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diff --git a/old/66308-0.txt b/old/66308-0.txt deleted file mode 100644 index e94a3fb..0000000 --- a/old/66308-0.txt +++ /dev/null @@ -1,2263 +0,0 @@ -The Project Gutenberg eBook of Guide to Rocks and Minerals of Illinois, by -Anonymous - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Guide to Rocks and Minerals of Illinois - Educational Series 5 - -Author: Anonymous - -Release Date: September 14, 2021 [eBook #66308] - -Language: English - -Character set encoding: UTF-8 - -Produced by: Stephen Hutcheson and the Online Distributed Proofreading - Team at https://www.pgdp.net - -*** START OF THE PROJECT GUTENBERG EBOOK GUIDE TO ROCKS AND MINERALS OF -ILLINOIS *** - - - - - _Educational Series 5_ - - - - - _Guide to_ - ROCKS AND MINERALS - OF ILLINOIS - - - _Illinois State Geological Survey_ - - [Illustration: uncaptioned] - - STATE of ILLINOIS - William G. Stratton, Governor - - DEPARTMENT of - REGISTRATION and EDUCATION - Vera M. Binks, Director - - - First printing 1959 - Second printing 1960 - - - ILLINOIS STATE GEOLOGICAL SURVEY - John C. Frye, Chief - URBANA, ILLINOIS - - -Printed by Authority of the State of Illinois - - - - - _Guide to_ ROCKS AND MINERALS OF ILLINOIS - - -Illinois has so long been known as the Prairie State that at first -glance it seems a most unlikely place in which to collect rock and -mineral specimens. - -But Illinois has a surprising wealth of rock and mineral resources, not -only to be collected as interesting specimens but to be put to practical -and profitable use. - -The rich prairies that gave the state its nickname are themselves -derived from ancient rocks, worn and changed by millions of years of -action by weather, water, wind, plants, and animals. Unmeasured depths -of rock underlie the prairies, hills, and valleys, and in some parts of -the state are exposed in outcrops, canyons, and river valleys. Boulders -and gravel brought in by the glaciers thousands of years ago are strewn -over many parts of the state. - -These resources are of great value. Besides the rich agriculture based -on the rock-derived soil, much of our industry, manufacturing, and -transportation is dependent on rock and mineral materials. Every county -in Illinois possesses some rocks and minerals that either are being used -or have potential future value. - -The Illinois State Geological Survey several years ago began to prepare -sets of typical rocks and minerals of Illinois for use by the schools -and other educational groups in Illinois. This booklet is designed to -furnish a brief geological background and explanation of these common -Illinois rocks and minerals. It also should be useful to the student or -amateur interested in making his own collection. - -Even though Illinois has no mountain ranges or deep canyons, the geology -of the state has many complexities. In fact, the very flatness of our -topography is a complicating factor because in order to study the -geology at many places in the state it is necessary to use information -from mines and descriptions (logs) and samples (cores) of the rock -penetrated during drilling of deep wells. There are also geophysical -methods of learning something about the rocks beneath the surface. - - [Illustration: Fig. 1.—Geologic map of Illinois.] - - CAMBRIAN - ORDOVICIAN - ORDOVICIAN-SILURIAN - ORDOVICIAN-SILURIAN UNDER DRIFT - DEVONIAN-MISSISSIPPIAN - PENNSYLVANIAN UNDER DRIFT - CRETACEOUS-TERTIARY - KEY - Wisconsin glacial drift - Illinoian glacial drift - Pennsylvanian boundary - -The complexity of Illinois geology is not produced by the upturning and -sharp folding of rock layers such as can be seen in the Rocky Mountains, -but rather by the changes in composition, thickness, and character of -the rock layers that are only gently warped or relatively flat. At -several places in the state, especially in the southern part, faults, or -breaks, in the rock layers do occur, but over much of our area this is -not common. - -The presence of usable minerals at considerable depth is known at many -places; coal is mined from depths greater than 800 feet, and oil is -produced from saturated rock layers, called pay zones, several thousand -feet below the surface. Lead and zinc ores, fluorspar, silica sand, -limestone, sand, gravel, clay, and shale are all produced at shallower -depths. However, the student can see only those rocks and minerals that -are to be found at or near the surface. For that reason the following -paragraphs describing their geologic occurrence deal only with surface -geology. - -The youngest of the major geologic divisions of our rocks is called the -Pleistocene, which is the scientific name for the “Ice Age” deposits. -During this relatively recent period of geologic time, which began about -a million years ago, glaciers flowed southward from Canada and spread a -layer of “glacial drift” over all of the state except the northwest -corner, the southwest edge of the state along the Mississippi River, and -extreme southern Illinois (fig. 1). - -Most of the glacial deposits that we see were formed by the last two of -the four major periods of glacial advance, the Illinoian and the -Wisconsin. The Illinoian was the most extensive, reaching as far south -as Carbondale and Harrisburg. The Wisconsin, so called because its -deposits are so widely spread in that state, reached only to Mattoon and -Peoria. - -The glacial drift is the youngest and uppermost of the divisions of the -rock column (fig. 2). Within the drift can be found the widest diversity -of rock and mineral types—quartzite, schist, and other metamorphic -rocks; granite, gabbro, and other igneous rocks; and of course the -sedimentary rocks, limestone, dolomite, sandstone, shale, and even -pieces of coal, which occur in bedded layers of the older rocks in -Illinois. - -Sand and gravel were carried and deposited by flowing streams before, -during, and after glaciation, but the major deposits were made while the -glaciers were melting. They contain a wide variety of rock and mineral -types. - - [Illustration: Figure 2—Diagram of layers of rocks in Illinois.] - - Era General Types of Rocks - Period or System and Thickness - Epoch - - CENOZOIC “Recent Life” - Age of Mammals - Quaternary - 0-500′ - Pleistocene or Glacial Age - Recent—alluvium in river valleys - Glacial till, glacial outwash, gravel, sand, - silt lake deposits of clay and silt, loess and - sand dunes; covers nearly all of state except - northwest corner and southern tip - Tertiary - 0-500′ - Pliocene Chert gravel; present in northern, southern, and - western Illinois - Eocene Mostly micaceous sand with some silt and clay; - present only in southern Illinois - Paleocene Mostly clay, little sand; present only in - southern Illinois - MESOZOIC “Middle Life” - Age of Reptiles - Cretaceous Mostly sand, some thin beds of clay and, - 0-300′ locally, gravel; present only in southern - Illinois - PALEOZOIC “Ancient Life” - Age of Amphibians and Early Plants - Pennsylvanian Largely shale and sandstone with beds of coal, - 0-3,000′ limestone, and clay - (“Coal Measures”) - Mississippian Black and gray shale at base; middle zone of - 0-3,500′ thick limestone that grades to siltstone, chert, - and shale; upper zone of interbedded sandstone, - shale and limestone - Age of Fishes - Devonian Thick limestone, minor sandstones and shales; - 0-1,500′ largely chert and cherty limestone in southern - Illinois - Age of Invertebrates - Silurian Principally dolomite and limestone - 0-1,000′ - Ordovician Largely dolomite and limestone but contains - 500-2,000′ sandstone, shale, and siltstone formations - Cambrian Chiefly sandstones with some dolomite and shale; - 1,500-3,000′ exposed only in small areas in north-central - Illinois - ARCHEOZOIC and PROTEROZOIC - Igneous and metamorphic rocks; known in Illinois - only from deep wells - -As shown by the diagrammatic rock column (fig. 2), rocks placed in the -divisions called early Cenozoic and Mesozoic are next in age to the -Pleistocene. The map (fig. 1) shows that the Cenozoic and Mesozoic rocks -occur only in the extreme southern tip of Illinois because only that -part of Illinois was covered by a northward extension of the forerunner -of the Gulf of Mexico in which the deposits of sand, gravel, and clay -were laid down. - -The next older division of Illinois rocks is called Pennsylvanian—or -“Coal Measures”—because during the last century they were first -extensively described by geologists working in Pennsylvania. - -The Pennsylvanian is one of our most important groups of rock strata -because it contains all of our minable coal beds, as well as important -deposits of limestone, shale, clay, sandstone, and some oil and gas. The -Pennsylvanian rocks are very widespread in Illinois, occurring under the -glacial drift from depths of a few feet to several hundred feet -throughout about two-thirds of the glaciated area (fig. 1). - -Next below the Pennsylvanian are the Mississippian rocks. We in Illinois -are particularly interested in this division of rocks because they take -their name from the excellent exposures along the Mississippi River -valley in western Illinois, southeastern Iowa, and eastern Missouri. -They are composed of extensive beds of limestone and cherty limestone, -sandstone, and shale. - -Mississippian rocks are of great economic importance in the structural -area known as the Illinois Basin, where they are the most important oil -producing rocks. They also contain our fluorspar deposits and along the -valley bluffs are an excellent source of limestone for quarrying. - -Rocks older than the Mississippian—except for small areas along the -Mississippi and Illinois River valleys—are found at the surface only in -the northern quarter of the state and locally in Hardin County near the -southern tip of the state. They are nonetheless economically important -because from these older rocks are produced lead and zinc, some oil and -gas, silica sand, limestone, dolomite, and shale. - -On the generalized rock column (fig. 2) these older rocks are grouped -into two units. The uppermost contains the Devonian and Silurian and the -lower contains the Ordovician and Cambrian. In general they include -dolomite, limestone, and shale, with sandstone at several places, -especially in the lower unit. - - - - - DESCRIPTION OF ROCKS AND MINERALS - - -The terms mineral and rock are often confused. They are frequently used -together and the materials they describe are closely related. In -general, a mineral is a naturally occurring chemical element or compound -formed by inorganic processes, whereas a rock is a mixture of particles -or grains of several minerals. - -However, when we refer to mineral resources or industrial minerals, we -generally include materials that are technically rocks—such as -limestone, dolomite, shale—and also coal and oil that are in fact -organic substances. On the other hand, we include in the rock category -high purity sandstone that is composed almost entirely of one mineral -(quartz) and high purity limestone that is composed largely of the -mineral calcite. - - - - - MINERALS - - -A few minerals are composed of only one element, such as diamond -(carbon) and native copper, but most minerals are chemical compounds -that contain several elements. - -Most minerals grow into distinctive shapes if they are free to grow. A -familiar example is the formation of salt crystals that grow on a saucer -of evaporating salt water. The distinctive shapes of crystals are called -their habits, and the flat surfaces that develop are called crystal -faces, the angles of which may be used to identify the mineral. - -The individual atoms of a crystal always arrange themselves in the same -way, so that each mineral breaks characteristically. Some minerals break -more easily in particular directions and present a flat, smooth surface. -This characteristic is called cleavage and the cleavage surfaces, -although sometimes confused with crystal faces, may be useful for -identifying a particular mineral. The manner in which a mineral breaks -when the broken surface does not include cleavage surfaces is called its -fracture, and this too may give a clue to the identification of the -mineral. - -There are other physical features useful in identifying minerals. Some -of them, such as color, are quite obvious. The color of the powder left -when a mineral is scraped on a rough, white surface, such as unglazed -porcelain or tile, is called streak. Luster refers to the brightness of -light reflected from the mineral’s surface. Transparency and -translucency refer to the mineral’s ability to transmit light, and -tenacity is a measure of its toughness. - -Two special physical characteristics of minerals are important to their -identification—specific gravity and hardness. Specific gravity simply -means the ratio of the weight of the mineral to an equivalent volume of -water. For example, if a mineral has a specific gravity of 4, then a -cubic inch of the mineral weighs as much as 4 cubic inches of water. - -Hardness is measured by the ability of one mineral to scratch another, -and a set of ten standard minerals has been selected for determining -this characteristic. The listing below, from soft to hard, is known as -Mohs scale. - - 1—Talc - 2—Gypsum - 3—Calcite - 4—Fluorite - 5—Apatite - 6—Orthoclase - 7—Quartz - 8—Topaz - 9—Corundum - 10—Diamond - -A rough measure of hardness can be made by using handy objects. Your -fingernail has a hardness ranging from 2 to 3, a penny is a little -harder than 3, window glass ranges from less than 5 to approximately 6 -in hardness, and a knife blade is generally in the range of 5 to 6. - - - - - ROCKS - - -Rocks, being mixtures of minerals, are more complex than minerals and -are therefore classified in a more complicated way. The broadest -grouping of rocks is based on the origin of the rock rather than on the -minerals that compose it. In this scheme all rocks are divided into -three general groups, igneous, sedimentary, and metamorphic. - -The igneous rocks are mentioned first because they are produced directly -from hot liquids that come from deep within the earth. These hot liquids -are essentially molten rock and are called magmas. When they cool, the -elements of the individual minerals come together and crystallize, as -water crystallizes into ice on a winter day. - -Different minerals crystallize, or “freeze”, at different temperatures -so that if the magma cools slowly some individual grains have -opportunity to grow larger than others. If the magma cools quickly, as -does lava or basalt, the separate mineral grains will be small. Igneous -rocks are classified on the basis of the size and arrangement of the -individual crystals and the kinds of minerals present. - -The glacial drift in Illinois contains many pieces of igneous and -metamorphic rocks, but most of the rocks native to Illinois are -sedimentary. - -Some sedimentary rocks are made up of weathered fragments of other rocks -that have been moved by rivers, waves, winds, or glaciers. These -sediments have been deposited and later compacted or cemented by the -mineral matter carried in water moving through them. Such sedimentary -rocks are called clastic (meaning broken pieces) rocks, as opposed to -those formed by chemical precipitation from water and those that consist -of fossil remains. - -Clastic sedimentary rocks are classified first on the basis of the size -of the grains of gravel, sand, silt, and clay of which they are -composed; on the type of deposition, such as glacial drift and -wind-blown silt or loess, that produced them; and then on the basis of -their mineral composition. Rocks such as gypsum and some of our -limestones were formed by chemical precipitation from sea water. - -Metamorphic rock literally means rock that has changed form. The change -of form has been caused by heat and pressures that occur below the -surface of the earth or by heat from upward moving hot magmas or melted -rocks. Examples of metamorphic rocks are found in Illinois as boulders -and pebbles in the glacial drift. - -Descriptions of the rocks and minerals represented by specimens in the -Geological Survey’s rock and mineral sets follow. The numbers preceding -the descriptions correspond to the numbers on the specimens in the sets. - -Although the set includes the rocks and minerals that are most commonly -found in Illinois, many others can be collected. For that reason, -following the descriptions is a key for identifying other Illinois -minerals and rocks. - -As a further aid to the beginner and the amateur, a list of equipment -useful in making a rock and mineral collection is given at the end of -the book. - - - - - GRANITE (1) - - -GRANITE is one of the most widespread intrusive (originating deep within -the earth) igneous rocks. It consists chiefly of feldspar and quartz -with small amounts of biotite, muscovite, or hornblende. Most granite is -light colored, but it can be white, gray, yellow, pink, or deep red. The -texture ranges from medium grained to coarse grained. - -Granite pebbles or boulders are the most common igneous rocks found in -glacial deposits in Illinois. They are not native to the state but were -brought here by the great ice sheets or glaciers that advanced from -southern Canada to cover much of northern United States during -Pleistocene time. - -Native granitic rock probably lies very deep beneath the entire state. -It has been found in deep oil-test drillings along the western and -northern margins of Illinois. - - - - - GABBRO (2) - - -GABBRO is another intrusive igneous rock, but it is heavier and darker -than granite. It is composed mainly of feldspar and dark iron-bearing -minerals that give the rock a dark color. It is coarse grained and -contains little or no quartz. - -Mineral crystals of gabbro are especially tightly interlocked, making -the rock very difficult to break. Weathered gabbro is a rusty color on -the surface, because the iron in gabbro changes color just as a piece of -metallic iron becomes coated with rust when left out of doors. - -Like other igneous rocks found near the surface in Illinois, gabbro was -carried into Illinois by the glaciers and deposited as glacial debris. - - - - - PORPHYRY (3) - - -PORPHYRY is an igneous rock identified by its texture rather than its -mineral content, which is variable. Distinct crystals (phenocrysts) of -minerals are embedded in a matrix of fine-grained rock. The phenocrysts -formed before the main mass of the rock hardened. - -Any igneous rock may have a porphyritic variety, such as granite -porphyry and rhyolite porphyry, although porphyries are most likely to -form in association with fine-grained igneous rocks. - -Porphyry is found in Illinois only in glacial drift. - - - - - BASALT (4) - - -BASALT is the most widely distributed volcanic rock. Pyroxene, feldspar, -magnetite (an iron ore), and in some instances olivine, biotite, and -hornblende, compose this rock. The dark green, gray, or black color is -due to the dark-colored minerals that make up much of the rock. The -minerals in basalt are fine grained and are packed closely together. -Phenocrysts of olivine, pyroxene, and hornblende may be present. Basalt -is easily identified by its color and fine-grained texture. - -The glaciers brought basalt into Illinois along with other igneous -rocks. - - - - - PERIDOTITE (5) - - -PERIDOTITE is the only igneous rock native to Illinois that crops out at -the surface. It is found as dikes (irregular veins) or sills (thin -sheets) that were formed when molten rock from deeper in the earth -intruded into cracks and fissures in the bedrock of southeastern -Illinois (Hardin, Pope, Gallatin, and Saline Counties). Peridotite -pebbles and boulders also may be found in the glacial drift. - -Peridotite ranges from very fine grained to medium grained and has an -even texture. It is dark gray to greenish gray, depending on the -minerals present. In general it is composed of olivine, hornblende, -pyroxene, and mica, with little or no feldspar or quartz. - - - - - GNEISS AND SCHIST (6) - - -GNEISS is a metamorphic rock composed of roughly parallel bands of -minerals. It is medium grained to coarse grained and is generally light -in color. The names given to gneiss emphasize a distinctive texture or -mineral or indicate composition. For example, biotite gneiss emphasizes -a mineral, and granite gneiss indicates the composition of the rock. - -SCHIST is much like gneiss but is fine grained and has a thinly layered -structure that makes the rock break with a wavy surface. Some common -types of schistose rocks are talc schist, chlorite schist, and -hornblende schist. As the names indicate, they are characterized by -their dominant mineral. Mica schist may be formed by the metamorphism of -either sedimentary or igneous rocks. - -Gneiss and schist are not native to Illinois but are found in the -glacial drift. - - - - - CONGLOMERATE (7) - - -CONGLOMERATE is a sedimentary rock made up of pebbles or other rock -fragments cemented in a background or matrix of finer material, -generally silica, calcium carbonate, clay, iron oxide, or a mixture of -these substances. The rounded rock fragments have been worn by being -rolled in streams and along beaches. - - [Illustration: uncaptioned] - -If the pebbles embedded in the matrix are sharp and angular, freshly -broken and not worn, the rock is called breccia and is generally found -near the place where the fragments originated. Conglomerate or breccia -may be made up of any type of rock or mineral, most commonly durable -material such as chert, quartz, quartzite, granite, and gneiss. - -In Illinois, conglomerates commonly are found at the base of sandstone -formations and as beds in the lower “Coal Measures.” They also are found -in some gravel deposits. - - - - - SANDSTONE (8) - - -SANDSTONE is a clastic sedimentary rock consisting of sand-sized grains -(one sixty-fourth to one-sixteenth inch in diameter) held together by a -cementing material. As sandstones become finer grained they grade into -siltstones; as they become coarser grained they grade into conglomerate. -The shape of sand grains in sandstones ranges from rounded to angular. - -Quartz is the dominant mineral in sandstone, but other rock grains and -mineral grains (especially chalcedony, feldspar, muscovite, hornblende, -magnetite, or garnet) generally are present. - -Sandstones are commonly cemented by carbonates, silica, iron oxides, or -clays. Most sandstones are a shade of gray or brown, but the color may -vary from gray or white to yellow, brown, or red. The color probably -depends on the type of cement, the amount of organic material present, -and the amount and degree of oxidation of iron in the rock. - -Durability of sandstones depends largely on the character of the cement. -Some sandstones crumble easily, but others, especially those cemented by -iron oxides or silica, are tough and durable. Sandstones break around -the grains, giving the broken surface a granular appearance. - -Sandstone crops out in many places throughout much of the state. In -LaSalle and Ogle Counties, large tonnages of sand are mined from the St. -Peter Sandstone and sold for a variety of uses, including abrasive sand, -molding sand, and sand for making glass. In extreme southern Illinois -attractively colored sandstones have been quarried for building stone. - - - - - QUARTZITE (9) - - -QUARTZITE is a metamorphic rock that originally was quartz sandstone. -Quartzites are produced by intense heat and/or pressure, probably aided -by hot silica-bearing solutions. The quartz grains may be so closely -interlocked that individual grains are no longer recognizable. The rock -fractures conchoidally through both the grains and cement, so the broken -surface, unlike that of sandstone, is smooth and may even be glassy like -quartz. - -Color depends upon the amount and kind of impurities present. A -quartzite that is all quartz is white or gray, but iron or other -elements may change the color to shades of purple, yellow, brown, or -red. Quartzite is a very resistant, hard rock and cannot be scratched by -a knife. - -Quartzite is abundant as boulders and pebbles in glacial drift of -Illinois, having been brought into the state during the “Ice Age.” - - - - - SHALE (10) - - -SHALE is a common and important sedimentary rock composed of compacted -clay or mud. It is so fine grained that the minerals forming it -generally cannot be identified without the aid of X-ray. - -Shales are composed mainly of clay minerals but, like other sedimentary -rocks, generally include other minerals. Shales containing calcium -carbonate are called calcareous shales. Most shales contain silt or sand -particles; if silt or sand is present in large quantity, the rock is -called silty shale or sandy shale. If mica minerals are present in -quantity in a shale it is called micaceous shale. - -The particles of most clay minerals are thin and flat and overlap each -other. - -Shales have a wide range of colors but most of those in Illinois are -gray. A gray, black, or blue-gray color is caused by organic matter in -the shale; shades of red, brown, yellow, or green are caused by iron -compounds. - -Shale is widely distributed in Illinois, especially in “Coal Measures” -rocks, and is used in manufacturing bricks, drain tile, building tile, -and lightweight aggregate. - - - - - CLAY (11) - - -CLAY is an unconsolidated rock made up of a group of hydrous aluminum -silicate minerals, of which chlorite, montmorillonite, kaolinite, and -illite are the most abundant. These minerals are formed by the -weathering or alteration of other rocks and minerals. - -Clays are very fine grained and their minerals have tiny, flat crystals -that can be distinguished from each other only by laboratory methods. -Although clays may appear to be similar, their compositions vary -greatly. - -Some clays are white, but most are colored by iron compounds and organic -matter. Wet clays have an earthy odor and generally are slick and -plastic, but dry clays are relatively hard and are greasy to the touch. - -Clays are abundant in Illinois, especially in soils, in shales, and as -clay deposits. In Illinois the underclays that occur beneath coal beds -are particularly well suited to the manufacture of bricks, pottery, -stoneware, and drain tile. - - - - - LIMESTONE (12) - - -LIMESTONE is a sedimentary rock composed of particles of calcite -(calcium carbonate). The crystals may range from fine to coarse. Many -limestones contain other minerals, such as chert, clay, or sand, and in -some places they grade into dolomite (calcium magnesium carbonate). - -Many limestones are white or gray. Yellow or brown shades are caused by -iron oxide impurities and dark gray to black colors by organic matter. - -Limestones form in various ways. Some are deposited when calcium -carbonate precipitates from solution; others are formed when the shells -or skeletons of organisms such as brachiopods, clams, and corals -accumulate on a sea floor. If such fossils are very abundant, the rock -is called fossiliferous limestone. Limestone composed of tiny, rounded -concretions is called oolite or oolitic limestone. - -Limestone effervesces freely in dilute hydrochloric acid, but dolomite -must be powdered before it will effervesce. In nature, limestones may be -dissolved by percolating water containing weak acid (such as carbonic -acid, composed of water and carbon dioxide). At many places in southern -and southwestern Illinois such solution of limestones has produced caves -and caverns. - -Limestone outcrops are abundant in Illinois, especially along the bluffs -of the Mississippi, Ohio, and Illinois Rivers. - -Limestone has many uses. It is used for building stone, road surfacing, -railroad ballast, in the manufacture of portland cement, and, if of high -purity, for making lime and chemicals and as a flux in smelting metals. -It also is used, as agricultural limestone, to add calcium to the soil. - - - - - PEAT (13) - - -PEAT is produced by the partial decomposition of plants that accumulate, -with varying amounts of mineral matter, in old ponds, swamps, and lakes, -and in abandoned channels in valley bottoms along many rivers and -streams. Peat may be an early stage in the formation of coal. - -Peat ranges from light to dark brown, the color, decomposition, and -compaction increasing with depth in the deposit. Some of the plant -remains are clearly distinguishable and appear as fibrous fragments held -together by the fine peat particles. The type of peat common in Illinois -has a high water content. Before it is dried it is soft and spongy to -the touch; upon drying, it loses much water and becomes harder. - -Peat is used as a fuel in some parts of the world, but its heating value -is low compared to that of coal. It burns with a long flame and leaves a -great deal of ash because of the silt and sand that were buried with the -vegetation. Peat and peat moss are used chiefly as an absorbent, as -stable litter, as insulating and packing material, and by gardeners to -increase the water-holding capacity of soils. - -Peat is found in many places in Illinois but the largest deposits are in -northeastern Illinois. - - - - - COAL (14) - - -COAL, an organic stratified rock, is formed from accumulated plant -material and partially decayed plants that were buried during the “Coal -Measures” period in Illinois more than 200 million years ago. - -Sediments deposited over the peat-like organic material compacted it. -Chemical changes gradually took place and resulted in the loss of water -and gases, leaving a higher percentage of carbon than the original -material contained. - -The amount of such change that has taken place determines the rank of -the coal. The lowest ranks are called lignite, the intermediate group is -called bituminous (soft) coal, and coals of the highest rank, with the -highest carbon content, are called anthracite (hard coal). Mineral -matter, such as shale, clay, or pyrite, generally is present in the coal -and becomes ash when the coal is burned. - -Most coal mined in the United States is bituminous coal. It is black, -brittle, breaks into angular blocks, has a shiny luster, and generally -shows a banded structure. - -Coal mining is an important industry in Illinois, and the state contains -the largest known reserves of bituminous coal in the United States. -Minable coal beds underlie about two-thirds of the state. As many as 20 -different coal beds have been mined in Illinois, the most important -being the Herrin (No. 6) and the Springfield (No. 5) or Harrisburg (No. -5). The coal in most mining areas averages 5 to 7 feet thick and in -places attains a thickness of 15 feet. - -In underground mines the coal is approached by vertical or inclined -shafts. In open cut, or strip, mines all of the overlying material -(overburden) is removed, leaving the coal exposed. Coal as much as 100 -feet deep is now being strip mined in Illinois. - -Illinois coal is now used mainly for generating electric power, for -industrial purposes, and for heating. In industry, coal is used -extensively for power, heating, burning cement, firing clay products -such as brick, tile, pottery, porcelain, and china, and making coke. - -Certain Illinois coals when mixed with coal from the eastern part of the -United States, produce metallurgical coke for making steel. Gases, oils, -and tars derived in processing coal for coke have been used for making -many chemical products, including dyes, perfumes, explosives, medicines, -insecticides, plastics, and road tar. - - - - - PETROLEUM (15) - - -PETROLEUM (crude oil) is classed as a mineral resource although it is a -liquid hydrocarbon and not technically a mineral or rock. It is, -however, found in the pores and fractures of rocks. The color of crude -oil ranges from yellow through green and brown to black. - - [Illustration: uncaptioned] - - Arch (anticline) - Gas - Porous rock - Nonporous rock - Oil saturated rock - Water - Break (fault) - Nonporous rock - Oil saturated porous rock - Water saturated porous rock - Trap formed by change in character of rock - Water - Nonporous rock - Porous rock - -Petroleum had its origin in the plants and animals buried in ancient -sediments. The organic matter changed slowly into the complex mixture of -hydrogen and carbon compounds that is petroleum. - -Because gas is lighter than oil, and oil in turn lighter than water, gas -and oil move upward in a porous rock containing all three. Gas moves to -the highest position with oil next below and water in the lowest part of -the rock. Oil pools exist where geologic barriers have stopped the -movement of gas and oil. - -Arches (upward folds or anticlines), breaks (faults), and lateral -changes from porous to nonporous rock are geologic features that serve -to localize oil pools within the reservoir rock. - -Petroleum is obtained by drilling wells into the reservoir rock. Gas -that is free or dissolved in the petroleum expands as pressure is -released when the well is drilled and drives the oil to the well. Water -in the reservoir rock also acts as a driving force. When this natural -pressure is no longer effective, other methods (secondary recovery) are -used to recover the oil remaining in the reservoir. - -Porous sandstones and limestones are the oil-bearing rocks. Illinois -also has deposits of oil shale from which petroleum may be produced in -the future. - -Thousands of products are derived from petroleum, including gasoline, -kerosene, naphtha, lubricating oils and waxes, medicinal oils, salves, -heavy fuels, road oils, tar, and asphalt. - - - - - GLACIATED PEBBLES (16) - - -GLACIATED PEBBLES are small stones whose shapes have been altered by the -grinding action of a glacier. Such pebbles commonly have at least one -flattened side that shows scratches (called striae). The striae were -produced when the pebbles were pushed over bedrock or ground against -other pieces of rock. - -Glaciers tore fragments from the bedrock over which they moved and the -fragments accumulated in, on, and under the mass of ice. The rock -fragments were transported, some of them far from their source, and were -deposited as the glacier moved along or when the ice melted. - -Soft rocks like limestone and dolomite are easily scratched, but soft -rocks cannot make grooves in hard igneous and metamorphic rocks. -Therefore soft rocks have more striae. - -Glaciated pebbles can be found in deposits in many parts of the state, -especially in northeastern and east-central Illinois. However, many -deposits near the surface have been weathered and striae have been -destroyed. An especially good place to look for striated pebbles is in -quarries and strip mines where glacial drift overburden has been -removed. Pebbles found in such deposits show good striae for they are -but little weathered. - - - - - SILICA SAND (17) - - -SILICA SAND is the commercial name for sand composed almost entirely of -grains of quartz. Sand of this kind is mined in Illinois from the St. -Peter Sandstone in LaSalle and Ogle Counties. The coarser grains of the -sand are characteristically rounded and frosted. The frosting causes the -sand to look white. - -Illinois silica sand has many uses. It is used just as it is mined for -molding sand in which metal castings are made, for lining industrial -furnaces, and for many other purposes. Some of the sand is washed to -remove the small amount of impurities present. The washed sand is used -for such purposes as making glass, for grinding plate glass smooth, for -sand blasting, for molding metal, as fracturing sand to increase the -production of oil wells. Some silica sand is ground to a fine powder and -used as a fine abrasive, as a filler in paint, and as an ingredient in -pottery, glazes, and enamel. - -A specially prepared St. Peter sand, known throughout the world as -Standard Ottawa Testing Sand, is used to test the strength of cements -and as a laboratory standard in physical tests of other sands. - - - - - MOLDING SAND (18) - - -MOLDING SAND is a mixture of sand and clay or other bonding material and -is used to make molds in which metal is cast into various useful shapes. -There are two kinds of molding sands, synthetic and natural bonded. The -first is an artificial mixture of silica sand and clay; the second is a -naturally occurring mixture of sand and a bonding material. - -For casting, molding sand is first moistened with water and shaped into -a mold of the metal part to be reproduced. The mold is then allowed to -dry and the molten metal poured into it. The bonding material in the -sand must be strong enough to keep the mold in shape during these -operations. - -Natural bonded molding sand is produced in Fayette, Bond, Bureau, -Carroll, Kankakee, and Rock Island Counties, Illinois. - - - - - TRIPOLI (19) - - -TRIPOLI, called amorphous silica in southern Illinois, is a white or -light brown, powdery substance that rubs off on the hands like chalk. It -consists mostly of very small particles of quartz that result from the -weathering of calcareous chert or highly siliceous limestone. - -It is finely ground and used as “white rouge” for polishing optical -lenses, as a filler in paints, in making ceramic products, as a -component of buffing compounds, and as a fine abrasive. - -Tripoli occurs in Alexander and Union Counties and is milled at Elco and -Tamms in Alexander County. - - - - - FULLER’S EARTH (20) - - -FULLER’S EARTH is clay or silty clay material that contains very fine -silica. It is soft, nonplastic, opaque, has a greasy feel when wet, and -does not readily break up in water. Its color varies from blue-gray to -yellow or buff. - -Fuller’s earth is valuable for its unique property of absorbing and -decolorizing substances. The material was first used to “full” or remove -grease from woolen cloth, hence its name. It also has been used to -filter and bleach mineral and vegetable oils by absorbing dark organic -matter. - -In Pulaski County in extreme southern Illinois the Porter’s Creek -Formation contains deposits of clay that were at one time the source of -fuller’s earth and still afford clay whose absorbent properties make it -useful as litter and as sweeping and cleaning compound. - - - - - QUARTZ (21) - - -QUARTZ is the most common of all minerals, making up about 12 percent of -the earth’s crust. There are two main types of quartz—crystalline quartz -and dense, crypto-crystalline (submicroscopic) quartz. Many dense -varieties occur in Illinois, probably the most common are chert or -flint. - - [Illustration: uncaptioned] - -Crystals of quartz are typically six-sided, elongated, have sharply -pointed pyramid-like ends, and are apt to grow together forming twins. -Good crystals are rare in Illinois, and the crystal structure is not -apparent in the commonly occurring grains and masses. - -Quartz is brittle and hard. It may be colorless or tinted, transparent -or translucent, but more commonly it is white and nearly opaque. -Transparent quartz looks much like ordinary glass, but it scratches -glass easily. It has a glassy to brilliant luster and breaks irregularly -or with a good conchoidal fracture. - -Some varieties of quartz that are used for semiprecious gems are -chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, -generally translucent, white to gray, blue, brown, or black. Agate is a -form of chalcedony that has a mottled or variegated banded appearance -and may be yellow, green, red, brown, blue, gray, or black. Onyx is -agate with parallel bands that as a rule are brown and white or black -and white. Jasper, an impure opaque quartz, generally is red. - -Quartz occurs as rock crystal (colorless, transparent), milky quartz -(white, nearly opaque), and smoky quartz (smoky yellow to gray or brown) -in geodes from the Warsaw and Keokuk Limestones of the -Nauvoo-Hamilton-Warsaw area and as vein and cavity fillings associated -locally with fluorite, sphalerite, and galena in extreme southern -Illinois. It also occurs as vug (cavity) fillings in limestones and -sandstones. - - - - - FELDSPAR (22) - - -FELDSPAR is the name applied to a group of minerals that are the second -most common of all the earth’s minerals. All feldspars are composed of -aluminum, silicon, and oxygen, combined with varying amounts of one or -more metals, particularly potassium, sodium, calcium, and lithium. - -The minerals are hard, have a smooth glassy or pearly luster, and cleave -along two planes nearly at right angles to each other. Feldspars are -fairly light weight. The streak is white, but the color of the mineral -is highly variable, although potassium and sodium-bearing feldspar are -commonly white or pink and most plagioclase feldspar is gray. - -Feldspars are essential parts of the crystalline igneous rocks. Their -decomposition products are present in most soils. In Illinois relatively -small feldspar crystals can be found associated with quartz and other -minerals in granite and gneiss boulders and pebbles in glacial drift. - - - - - MICA (23) - - -MICA is the name of a family of complex aluminum silicate minerals that -can be split easily into paper-thin, flexible sheets. If broken across -the grain at right angles to the flat, smooth surface they fracture -raggedly. In a single mica crystal the sheets range from more or less -transparent to translucent and are arranged one on top of another like a -deck of cards. - -Micas are tough and somewhat elastic, soft enough to be split and -scratched by a fingernail, and are light weight. They have a -nonmetallic, glassy or pearly luster, although yellow mica may appear to -be metallic. Color and streak depend upon the chemical composition of -the mineral. Muscovite, or white mica, contains potassium and makes a -colorless or white streak. Biotite, or black mica, contains iron and -magnesium and is commonly dark green or black, although it may be shades -of yellow or brown; its streak is uncolored. - -Mica is abundant as tiny, shimmering flakes in Illinois sands, -sandstones, and shales (which are then said to be micaceous). It also is -common in many varieties of igneous and metamorphic rocks. White or -yellow flakes may show a brilliant luster and may be mistaken for -silver, platinum, or gold, but those minerals are heavy and malleable -whereas mica is not. - - - - - CALCITE (24) - - -CALCITE, a common rock-forming mineral, consists of calcium carbonate. -The mineral is white or colorless, but impurities may tint it shades of -yellow or gray. Transparent calcite is more rare than the tinted -varieties. - -Transparent calcite possesses the property of double refraction; an -image appears double when viewed through a calcite cleavage block. - -Calcite has a glassy luster, its streak is white or colorless. The -mineral is of medium hardness and can be scratched by a penny or a piece -of window glass but not by the fingernail. It is fairly light weight and -effervesces freely in cold dilute hydrochloric acid. - - [Illustration: uncaptioned] - -Calcite has a variety of crystal forms but in Illinois flattened -block-shaped crystals and elongate crystals with tapering points -(“dogtooth spar”) are the most common. When broken, calcite cleaves into -six-sided blocks called rhombs. - -Crystals of calcite are found in Illinois as linings in geodes in -certain limestones and shales, especially in the Nauvoo-Hamilton-Warsaw -area, and as crystalline masses in limestone and dolomite. Small amounts -of clear crystalline calcite are associated with various ores in -northwestern and extreme southern Illinois. - -Calcite is the principal mineral in limestones and occurs as a component -of many concretions. - - - - - FLUORITE (25) - - -FLUORITE, or fluorspar, is made up of the elements calcium and fluorine. -The mineral is easily identified by its perfect cleavage, color, and -hardness. - -It occurs in cubic crystals that may be twinned but is more often found -as irregular masses. It can be split into diamond-shaped, eight-sided -forms (octahedrons). Fluorite is commonly gray, white, or colorless, but -it may be green, blue, purple, pink, or yellow. The streak is colorless -and the luster glassy. It can be scratched by a knife or a piece of -window glass, is fairly light weight, and is transparent to translucent. - -Extensive deposits of fluorite, one of Illinois’ important commercial -minerals, occur in Hardin and Pope Counties in extreme southern -Illinois, where it is associated with galena, sphalerite, calcite, -barite, and other less abundant minerals. - -Fluorite is used to make hydrofluoric acid, to form a fluid slag in the -production of iron and steel, in the manufacture of aluminum, to make -many chemical products, and in the ceramic industry, in which it is used -to make colored glass, enamels, and glazes. - - - - - GYPSUM (26) - - -GYPSUM, hydrous calcium sulfate, is a colorless, transparent to -translucent mineral when pure, but it often is stained yellow by -impurities. It has a white streak, is soft enough to be scratched by a -fingernail, and is light weight. - - [Illustration: uncaptioned] - -Gypsum occurs in several forms. Selenite is a coarsely crystalline, -transparent variety, composed of flat, nearly diamond-shaped crystals -that can be split easily into thin sheets, have a glassy luster, and -often grow together to form “fishtail twins.” Crystals of selenite occur -in shales of the “Coal Measures” of southern, north-central, and western -Illinois, and can be picked up at the surface. - -Satin spar has crystals like silky threads closely packed together, -splits parallel to the fibers, and is found as fillings in rock cracks -and as thin layers in shales. Massive gypsum is granular. - -Gypsum deposits occur deep underground in Illinois but thus far have not -been mined. - - - - - CHERT (27) - - -CHERT, one of the main forms of silicon dioxide, is cryptocrystalline -(submicroscopic) quartz. Most of the chert in Illinois is white or gray, -but impurities stain many deposits yellow, brown, or even pink. Chert is -so hard that it can scratch glass and ordinary steel. It is fairly light -weight, dense, opaque, and brittle; the luster is dull. - -Flint, a variety of chert, is generally dark colored, more dense, may -have a glassy luster and be transparent in thin flakes. Both chert and -flint have a smooth, curved (conchoidal) fracture, but flint tends to -break with thinner, sharper edges. Indians used flint and chert to make -arrow points and spearheads. - -Chert occurs as rounded masses (nodules and concretions) or as irregular -layers in limestones and dolomites in northern, western, southwestern, -and southern Illinois. Because chert is hard and more resistant to -weathering than limestone or dolomite, it often remains after the rest -of the rock has weathered away. - -Chert also is abundant in many glacial deposits because it is hard and -resists solution. Streams that flow through cherty bedrock or glacial -deposits carry pebbles along and concentrate them as gravel in stream -channels. Cherty stream gravels are especially abundant in western and -southern Illinois. - -Brown chert gravels in the southern part of the state are used for road -gravel. Other deposits in extreme southern Illinois, consisting of -angular fragments of chert and a small amount of clay (known locally as -novaculite gravel) also are used for road surfacing. - -White and gray chert occur as massive bedrock deposits several hundred -feet thick in Union and Alexander Counties. - - - - - PYRITE AND MARCASITE (28) - - -PYRITE and MARCASITE are iron disulfide compounds. They look much alike -but have different crystal forms. Both are brittle, hard, brassy yellow -with metallic luster, and opaque. The best distinguishing feature is -crystal shape. The pyrite crystals are cubes, but the marcasite crystals -are blade- or needle-shaped. - -Pyrite and marcasite have been mistaken for gold because they are yellow -and metallic and hence are sometimes referred to as “fool’s gold”. They, -however, are harder than gold, tarnish, and leave a dark streak, whereas -gold is soft, very heavy, does not tarnish, and leaves a yellow streak. -Gold is malleable, but pyrite or marcasite are reduced to powder if they -are pounded and give off a noticeable odor of sulfur dioxide gas if they -are heated. - -Pyrite and marcasite are found in many deposits in Illinois. They occur -as grains or larger masses in some clays, shales, and limestones. They -also occur with the lead and zinc ores of northwestern Illinois and, in -small amounts, with the fluorite and associated minerals in the extreme -southern part of the state. - -Both pyrite and marcasite are common as surface coatings, veins, and -concretionary structures in coal and in dark shales associated with -coal. They are referred to as “coal brasses” or “sulfur” when found as -impurities in coal. - -A potential use for pyrite and marcasite is in the manufacture of -sulfuric acid for industrial use. Coal brasses recovered from Illinois -coal have been so used. - - - - - LIMONITE (29) - - -LIMONITE is an iron oxide containing water and has a complex chemical -composition. The limonite found in Illinois may be yellow, orange, red, -brown, or black, but its streak is always yellowish brown. The mineral -may have a glassy or an earthy luster. It may be too hard to be -scratched by a knife. It is of medium weight. - -Limonite is common and occurs as concretions and cavity fillings in -sedimentary rocks, and as coatings on them, especially sandstone. It -also occurs as iron rust, as scum on stagnant water, and it accumulates -around rootlets in soils. Small amounts color limestone, dolomite, clay, -shale, sandstone, and gravel. Some sands are firmly cemented by brown or -black limonite and look much like iron ore. Clays containing a high -percentage of limonite are called ocher. - -In some states limonite is mined as an iron ore, and in Illinois it was -so used in Hardin County in the middle 1800’s, but deposits are not -large enough for profitable use now. - - - - - SPHALERITE (30) - - -SPHALERITE, zinc sulfide, is a major ore of zinc. It has a resinous -luster and a white, yellow, or brown streak. Illinois sphalerite is -generally yellow, yellowish brown, reddish brown, or brownish black. It -is of medium weight, brittle, can be scratched by a piece of window -glass but not by a penny. It is commonly opaque but may be translucent -on thin edges. - -Sphalerite is mined with galena in northwestern Illinois and in extreme -southern Illinois with galena and fluorite. Small crystals occasionally -are found in limestones and as crystalline masses in clay-ironstone -concretions. - - - - - GALENA (31) - - -GALENA, lead sulfide, is the principal ore of lead. It is steel gray, -heavy, opaque, and has a bright metallic luster, though the shiny -surface may be dulled by a coating of lead carbonate. It has a gray or -black streak, is soft enough to mark paper, and can be scratched by a -penny. The cube-shaped crystals readily break into cubic, right-angled -fragments. Probably the most obvious features of the mineral are its -bright metallic luster on fresh surfaces, high specific gravity, and -cubic cleavage. - - [Illustration: uncaptioned] - -At many places galena is argentiferous (silver-bearing), but Illinois -galena is relatively unimportant for its silver content. As a source of -lead, however, it is an important commercial product of the state. - -Scattered pieces of galena are found at many places in Illinois. Some -occur in the glacial deposits, others occur as small pockets and as -crystals in limestones and geodes. In only two areas of the state are -deposits of commercial value. In northwestern Illinois galena occurs in -association with sphalerite; in extreme southern Illinois it occurs in -association with fluorite and sphalerite. - - - - - CONCRETIONS (32) - - -CONCRETIONS are concentrations of inorganic sedimentary material within -other sediments. Minerals that commonly form concretions are silica (in -the form of opal, chert, chalcedony, and quartz), calcite, siderite, -pyrite, marcasite, and limonite. - -Concretions may form either as the sediment around them is forming or -after the sediment around them has hardened. They may be formed when -water containing dissolved minerals seeps through the sediment or rock -and leaves a concentration of mineral matter in a cavity or around a -central particle (nucleus) such as the remains of a plant or animal. -Portions of rock may also become firmly cemented by such mineral matter. - -Concretions range in size from minute particles to objects several feet -in diameter. Shapes range from spheres to tubes. Many are globular or -lumpy-surfaced, some are smooth. Because concretions generally are -harder than the surrounding rock in which they have formed, they do not -weather away as readily and may remain after the surrounding material -has been eroded. - -Concentrations of calcite are found in loess deposits. They may look -like bizarre, knobby figurines, and the Germans called them loess -kindchen (little children of the loess). - -Ironstone concretions, especially common in many Illinois shales, are -formed by a local concentration of the mineral siderite (iron carbonate) -in the rock. The concretions found in weathered outcrops commonly are -partly or entirely weathered to limonite. Some ironstone concretions -grow together into odd shapes. Mazon Creek ironstone concretions of -northeastern Illinois, world famous for their fossils, are sideritic. -The concretions are commonly covered with limonite, the result of -oxidation. - -Limonite concretions, generally with a high content of clay, silt, or -sand, occur in loess, shale, and sandstone. - -Concretions of chert and other forms of silica are common in limestones. -In many places, because of their greater resistance to weathering, -lenses and nodules of chert protrude from the beds. - -Pyrite or marcasite occur as concretions or concretion-like masses in -some coal beds and in the black shales, sometimes popularly called -“slates,” above coal beds. Some other Pennsylvanian clays and shales -also contain concretions or coarsely crystalline aggregates of these -minerals. - - - - - GEODES (33) - - -GEODES are roughly spherical bodies that may be filled with layers of -minerals, lined with crystals, or both. The outer layer of geodes found -in Illinois as a rule is composed of chalcedony, a form of finely -crystalline silica. - -Geodes differ from concretions in that they form inward from the outer -shell, whereas concretions develop outward from a center. Even if geodes -have been completely filled by mineral matter, their inward-projecting -crystals prove that they formed within a cavity. - -In a partly filled cavity, crystals generally are well formed because -they grew without being crowded. Some of the best mineral specimens -known in Illinois are found as crystal linings in geodes. - - [Illustration: uncaptioned] - -Quartz is the most common mineral deposited in geodes, but calcite, -aragonite, dolomite, siderite, pyrite, galena, fluorite, and sphalerite -also are found. - -Geodes ranging in size from less than one inch to a foot or more in -diameter can be gathered from streams where they have accumulated as -residual boulders after the rock in which they were enclosed has been -eroded. - -Hollow geodes are the most desirable because they have better crystals. -They can be distinguished from solid ones by their comparative lightness -of weight. - -Geodes are commonly associated with limestone and dolomite, at some -places with shale. In Illinois they can be found most easily in the -Warsaw Formation in the area of Nauvoo, Hamilton, and Warsaw, but they -also occur in other areas and other formations. - - - - - ANIMAL FOSSILS (34) - - -Prehistoric animals lived in water, on land, and in the air, and left -both direct and indirect evidence of their existence, evidence we now -call fossils. - -Millions of ancient animals died without leaving a trace, but some, -especially those that had hard parts such as shells, bones, or teeth, -may be found preserved in rocks much as they were when buried beneath -sediment on the floor of an ancient sea. Sometimes only imprints of the -outside or fillings of the inside of the shells remain, the original -material having been completely dissolved. Footprints of land or -amphibious animals, burrows made by clams, or holes made by worms also -are fossils. - - [Illustration: uncaptioned] - -The animals whose remains are fossilized lived and died while the -sediments that contained them were being deposited, and they provide -clues to the types of life and climate then existing. Fossils of animals -characteristic of a certain time are an index to the age of formations -where they occur. For example, if a certain trilobite (an ancient -relative of the crayfish and lobster) is known to have lived only during -a definite time, then all rocks in which it is found are the same age. - -Fossils of animals that lived in the sea are exposed in rocks in many -parts of Illinois, especially in quarries, river bluffs, and road cuts. - -The oldest fossils found in Illinois are shells of marine -animals—snails, corals, crinoids, brachiopods, trilobites, pelecypods -(clams), cephalopods, bryozoa, arthropods, and others. The youngest -fossils are teeth and bones of prehistoric bison, giant beavers, deer, -mammoths and mastodons of the “Ice Age,” and snails found in glacial -loess. - - - - - PLANT FOSSILS (35) - - -PLANT FOSSILS are the remains of prehistoric plants. Woody structures of -plants aid preservation just as hard parts of animals do. Leaves and -plants without much woody material generally were well preserved only if -they were buried quickly in fine, soft sediment. - -The most famous Illinois plant fossils are those from the Mazon Creek -area in Grundy and Will Counties of northeastern Illinois. The plant -material acted as a nucleus around which iron minerals accumulated to -form concretions. Many good fossils—of trunks, branches, leaves, and -seeds—are found in coals and in shale directly overlying coals. -Descendants of “Coal Measures” plants, such as ferns, mosses, and -rushes, are still living today, but they no longer thrive as they did in -the warm, moist climate of the Pennsylvanian forests. - -Some plants of Pennsylvanian age are petrified, and occasionally such -trees or stumps are found. Petrified trees are found also in the upper -Mesozoic deposits of southern Illinois. Fossils of “Ice Age” plants -closely related to forms living at the present time are occasionally -found in peat bogs or scattered throughout glacial deposits. - - [Illustration: uncaptioned] - - - - - KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS - - -Two keys, one for minerals and one for rocks, briefly present clues that -may aid the collector in identifying rocks and minerals found in -Illinois. In outline form, the keys are a guide to some of the easily -observable properties that various rocks and minerals display. - -The rocks and minerals in the school set of “Typical Rocks and Minerals -of Illinois” are included, plus other relatively common ones you might -find in Illinois. Because of the great diversity of rocks and minerals -in this state, the keys are not conclusive. It is therefore suggested -you consult other more complete keys (such as that in Dana’s _Manual of -Mineralogy_) when identifying rocks and minerals that are either from -other states or are difficult to identify. - -The minerals (p. 30-35) are arranged in two groups: 1) those with a -metallic luster, and 2) those with a nonmetallic luster. Each group is -arranged according to increasing hardness. Other characteristics such as -color, streak, cleavage, fracture, and composition are listed. - -The rocks (p. 36-39) are arranged according to their reaction to dilute -hydrochloric acid applied to a scratched surface. (The acid reacts more -readily to powdered material produced by scratching the rock.) After the -reaction to acid has been determined, the texture and components of the -rock should be noted. Because rocks grade into one another, clear -distinctions are not always possible. - - -MINERAL IDENTIFICATION KEY - - I. METALLIC LUSTER, STREAK COLORED - C—color H—hardness Remarks Name and - S—streak Cl—cleavage composition - F—fracture - A. Hardness not more than 2.5 - - C—lead gray H—2.5 Very heavy; occurs as Galena (31) - S—black Cl—cubic; crystals, grains, or PbS - perfect in 3 masses; easily - directions identified by color and - F—subconchoidal cleavage - or even - C—copper red H—2.5 Very heavy; apt to have Native copper - S—metallic, Cl—none green coating; distorted Cu - shiny F—jagged or wirelike forms; - malleable - - B. Hardness greater than 2.5 but not greater than 6.5 - - C—yellow-brown H—5.5 (may be as In earthy masses; Limonite (29) - to black low as 1) coloring material in FeO(OH)·H₂O - S—yellow-brown Cl—none many sandstones, - F—uneven conglomerates, and - soils; often mixed with - and difficult to - distinguish from - goethite and other iron - minerals - C—brassy yellow H—6 As compact masses, Pyrite (28) - S—greenish black Cl—poor grains, cubes, and in 8- FeS₂ - F—conchoidal to and 12-sided crystals; - uneven commonly associated with - coal, and with lead-zinc - ores of northwestern - Illinois - C—pale brassy H—6 As fibrous, radiating, Marcasite - yellow to Cl—poor tabular, and cocks-comb FeS₂ (28) - silver white F—uneven crystals or compact - S—greenish gray masses; usually lighter - colored than pyrite, but - difficult to distinguish - from pyrite; associated - with coal, and with - lead-zinc ores of - northwestern Illinois - - II. NONMETALLIC LUSTER, STREAK WHITE - A. Hardness not greater than 2 (can be scratched by fingernail) - - C—usually white H—2 Commonly found in Gypsum (26) - but may be Cl—perfect in Illinois as twinned or CaSO₄·2H₂O - almost any color one direction, needle-shaped crystals - less perfect in in weathered shales - two others containing pyrite and - calcium carbonate - C—white or a H—2 As needle-shaped Melanterite - shade of green crystals or powdery FeSO₄·7H₂O - coating on pyrite or - marcasite; has an - astringent taste - - B. Hardness greater than 2 but not greater than 3 (Can be scratched by a - penny) - - C—colorless, H—2-2.5 In scales or “books”; in Muscovite - silver white, Cl—perfect in splits into thin sheets; (white mica) - gray, brown one direction common in sandstones, (23) - shales, and in igneous (OH)₂KAl₂ - and metamorphic rocks AlSi₃O₈ - C—brown or black H—2.5-3 As scales or “books”; Biotite (23) - Cl—perfect in splits into thin sheets; (black mica) - one direction common in igneous and (OH)₂K(Mg,Fe)₃ - metamorphic rocks but AlSi₃O₈ - not in sedimentary rocks - such as sandstone or - shale - C—colorless, H—3 Common mineral; Calcite (24) - white, gray, Cl—perfect in effervesces vigorously CaCO₃ - and various three in cold acid; occurs in - tints directions, not many crystal forms and - at right angles as fibrous, banded, and - (rhombohedral) compact masses; chief - mineral in limestones - C—white, gray, H—3 Very heavy; commonly in Barite - red, or almost Cl—perfect in tabular crystals united BaSO₄ - any color one direction, in diverging groups, as - less perfect in laminated or granular - two other masses; associated with - directions fluorite in southern - Illinois - - C. Hardness greater than 3 but not greater than 5 - (Cannot be scratched by penny; can be scratched by knife) - - C—white, gray, H—3.5 Relatively heavy; Witherite - light yellow Cl—in one effervesces in acid; BaCO₃ - direction associated with fluorite - F—uneven and barite in southern - Illinois but is not - abundant - C—white, pink, H—3.5 In grains, rhombohedral Dolomite - gray, or light Cl—perfect in crystals and cleavable CaMg(CO₃)₂ - brown three or granular masses; - directions, not effervesces slowly in - at right angles cold acid when powdered, - (rhombohedral) more vigorously in warm - acid; principal mineral - in rock called dolomite - C—colorless, H—3.5 In fibrous or compact Cerussite - white, gray, masses or may be in PbCO₃ - grayish black orthorhombic crystals as - a coating on galena; - very heavy; effervesces - in acid; formed by - alteration of galena - C—brown to gray H—3.5 In fibrous or botryoidal Siderite - S—usually white Cl—in three masses or rhombohedral FeCO₃ - but may tend directions not crystals; effervesces in - toward brown at right angles hot acid - when weathered (rhombohedral) - slightly curved - surfaces - C—yellow, H—3.5 In crystals, in fibrous Sphalerite - yellow-brown to Cl—parallel to or layered masses; ZnS (30) - almost black dodecahedral associated with galena - S—light yellow faces; in six in northwestern - to brown directions Illinois, with fluorite - and galena in southern - Illinois - C—colorless, H—4 In cubes and cleavable Fluorite (25) - white, yellow, Cl—perfect, masses; many colors; (Fluorspar) - purple, green, parallel to mined in Hardin and Pope CaF₂ - blue octahedral counties - faces; in four - directions - C—white, tinted H—5 As crystalline Smithsonite - yellow, blue, incrustations or in ZnCO₃ - or green earthy or compact - masses; associated with - fluorite-sphalerite ores - in southern Illinois, - with galena and - sphalerite in - northwestern Illinois - - D. Hardness greater than 5 but not greater than 7 - - C—white, green, H—5-6 In long, slender 6-sided Amphibole Group - brown, black Cl—in two crystals; cleavage angle (Mg,Fe,Ca)₇ - directions important in (Si₈O₂₂)(OH)₂ - intersecting at differentiating from (may also - about 60° and pyroxenes; common in contain Na or - 120° metamorphic and some Al) - igneous rocks - C—gray, dark H—5-6 Crystals short, stout, Pyroxene Group - green, black, Cl—in two and 8-sided; cleavage (Mg,Ca,Fe)₂ - dark brown, directions angle important in (Si₂O₆) - bronze intersecting at differentiating from - about 90° amphiboles; common in - igneous and some - metamorphic rocks - C—white, gray, H—6 As crystals, cleavable Feldspar Group - pink, light Cl—in two masses and grains; (22) - blue, green directions common in igneous and K, Na, Ca, Ba - nearly at right metamorphic rocks, also (Al, Si)₄O₈ - angles in stream gravel and - sand; many varieties - C—white when H—7 Finely crystalline Chalcedony - pure; may be Cl—none variety of quartz; SiO₂ - colored by F—conchoidal botryoidal or - impurities concretionary masses; - lining in geodes - C—colorless, H—7 Most abundant mineral; Quartz (21) - white, or F—conchoidal occurs in 6-sided SiO₂ - almost any color crystals capped by - pyramids, in grains or - masses; principal - mineral in sandstone, - also abundant in igneous - and metamorphic rocks; - is a variety of silica - C—red H—7 A variety of quartz Jasper - F—conchoidal usually colored red by SiO₂ - hematite inclusions; - common in glacial and - river sand and gravel - found along Lake - Michigan shores and in - the Mississippi River - C—many; H—7 Cloudy banded variety of Agate - arranged in F—conchoidal silica; widely used as SiO₂ - bands semi-precious stones. - Onyx and silicified wood - are forms of agate; - found in glacial gravels - and upper Mesozoic - sediments in southern - Illinois - - E. Hardness greater than 7 (cannot be scratched by quartz) - - C—red, brown, H—7.5 Irregular grains or Garnet Group - yellow, green, Cl—poor masses; sometimes as (Ca,Mn,Fe,Mg)₃ - black, white F—even 12-, 24-, and 36-sided (Al,Cr)₂(SiO₄)₃ - crystals; abundant in - glacial sands and Lake - Michigan beach sands; - common in metamorphic - rocks - - - - - ROCK IDENTIFICATION KEY - - - SAMPLE - Scratch with a knife and apply dilute acid (HCl) - If rock does not scratch, go directly to I, II, or III - - No effervescence or very slight effervescence - I Coarse-grained (p. 37) - II Fine-grained (p. 38) - III Organic (p. 39) - Slight effervescence - gray, light gray, white, or brown: Dolomite - Vigorous effervescence - Composed of pebbles that effervesce - Rounded pebbles: Limestone conglomerate - Angular pebbles: Limestone breccia - Composed of crystals of calcite, fossil shells, or oolites: - Limestone - Composed of banded layers of crystalline calcite; commonly found in - caves, forming stalactites and stalagmites: Travertine - Large amount of insoluble residue left on acid-treated surface - Individual grains seen with unaided eye: Calcareous sandstone - Individual grains not seen with unaided eye: Calcareous shale - Composed of porous or cellular mass of calcite; commonly found near - springs and waterfalls: Tufa - - -I COARSE-GRAINED ROCKS - - A. Rock consists of interlocking grains or crystals, easily seen; too - hard to scratch with a knife - 1. Crystals aligned in one direction - a) Crystals in parallel bands with layers of quartz and feldspar - separated by mica and other minerals - Gneiss (6) - b) Crystals in thin parallel bands; tends to split into thin - sheets parallel to banding; some varieties may be scratched - with a knife - Schist (6) - 2. Crystals not aligned in any particular direction - a) Light gray, pink, red, or tan with only a few dark minerals; - feldspar and quartz principal minerals - Granite (1) - b) Dark to medium gray; composed of feldspar and dark minerals - with little quartz - Gabbro (2) - c) Dark green to black; essentially dark minerals, may have some - feldspar; quartz generally lacking - Peridotite (5) - d) Light color; similar to granite in texture but lacks quartz; - composed of feldspar and some dark minerals - Syenite - e) Large, easily seen crystals set in a fine- to extremely - fine-grained background; any color - Porphyry (3) - f) Essentially quartz; grains may be identifiable; specimens break - through rather than around grains - Quartzite (9) - B. Rock composed of individual rock particles or fragments, - non-interlocking crystals, cemented or not cemented together; may - or may not be scratched with a knife - 1. Particles or fragments not uniform in size; a mixture of pebbles, - sand, and smaller materials - a) Solid rock consisting of particles or fragments generally - rounded and cemented together - Conglomerate (7) - b) Solid rock consisting of particles or fragments, generally - angular and cemented together - Breccia - c) Fragments ranging in size from clay to large boulders; may be - compacted, but not cemented; much clay generally present; may - effervesce - Glacial till - d) Loose particles of many sizes, not cemented together; some - particles may effervesce - Gravel - 2. Rock particles or fragments, about the size of grains of sugar (2 - to .05 mm) - a) Loose particles consisting largely of quartz - Sand - b) Solid rock consisting largely of quartz; can be separated - easily into individual particles; granular; breaks around - rather than through grains - Sandstone (8) - - -II FINE-GRAINED ROCKS - - A. Cannot be scratched easily with a knife; crystals or particles not - easily seen with the unaided eye; very hard, difficult to - break; may contain a few crystals or particles large enough to - see; granular - 1) Dense; brittle; splintery or conchoidal fracture; sharp edges and - corners when broken; often associated with limestone; usually - white or gray; very dense, dull varieties called flint - Chert (27) - 2) Light gray, pink, red, or tan varieties common; boulders or - fragments in the glacial drift - Felsite - 3) Dark gray, greenish, black, or maroon varieties common; may have - small mineral-filled cavities; occurs as boulders or fragments - in the glacial drift - Basalt (4) - 4) Essentially quartz; grains may be identifiable; specimens break - through rather than around grains - Quartzite (9) - - B. May or may not be scratched with a knife; fairly uniformly fine - grained - 1) Soft; feels slippery or soapy when wet; may disintegrate in - water; gives off an earthy odor when breathed upon - Clay - 2) Loose; gritty; particles smaller than table salt - Silt - 3) Solid rock; often in thin beds or sheets; separates into silt; - mica flakes may be present; may contain fossils; may effervesce - slightly - Siltstone - 4) Solid rock; breaks into thin platy sheets; may feel slippery when - wet; black to gray; may contain fossils; shows thin laminations; - may effervesce - Shale (10) - 5) Solid rock: does not break into thin platy fragments; may - effervesce slightly - Mudstone - 6) Solid rock; usually gray or black; splits into platy sheets or - slabs; harder than shale - Slate - 7) Powdery; white or light brown; commonly associated with chert and - quartz from which it forms - Tripoli (19) - - -III ORGANIC ROCKS (DARK COLORED) - - A. Soft; spongy when wet; very lightweight when dry; forms in swampy - places - 1) Fine mass with coarse plant fragments; dark gray to black - Peat (13) - 2) Plant fragments small and not easily recognized; fine-grained; - black to dark gray; earthy - Muck - B. Hard but can be scratched with a knife - 1) Black; contains bands of shiny and dull material; burns well - Coal (14) - 2) Dark gray to black; does not contain shiny bands; splits into - thin sheets; burns poorly or not at all - Bituminous shale - - - - - EQUIPMENT FOR COLLECTING - - -1. Hammer (bricklayer’s) with one chisel or pick head. - -2. Cold chisel about 6 inches long with an edge about ½-inch wide. - -3. Dilute hydrochloric (muriatic) acid (10 percent solution) in a - dropper bottle for testing the presence of carbonate minerals. - Mark the bottle POISON. If acid is spilled on skin or clothing, - wipe immediately and, if possible, rinse with water. - -4. Magnifying glass or hand lens—10 power is probably most useful. - -5. Hardness testers—penny, square of window glass, pocket knife, or - nail. - -6. Streak plate—piece of unglazed white porcelain (such as the back of a - tile) for testing the color of the streak of minerals. - -7. Notebook and pencil for keeping records of the locality and bed from - which specimens are collected. - -8. Collecting bag—a musette bag, a knapsack, or similar bag of strong - material. - -9. Heavy gloves and goggles to protect hands and eyes. - -10. Labels and wrappings. Field identification of specimens may be - written on adhesive tape and attached to the specimen or on a slip - of paper enclosed in the wrapping. Newspaper, brown paper, or - paper bags can be used for wrapping specimens. Label the outside - of the wrapped specimen too. Take only the best specimens home - with you. Trim specimens to hand size (about 2 by 3 inches). - -All specimens should be labeled with the following information: name of - mineral or rock type, where found, collector’s name, and date. As - your collection grows, you may want to set up a system of - cataloging. List specimens and assign a number to each one. Place - a small amount of white enamel on a corner of each specimen; when - the enamel dries, number the sample with India ink; coat number - with lacquer. Corresponding numbers should be entered on your list - of specimens. - - - - - EDUCATIONAL EXTENSION PROGRAM - - -This book was prepared by the Educational Extension Section of the -Illinois State Geological Survey, principally Betty Jean Hanagan, I. -Edgar Odom, and Shirley J. Trueblood, under the direction of George M. -Wilson. They were assisted by other members of the Survey staff, -especially J. E. Lamar and J. C. Bradbury of the Industrial Minerals -Section. - -Educational Extension also serves the public by assembling and -distributing rock and mineral collections for Illinois educational -groups, giving lectures, preparing exhibits, answering queries about -identification of rocks and minerals, reporting Survey news, and -conducting earth science field trips. - -During each year six field trips are conducted in widely separated parts -of the state for teachers, students, and laymen. The general program is -especially designed to assist in teaching the earth sciences and to help -make Illinois citizens aware of the state’s great mineral wealth. - - Illinois State Geological Survey - Urbana, Illinois - - [Illustration: ILLINOIS _Land of Lincoln_] - - - - - Transcriber’s Notes - - -—Silently corrected a few typos. - -—Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - -—In the text versions only, text in italics is delimited by - _underscores_. - - - -*** END OF THE PROJECT GUTENBERG EBOOK GUIDE TO ROCKS AND MINERALS OF -ILLINOIS *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. 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