equal to half its own
+weight until it approaches within a centimetre of the conducting
+surface, and the grain will have to approach within 7 millimetres of the
+surface before the attraction is equal to the weight of the grain. But
+the important thing to note is that the attraction is independent both
+of the sign of the charge and of the direction of the wind which carries
+the particle. Another way of looking at the matter is to consider the
+probable distribution of the equipotential surfaces in the air about a
+dune, as in the diagram below, which represents a section taken
+perpendicular to the direction of the wind and to the axis of the line
+of dunes:
+
+[Illustration: _Diagram showing suggested distribution of equipotential
+surfaces and directions of lines of force in the neighbourhood of a
+sand-dune_]
+
+The equipotential surfaces, shown by dotted lines, will be squeezed
+together over the dune, and the electric forces, shown by full lines and
+arrows, being everywhere normal to those surfaces, will converge to the
+dune. Thus the dune will attract any particles carried by the wind and
+charged by conduction to the potential of the air conveying them. If the
+wind changes its direction, the attraction will still persist unaltered.
+Another point worth mention is that a sand-dune may be far more
+effective in its attraction of sand than a rocky ridge of the same size
+and shape would be; for when the electrified grains approach the surface
+of the dune, uncharged grains can rise from the dune to meet them;
+possibly some of the dancing of sand-grains which is frequently
+observable along the crest of a dune during a wind may be due to this
+cause.[56] During violent sandstorms, particles of sand may be whirled
+aloft and kept suspended at high altitudes for a period long enough for
+them to acquire very high potentials, and a sudden cessation of the
+storm may cause them to fall so rapidly that much of the charge may be
+retained; in which case, of course, electrical attraction may cause a
+very considerable deviation from the purely gravitational paths of the
+falling particles, the deviation being always towards the dune. Since
+the closeness of the equipotential surfaces and the mobility of the
+sand-grains composing the dune are greatest along the dune-crest, the
+maximum rate of deposition will be at the top of the dune, which is also
+the place where the wind exerts its greatest action in conveying the
+sand of the dunes southward. That the long axes of the lines of dunes do
+really correspond with the direction of the prevailing wind is certain,
+both from Mr. Beadnell’s observations of wind-direction[57] and from the
+fact that the scoring of the limestones in the Libyan Desert by wind-
+borne sand coincides in direction with the lines of dunes.[58] Thus I
+suggest that while the extension of the lines of dunes southwards is
+purely the result of the prevailing north-north-west wind, their clean-
+cut character and narrow width is the consequence of lateral attraction
+by the dunes themselves on the flying electrified particles of sand.
+
+If the hypothesis I have ventured to put forward is regarded as likely
+to furnish the true explanation for the formation of the lines of dunes,
+I hope that someone possessing the necessary skill in electrostatic
+measurement will put it to a crucial test, by carrying out observations
+on and near the dunes themselves. If a number of flying sand-grains
+could be caught from different levels in an insulated metallic receiver
+connected to an electrometer, the readings of the electrometer would
+enable the potentials to which the grains were charged at different
+levels to be calculated, provided the electrostatic capacity of the
+receiving system and the number and diameter of the grains giving up
+their charge were measured. Allowance would, of course, have to be made
+for any electrification of the receiver by the air, or by uncaught sand-
+grains, as well as for losses by imperfect insulation of the collecting
+system. In a preliminary experiment of this kind which I made in Cairo
+during a sandstorm, leakage from the electrometer was found to be so
+great as to vitiate any attempts at accurate measurement; but I think
+this particular difficulty could be overcome with a specially designed
+apparatus, or that at least the leakage could be measured and allowed
+for. The taking of observations of any kind in dune areas during windy
+weather is a most difficult and trying operation, and commonly means, in
+addition, a sojourn of some weeks in isolated and otherwise
+uninteresting areas. But the advent of the motor-car has so greatly
+facilitated communication in the deserts that difficulties of access are
+now not so great as they used to be; the south end of the 35-kilometre
+line of the Kattania Dunes, for instance, about 90 kilometres west-
+south-west of Cairo, which would form an excellent site for detailed
+observations, can now be reached by a four or five hours’ car journey
+from Cairo instead of the several days’ journey by camel which was
+formerly necessary.
+
+
+12. _The Distribution of Stone Implements._
+
+
+I have not made any special study of stone implements, nor have I been
+able to pay any considerable attention to their collection during my
+journeys, having usually found my time very fully taken up with other
+matters. I have, however, been much struck by the wide distribution of
+stone implements in the desert. I have found them, for instance, not
+only in Siwa Oasis and near the wells of Abu Mungar and Sheb, but also
+on the plateau between Baharia and Farafra, on the open desert between
+Terfawi and Owenat, and to the south-west of Owenat near the Anglo-
+French boundary. This wide range of occurrence, coupled with the finds
+of Schweinfurth and others on the plateaux nearer the Nile, inclines me
+to think that there is scarcely any part of the Libyan Desert in which
+stone implements might not be found by an expedition which would make
+the collection and study of them one of its main objects. The likeliest
+places in which to search (besides the neighbourhood of old wells and
+springs) would appear to be the shores of the various lakes and around
+the feet of hills affording shelter from wind and sun; for it is in such
+localities that I have most usually come across implements and pottery.
+Grinding-stones, often with a sort of stone rolling-pin, unpolished
+celts, knives, and arrow-heads are the principal forms of implement I
+have met with. Many of the grinding-stones must have been carried for a
+considerable distance, for they are made out of rocks which do not
+naturally occur in the localities in which they are found.
+
+Whether this wide distribution of stone implements would justify the
+view of Blanckhenhorn[59] that primitive man lived on the desert
+plateaux rather than in the Nile Valley must, I think, be settled by the
+further collection of specimens and by a careful comparison of the forms
+to be found in the two situations. In this connection I may remark that
+the grinding-stones I have seen in the desert seemed to me to be very
+similar to those used by the Nubians of the Nile Valley to-day for
+grinding corn.
+
+
+                                APPENDIX
+
+        SOME RECENTLY DETERMINED POSITIONS IN THE LIBYAN DESERT
+
+
+In view of their possible utility in connection with future exploratory
+surveys, I give below a list of some of the more important of the
+positions which I have recently determined by astronomical observation
+in the southern part of the Libyan Desert, with brief descriptive notes
+on the places to which they refer. The observations were made whilst
+accompanying H.R.H. Prince Kemal el Din Hussein on his exploratory
+motor-car expeditions of 1923, 1924, and 1925. The longitudes of Qasr
+Farafra, Abu Mungar, Pottery Hill, and Regenfeld depend on the transport
+of a box-chronometer; the others on wireless time-signals from Europe.
+The positions of peaks were found by triangulation from the actual
+observation-spots. The altitudes of camps are from careful barometric
+determinations; those of the peaks depend on trigonometric levelling
+from the camps.
+
+                                                             _Altitude
+              _Place._                _Lat. N._  _Long. E._  above sea.
+                                                             Metres._
+
+                                        °  ′  ″    °  ′  ″
+
+
+
+  Merga, Camp 600 metres S.W. of west  19  2 29   26 18 32      526
+  corner of salt lake. (Lake surface
+  is 17 metres below level of camp.)
+
+  Bir Terfawi, Camp close to well and  22 55 12   28 52 51      244
+  palms
+
+  Gebel Kissu, Summit                  21 34 59   25  8 26     1726
+
+  Gebel Owenat, highest point          21 54 34   25  0 47     1907
+
+    „   „ remarkable triple peak       21 53 51   25  1 39     1718
+
+    „   „ S.W. peak                    21 49 35   24 53 52     1450
+
+  Chunk Hill, Summit                   21 52 46   25 13 56      985
+
+  Owenat, Camp 600 metres S.E. of      21 48 35   24 51 45      568
+  western spring, on plain at foot of
+  mountain
+
+  Owenat, Camp at mouth of gully,      21 53  8   25  7 58      626
+  about 800 metres S.S.E. of eastern
+  springs
+
+  Gebel Kamil, Summit (160 metres      22 16 31   26 38 11      800
+  above plain)
+
+  Regenfeld, Rohlfs’ cairn of 1874,    25 10 49   27 24 22      470
+  rediscovered 1924
+
+  Pottery Hill, Summit (39 metres      24 26 27   27 38 54      506
+  above plain)
+
+  Sheb, Camp close to well, 240        22 19 48   29 45 46      228
+  metres N.N.E. of fort
+
+  Mut, Government Rest-house           25 28 37   28 58 24      119
+
+  Abu Mungar, Camp at small ruin, 400  26 30 22   27 35 29      117
+  metres E.S.E. of well
+
+  Qasr Farafra, Camp E. of village,    27  3 26   27 57 52       90
+  200 metres S.W. of Tomb of Sheikh
+  Dakhil
+
+_Merga._—To geographers the most important of the new determinations
+will undoubtedly be that of the uninhabited oasis of Merga, which had
+not previously been visited by a European, and of which the situation
+could hitherto only be guessed at from Arab statements. Merga lies in a
+shallow depression about 50 metres deep, broken by sandstone hills and
+sand-dunes, and extending for some 20 kilometres north-east and south-
+west. The salt-lake, near the centre of the depression, measures about
+300 metres by 150. It is surrounded by tall rushes and sandhills except
+at its south-western end. There are numerous date-palms, both near the
+lake and at considerable distances from it, as well as acacia trees and
+tamarisk bushes. The neighbourhood of the lake swarms with mosquitoes.
+Good and plentiful water was got by shallow digging about 1 kilometre
+south-south-east of the observation-spot, and could probably be obtained
+by digging almost anywhere in the depression. It is possible that the
+names Bidi and Tura el Bedai, shown with a question mark on some Sudan
+maps, may refer to different spots within the same depression. Owing to
+the plentifully scattered vegetation, the place cannot easily be missed,
+either by travellers passing within several miles of the lake, or by
+aircraft; but the landing of aeroplanes in the neighbourhood of the lake
+might be difficult owing to the extensive sand-drifts.
+
+_Bir Terfawi._—Scarcely less important than the accurate fixation of
+Merga is that of Bir Terfawi, the farthest south-west of all the
+Egyptian artesian water-sources hitherto known. It will be remarked that
+this latest determination places the well some 22′ south and 15′ east of
+the position which I had provisionally assigned to it from the rough
+data furnished by Lieut. Moore’s traverse of 1916.[60] A knowledge of
+the true position of Terfawi is specially desirable for a traveller who
+wishes to reach it, owing to the absence of any conspicuous landmark
+near it and to the sandy nature of the surrounding country, which causes
+tracks to be soon obliterated. There are numerous sandhills covered with
+tamarisk bushes around Terfawi, and a little grazing for camels. Besides
+the well at which observations were taken, water was found by digging in
+the sand at the foot of some tamarisk mounds about 13 kilometres farther
+west, and it is probable that good and plentiful supplies could be
+obtained at shallow depths near any of the other mounds. The palmtrees
+at Terfawi are few and small, and are less conspicuous than the
+tamarisk-bushes. These latter should enable the place to be easily found
+by aircraft; but landing would require some caution owing to the
+prevalence of drifted sand.
+
+_Gebels Kissu and Owenat._—The peaks of these mountains (especially that
+of Kissu, because of its isolated character and sharply marked summit)
+will form useful points for the connection of future surveys, being
+visible from very long distances.
+
+_Chunk Hill_ is a prominent isolated hill, nearly conical and of dark
+colour, which forms a good landmark in the broken country to the east of
+Gebel Owenat. It rises some 335 metres above the ground at its foot.
+
+_The Springs of Owenat._—Of the two water-sources of Owenat whose
+positions are given, the western one is the better, and is moreover very
+easy of access, being practically on the level of the plain which
+extends southwards from the mountain mass, and easily discoverable by
+the numerous animal-tracks converging to it; it is a pool among great
+boulders, obviously fed by percolation through cracks and fissures in
+the granite mountain which towers above it. The eastern water-source is
+less easy of access; it lies about 1 kilometre up a stony gully, and
+consists of a series of pools in the rocky floor of the gully, fed by
+trickling springs at the level where the granite is overlain by
+sandstone. The plain to the south of the western spring would form an
+excellent landing-ground for aircraft.
+
+Whilst in the neighbourhood of Owenat, I had hoped to re-determine the
+longitude of Hassanein Bey’s camp of 1923 with the aid of wireless time-
+signals, or at least by triangulation-connection to one of my
+observation-points, because a really accurate fixation of the longitude
+of a point about midway along his route from Jarabub to Furawia would
+possibly furnish the means of slightly correcting the longitudes
+assigned to Hassanein’s camps at Kufra and Erdi. I was unfortunately
+prevented from carrying out the desired connection; but from a hurried
+car-traverse which I made, skirting the western side of the mountain
+mass, in the course of which I must have passed pretty close to the site
+of Hassanein’s Owenat camp, I think that camp really lies about in
+longitude 24° 49′, or some 5 miles to the west of where I had previously
+calculated it to be from Hassanein’s traverse data;[61] and that in
+consequence the longitudes assigned to Kufra and Erdi from the same data
+may be some 2 or 3 miles too great.
+
+_Gebel Kamil_ is a sharply pointed hill of dark sandstone, forming a
+useful landmark between Terfawi and Owenat. It was visible from the east
+for more than 40, or from the west for about 20, kilometres. The name
+was given to it by Prince Kemal el Din in honour of his father, Hussein
+Kamil, the late Sultan of Egypt.
+
+_Regenfeld_, it will be remarked, was found to be very nearly in the
+position assigned to it by Jordan in 1876,[62] and my estimation of the
+level of the place is only 20 metres higher than Jordan’s. The
+neighbouring dune I found to be 30 metres high, agreeing exactly with
+Jordan’s measurement of fifty years previously; and as far as I could
+judge, the situation of the dune relative to the cairn seems to have
+remained unchanged through this long interval, showing that there has
+been at any rate no great lateral displacement of the dunes. The iron
+tanks left by Rohlfs were quite intact, in spite of their half-century
+of exposure; they had become covered with a hard dark brown film,
+apparently of magnetic oxide of iron, not rusted in the ordinary way, a
+fact which speaks strongly for the dryness of the region. Empty wine-
+bottles left by the Rohlfs expedition were frosted by the sand-blast
+wherever they were exposed; but surprisingly little of the glass had
+been removed in this way.
+
+_Pottery Hill_ is the northern one of a pair of nearly conical dark
+sandstone hills about 40 metres high, which are conspicuous from some
+distance owing to their situation on a nearly level sand-plain. I gave
+it its name from the numerous jars which I discovered at its foot in
+1917.
+
+_Sheb Well_ is a pool of fairly good water in a shallow excavation at
+the foot of a clump of dom palms, half surrounded by sand-dunes and
+tamarisk mounds, 240 metres north-north-west of a fort; the latter
+occupies a fairly conspicuous position on a sandstone hill about 25
+metres high. Another well, situated at the foot of a clump of dom palms
+about 11 kilometres farther north, is called Bir Terfau; this should
+not, of course, be confused with the Bir Terfawi already mentioned.
+
+To the north of Sheb, on the Arbain road, are three other water-sources
+which I visited in 1925 and of which I determined the approximate
+positions by a carefully controlled car-traverse: Bir Kassaba, in lat.
+22° 41′, long. 29° 55′; Bir Abu Hussein, in lat. 22° 53′, long. 29° 55′;
+and Bir Murr, in lat. 23° 22′, long. 30° 5′. Bir Kassaba is a pool of
+good water at the foot of a clump of palms; Bir Abu Hussein, which
+likewise yields good water, is a small excavation in sand near a hill of
+pink coarse-grained granite, and usually requires to be dug out afresh
+by each traveller; Bir Murr, on a little plain surrounded by hills,
+consists of several holes, with fairly good water, excavated in the sand
+at the south side of the outcrop of a steeply inclined bed of speckled
+calcareous sandstone.
+
+I have not myself traversed the desert to the east of Sheb, but
+according to an intelligent Arab who has recently made the journey from
+Sheb to Dungul, there are only two spots between these places at which
+water is obtainable; they are Bir Abu Seifa, a small well of good water
+about 50 kilometres from Sheb, and Bir Haleifa, about 15 kilometres
+farther on, where there is a fort and numerous wells. This latter place
+must, I think, be the same as the Bir Nakhlai, of which the position
+found astronomically by Colonel Talbot in 1893 was lat. 22° 29′ 1″,
+long. 30° 19′ 36″.
+
+_The Rest-house at Mut_, in Dakhla Oasis, is a low whitewashed building
+of three rooms, situated on open ground a little to the south of the
+village. It is conspicuous by reason of its isolation from other
+buildings, and forms a convenient starting-point for car journeys to the
+south-west of Dakhla, as well as for journeys to Kharga by the southern
+track which was mostly followed by cars during the war.
+
+_Abu Mungar_ proved to be very close to the position given by Mr.
+Harding King’s observations of 1912.[63] To the south-east of Abu
+Mungar, about halfway to Dakhla, are two remarkable hills forming
+outliers of the plateau. These hills, which are good landmarks, were
+first discovered by Mr. Harding King, and as they appear to possess no
+native name, I have called them “King’s Hills” on the map. The farther
+one of the two from the plateau lies in lat. 25° 58′, long. 28° 11′,
+from compass-bearings which I took to it on a careful car-traverse in
+1924. The other is about 2 kilometres farther north.
+
+_Qasr Farafra._—The position of the village of Qasr Farafra which I gave
+in my paper of 1919[64] is proved to be very nearly correct by the
+latest observations. The main advantage of the new determination is that
+it is referred to an easily identifiable landmark, the tomb of Sheikh
+Dakhil, instead of to the ill-defined centre of the village.
+
+_Levels in or near Baharia Oasis._—I think it may be well to record here
+that owing to an unfortunate mistake about the datum to which the level
+of the old military railway-terminus at B6 was referred, all the
+altitudes given on pp. 10 and 11 of my above-mentioned Survey Department
+Paper of 1919 require a correction of 19 metres to be added. At the time
+when I triangulated Baharia in 1917, I was informed that the level found
+by the military authorities for the terminus was measured from the sea-
+level datum, and I employed the value given for it as my fundamental
+level. But I have since ascertained that an arbitrary datum was employed
+by the military engineers; and trigonometric levelling from the Nile
+Valley has shown that this arbitrary datum was about 19 metres above
+sea-level.
+
+
+FOOTNOTES:
+
+
+[Footnote 1: The use of motor cars for reconnaissances in the Libyan
+Desert, initiated by the British Army during the Great War, has been
+considerably developed in the last few years by Prince Kemal el Din
+Hussein (son of the late Sultan of Egypt), who has made several long
+exploratory journeys with a fleet of cars of the caterpillar type,
+specially constructed so as to be capable of carrying considerable loads
+over sandy tracts which were impassable to the Army patrols. The
+expeditions organized and led by His Royal Highness have resulted in
+important additions to geographical knowledge concerning the Libyan
+Desert, especially the southern and western regions, which have hitherto
+been practically inaccessible. I had the pleasure of accompanying the
+Prince on three of his journeys, and am much indebted to him for thus
+affording me the opportunity of making many observations which would
+otherwise have been impossible to me. I have further to thank him for
+kind permission to illustrate this paper by reproductions from a
+selection of the excellent photographs taken by him during our travels.]
+
+[Footnote 2: Kindly communicated by Mr. A. R. Boyce, Director of Sudan
+Surveys.]
+
+[Footnote 3: Printed by the French Army Staff, 1926.]
+
+[Footnote 4: By way of comparison, it may be remarked that the area (at
+the sea-level contour) of the Qattara depression is about four times
+that of the Dead Sea and Jordan Valley depression; but the latter
+(reckoning to the lowest point of the bed of the Dead Sea) is about six
+times the deeper of the two.]
+
+[Footnote 5: A similar ambiguity of meaning very commonly attaches to
+Arabic geographical terms. Thus _beled_ may mean either a village or an
+entire country; _gebel_, either a mountain or a desert; _gezira_, either
+an island or a part of the Nile bank which is alternately covered and
+uncovered by the rise and fall of the river; _bir_, which properly
+signifies a well, is often used to denote a spring or a water-cistern;
+_wadi_, though its proper meaning is a valley, is also sometimes
+employed for a closed-in depression, as in the case of the Wadi Natrun.]
+
+[Footnote 6: ‘Nouveaux Mémoires des Missions de la Compagnie de Jesus
+dans le Levant,’ Tome ii. (Paris, 1717), p. 74. This little book
+contains a map of Egypt, prepared by Father Sicard at Cairo in 1715, on
+which the “Mer sans eau” is shown with “bateaux petrifiez” depicted
+alongside it.]
+
+[Footnote 7: See “Particulars concerning the Valley of the Natron Lakes,
+and that of the Old Bed of the River” (with map) by Andreossi, General
+of Artillery, in ‘Memoirs Relative to Egypt, Written in That Country
+during the Campaigns of General Bonaparte in the Years 1798 and 1799, by
+the Learned and Scientific Men who Accompanied the French Expedition’
+(London, 1800), p. 270.]
+
+[Footnote 8: See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (Cassel,
+1875), p. 337; and Jordan’s ‘Physische Geographie und Meteorologie der
+libyschen Wüste’ (Cassel, 1876), p. 214.]
+
+[Footnote 9: ‘Zeitschrift der Gesellschaft für Erdkunde zu Berlin,’
+1902, Tafel III.]
+
+[Footnote 10: ‘Topography and Geology of the Fayum Province of Egypt’
+(Cairo, 1905), p. 67.]
+
+[Footnote 11: Since the above was written, the Professor of Geography at
+the new University of Cairo, M. Lorin, has suggested that at a former
+geological epoch the Nile flowed through Siwa Oasis and thence westward
+so as to make Cyrenaica an island. (See ‘L’Egypte,’ published by the
+Institut Français d’Archéologie Orientale, Cairo, in December 1926, p.
+153.) M. Lorin does not give any grounds for this suggestion, and I know
+of no evidence that would justify its acceptance. As will be gathered
+from my remarks elsewhere in this paper, it seems to me far more likely
+that Siwa and the other northern depressions were in recent geological
+times regions of central drainage, than that they were local deepenings
+of a single channel; and in regard to earlier geological periods we
+possess too little information to form any conclusions as to drainage-
+lines.]
+
+[Footnote 12: See Sir Hanbury Brown’s ‘The Fayum and Lake Moeris’
+(London, 1892), pp. 40-48 and 106-110; Sir W. Willcocks’ ‘Perennial
+Irrigation and Flood Protection for Egypt’ (Cairo, 1894), also the same
+author’s ‘Egyptian Irrigation,’ third edition (London, 1913); Sir W. E.
+Garstin’s ‘Report on the Basin of the Upper Nile’ (Cairo, 1904),
+Appendix I. pp. 6-9; and Beadnell’s ‘Topography and Geology of the Fayum
+Province of Egypt’ (Cairo, 1905), pp. 16-24.]
+
+[Footnote 13: ‘Report on the Administration of the Irrigation Department
+for 1895’ (Cairo, 1896), pp. 25-28.]
+
+[Footnote 14: ‘Report on the Administration of the Irrigation Department
+for 1896’ (Cairo, 1897), p. 129.]
+
+[Footnote 15: It may be remarked that even if a channel were to be cut
+from Lake Mariut to the Wadi Natrun it would still be necessary to
+continue pumping on a large scale from Lake Mariut to maintain the
+present-day drainage. According to returns communicated to me by the
+Irrigation Service, the quantity of water removed from Lake Mariut by
+pumping during the last nine years has averaged nearly 650 million cubic
+metres a year; and even on the most optimistic estimate of evaporation
+and seepage the Wadi Natrun could not be expected to dispose of half
+this quantity after the lapse of the few years which would be occupied
+in filling it to a permanent level.]
+
+[Footnote 16: The average discharge of the Nile past Cairo in a year is
+67 cubic kilometres. The capacity of the Qattara depression at the sea-
+level contour is roughly about 900 cubic kilometres.]
+
+[Footnote 17: _Geogr. Journ._, 56 (1920), pp. 97-99 and 161-163.]
+
+[Footnote 18: Lucas (‘Natural Soda Deposits in Egypt’ (Cairo, 1912), p.
+15) regarded the water of the Wadi Natrun as being due to infiltration
+from the Nile, mainly on the ground that he supposed the water visibly
+entering the wadi (as small streams and trickles) to do so on the north-
+eastern side. But this, I think, is a mistake, as it neglects
+consideration of the large springs of warm fresh water which bubble up
+in and near the salt-lakes themselves, and also the large wells of the
+monasteries which lie south-west of the lakes.]
+
+[Footnote 19: A far better site for this well would have been in the
+depression some 20 kilometres to the north-west, the ground-level at the
+lowest point of which is only 61 metres above sea; but at the time when
+the site of the well was chosen the existence of this depression was
+unknown.]
+
+[Footnote 20: See Appendix III. in the second volume of Willcocks’
+‘Egyptian Irrigation,’ 1913, p. 853.]
+
+[Footnote 21: Schweinfurth (_op. cit._, p. 855) even thought it might
+have percolated to Siwa, in spite of the great distance (480 kilometres)
+and the adverse direction of the difference of level between the two
+places.]
+
+[Footnote 22: Mr. Beadnell (‘An Egyptian Oasis’ (London, 1909), p. 139)
+estimates the total discharge of the Kharga wells at 53,000 cubic metres
+per diem. We should not, I think, err greatly in estimating the total
+output of all the Egyptian oasis wells and springs at about five times
+this, or 250,000 cubic metres per diem. As the average daily discharge
+of the Nile in Egypt is about 180 million tons, a local influx into it
+several times as great as the total discharge of the oasis wells would
+represent but an insignificant addition to the discharge of the Nile. I
+think, however, that the influx of the warm water might possibly be
+detected by careful thermometric measurements made in the river near its
+banks at the time of low Nile.]
+
+[Footnote 23: This was the lowest level recorded at the time when I drew
+the static contours on the map. The further exploration of the
+depression by Mr. Walpole in the present year has shown that at one
+place the ground-level descends to − 134 metres; the situation of this
+local deepening is, however, such that the drawing of the static
+contours would not have been affected by its inclusion in the list of
+adopted data.]
+
+[Footnote 24: ‘An Egyptian Oasis’ (London, 1909), p. 156.]
+
+[Footnote 25: “The Mutual Interference of Artesian Wells,” _Geol. Mag._
+(London, 1909), pp. 23-26; also ‘An Egyptian Oasis,’ chap. x.]
+
+[Footnote 26: ‘An Egyptian Oasis,’ p. 140.]
+
+[Footnote 27: ‘Drei Monate in der libyschen Wüste’ (Cassel, 1875), p.
+115.]
+
+[Footnote 28: See Jordan’s ‘Physische Geographie und Meteorologie der
+libyschen Wüste’ (Cassel, 1875), p. 204. Jordan states that there were
+on an average not more than two to four plants per acre in this
+locality.]
+
+[Footnote 29: See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (chapter
+ix. of which was written by Ascherson), p. 250.]
+
+[Footnote 30: In my paper on ‘Recent Determinations of Geographical
+Positions in the Libyan Desert,’ published in Cairo in 1919, I gave (p.
+14) the approximate position of the hill as lat. 24° 28′, long. 27° 45′.
+The position given above is from a more accurate determination which I
+made on revisiting the place with Prince Kemal el Din in 1923. On this
+latter occasion the excavations made by the Prince in the sand around
+the foot of the hill revealed the existence of hundreds of additional
+jars, many of them intact, set in regular order in the sand and
+obviously forming a water-dump. The majority of the jars bore clearly
+incised markings, which Prince Kemal el Din was able later to identify
+positively as being tribal marks of the Tebus, the marks being in fact
+exactly the same as those used by the Tebu tribes of the present day.]
+
+[Footnote 31: See Jordan’s ‘Physische Geographie und Meteorologie der
+libyschen Wüste’ (Cassel, 1876), p. 215.]
+
+[Footnote 32: ‘Kharga Oasis’ (Cairo, 1900), p. 57.]
+
+[Footnote 33: _Geogr. Journ._, 42 (1913), p. 283; also ‘Mysteries of the
+Libyan Desert’ (London, 1925), p. 304.]
+
+[Footnote 34: This “locus” is of course given by joining up the points
+of intersection of the 100-metre water-contour with the 200-metre
+ground-contour, the 200-metre water-contour with the 300-metre ground-
+contour, and so on.]
+
+[Footnote 35: It is possible that the “Nubian Sandstone” in South-
+Western Egypt may represent more than one geological system, as has been
+found to be the case in Sinai. Col. Tilho (_Geogr. Journ._, 56 (1920),
+p. 259) records that the sandstone at Ennedi proved to be of Upper
+Silurian age by the fossils found in it.]
+
+[Footnote 36: Though we passed within 10 kilometres to the east of Kissu
+on the return journey from Merga to Owenat in 1925, I could not observe
+the structure of the mountain very clearly, owing to haze. But it
+appeared to me to be a huge granitic intrusion rising through the gneiss
+which covers a large area hereabouts. The sandstone over which we had
+been travelling since leaving Merga gave place to gneiss about 40
+kilometres before we came abreast of Kissu, and gneiss continued to be
+the country rock until we came within 30 kilometres of Gebel Owenat,
+when sandstone reappeared, at first capping low hills and then forming
+the plain.]
+
+[Footnote 37: There exists no published English translation of Ptolemy’s
+‘Geographia.’ Müller’s edition (Paris, 1883-1901) gives Greek and Latin
+texts, with a Latin commentary.]
+
+[Footnote 38: Quoted in Müller’s edition of Ptolemy’s Geography, p.
+739.]
+
+[Footnote 39: For the latest delineation of this region, see the
+hypsometric map in the ‘Atlas de l’Algérie et de Tunisie,’ published by
+the Cartographic Service of the Algerian Government, Paris, 1924,
+Fascicule II.]
+
+[Footnote 40: ‘Manual of Ancient Geography’ (London, 1891), p. 311.]
+
+[Footnote 41: _Geogr. Journ._, 56 (1920), p. 94.]
+
+[Footnote 42: ‘Mysteries of the Libyan Desert’ (London, 1925), p. 303.]
+
+[Footnote 43: _Geogr. Journ._, 43 (1924), p. 288.]
+
+[Footnote 44: ‘The Secret of the Sahara—Kufara’ (London, 1921), p. 235.]
+
+[Footnote 45: Beadnell, “The Sand-dunes of the Libyan Desert,” _Geogr.
+Journ._ 25 (1910), pp. 379-395; Harding King, “The Nature and Formation
+of Sand Ripples and Dunes,” _Geogr. Journ._, 47 (1916), pp. 189-209;
+Harding King, “Study of a Dune Belt,” _Geogr. Journ._, 51 (1918), pp.
+16-33, and Discussion, pp. 250-258.]
+
+[Footnote 46: For particulars of the distribution and directions of the
+dune-lines near Gebel Owenat, and between that mountain and Sarra, I am
+indebted to Prince Kemal el Din Hussein, who devoted special attention
+to observing them on his expedition to Sarra in 1926.]
+
+[Footnote 47: It may be remarked that in the desert of Northern Sinai
+the dune-lines follow curved directions, instead of maintaining a single
+direction as in the Egyptian Libyan Desert, and this curvature of the
+Sinai dune-lines has generally been thought to be caused by the
+deflecting influence of the mountain masses of Moghara, Yelleg, and
+Hellal on the wind; but here also there may be regional influences at
+work.]
+
+[Footnote 48: See Jordan’s notes on pp. 204 and 208 of his “Physische
+Geographie und Meteorologie der libyschen Wüste,” Cassel, 1876.]
+
+[Footnote 49: _Geogr. Journ._, 49 (1917), p. 55.]
+
+[Footnote 50: The steep Miocene escarpment, some 200 metres high, which
+bounds the Qattara depression on the north, probably produces a great
+eddy in the wind sweeping southwards over it, causing a whirl round a
+horizontal axis south of the scarp; this may be very influential both in
+excavation and in lifting the sand to great heights. As a large portion
+of the floor of the depression at the foot of the escarpment is formed
+of a salty sludge (sabakha), it is likely that a considerable quantity
+of finely divided salt crystals are carried up with the sand in summer,
+and this may conceivably act to some extent as a binding material in the
+dunes.]
+
+[Footnote 51: _Geogr. Journ._, 47 (1916), p. 209.]
+
+[Footnote 52: _Geogr. Journ._, 51 (1918), p. 252.]
+
+[Footnote 53: Many years ago I discovered that this film can be removed
+by treating the sand with warm hydrochloric acid, leaving the grains of
+snowy whiteness (see my ‘Description of the First or Aswan Cataract’
+(Cairo, 1907), p. 57). But I think traces of iron in the ferrous state
+must exist even within the white siliceous body of the grains, and be
+continually passing into the ferric state at the exposed surface; for
+otherwise it is difficult to account for the persistence of the yellow
+colour in spite of the mutual rubbing of the grains during their
+transport by wind. The film may possibly have an influence on the
+electrical behaviour of the grains, by reason of its possessing a higher
+conductivity than the silica of the interior.]
+
+[Footnote 54: Professor P. E. Shaw has quite recently (_Nature_, vol.
+118 (1926), p. 659) suggested from laboratory experiments that the
+mutual _impact_ of particles of _identical_ nature in sandstorms may
+result in electrification of the particles and of the air with charges
+of opposite signs. I think most travellers in the desert have, like
+myself, formed the impression that electrification of the air is more
+pronounced during sandstorms (khamsins) than at other times. But it is
+curious that observations made with the electrograph at Helwan
+Observatory do not seem altogether to bear this out. Dr. Hurst finds
+that khamsins are usually accompanied by low values of potential
+gradient in their early stages, with great disturbances both positive
+and negative for some hours before and after the break of the khamsin
+(‘Observations on Atmospheric Electricity at Helwan Observatory,’ Survey
+Dept. Paper No. 10 (Cairo, 1909), pp. 52-62).]
+
+[Footnote 55: Hurst, _op. cit._, p. 28.]
+
+[Footnote 56: Mr. Harding King records in his paper of 1915 that he
+found sand-grains blown off the dunes to be electrified; can these have
+been grains which became charged by jumping up from the dune to meet
+others and thus sharing their charge?]
+
+[Footnote 57: _Geogr. Journ._, 35 (1910), p. 385.]
+
+[Footnote 58: The hot and disagreeable south and south-west winds which
+blow during the “khamsins” of the spring are much less constant in their
+direction; and though they produce thick sandstorms, they have always
+seemed to me to occasion rather a general scattering of the sand over
+the surface of the desert than to form deposits having very definite
+arrangement; moreover, they blow on relatively few days of the year. The
+far more prevalent cool strong north-north-west winds, on the other
+hand, often scarcely vary a point in direction for days together, and
+their coincidence with the direction of the dune-lines is very
+pronounced.]
+
+[Footnote 59: _Zeitschrift der Gesellschaft für Erdkunde zu Berlin_,
+1902, p. 760.]
+
+[Footnote 60: ‘Recent Determinations of Geographical Positions in the
+Libyan Desert,’ Survey Department Paper No. 34, 1919. p. 14.]
+
+[Footnote 61: _Geogr. Journ._, 44 (1924), p. 377.]
+
+[Footnote 62: Jordan’s final position was lat. 25° 11′ 7″, long. 27° 25′
+0″. The longitude given on the document found in the bottle at the cairn
+(14° 42′ E. of Berlin, or 28° 3′ 42″ E. of Greenwich) was a preliminary
+value resulting from Jordan’s observation of a lunar distance, and was
+wrong by some 40 miles.]
+
+[Footnote 63: Mr. Harding King’s observations gave the latitude as 26°
+30′ 46″, and the longitude as 27° 36′ (see his map in _Geogr. Journ._,
+42 (1913), p. 516).]
+
+[Footnote 64: ‘Recent Determinations of Positions in the Libyan Desert,’
+Survey Department Paper No. 34 (Cairo, 1919), p. 12.]
+
+
+
+*** END OF THE PROJECT GUTENBERG EBOOK 77091 ***