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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..ad1fda0 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #66308 (https://www.gutenberg.org/ebooks/66308) 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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} -p.pcapc { margin-left:4.7em; text-indent:0em; text-align:justify; } -dl.pcap { margin-left:1em; font-family:sans-serif; font-size:90%; } -span.attr { font-size:80%; font-family:sans-serif; } -span.pn { display:inline-block; width:4.7em; text-align:left; margin-left:0; text-indent:0; } -</style> -</head> -<body> - -<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of Guide to Rocks and Minerals of Illinois, by Anonymous</div> - -<div style='display:block; margin:1em 0'> -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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. 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. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Title: Guide to Rocks and Minerals of Illinois</p> -<p style='display:block; margin-top:0; margin-bottom:1em; margin-left:2em; text-indent:0;'>Educational Series 5</p> - -<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Anonymous</div> - -<div style='display:block; margin:1em 0'>Release Date: September 14, 2021 [eBook #66308]</div> - -<div style='display:block; margin:1em 0'>Language: English</div> - -<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> - -<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Stephen Hutcheson and the Online Distributed Proofreading Team at https://www.pgdp.net</div> - -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK GUIDE TO ROCKS AND MINERALS OF ILLINOIS ***</div> -<div id="cover" class="img"> -<img id="coverpage" src="images/cover.jpg" alt="Guide to Rocks and Minerals of Illinois" width="1000" height="1515" /> -</div> -<div class="box"> -<p class="center"><i>Educational Series 5</i></p> -<h1><i>Guide to</i> -<br /><span class="ss">ROCKS AND MINERALS -<br />OF ILLINOIS</span></h1> -<p class="center"><i>Illinois State Geological Survey</i></p> -</div> -<div class="pb" id="Page_i">i</div> -<div class="img"> -<img src="images/p01.jpg" id="ncfig1" alt="uncaptioned" width="314" height="324" /> -</div> -<p class="center">STATE of ILLINOIS -<br />William G. Stratton, Governor</p> -<p class="center">DEPARTMENT of -<br />REGISTRATION and EDUCATION -<br />Vera M. Binks, Director</p> -<p class="tbcenter">First printing 1959 -<br />Second printing 1960</p> -<p class="tbcenter">ILLINOIS STATE GEOLOGICAL SURVEY -<br />John C. Frye, Chief -<br />URBANA, ILLINOIS</p> -<p class="tb"><span class="smaller">Printed by Authority of the State of Illinois</span></p> -<div class="pb" id="Page_1">1</div> -<h1 title=""><span class="smaller"><i>Guide to</i> <span class="ss">ROCKS AND MINERALS OF ILLINOIS</span></span></h1> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_2">2</div> -<div class="img" id="fig1"> -<img src="images/p02.jpg" alt="" width="554" height="999" /> -<p class="pcap">Fig. 1.—Geologic map of Illinois.</p> -</div> -<dl class="undent pcap"><dt>CAMBRIAN</dt> -<dt>ORDOVICIAN</dt> -<dt>ORDOVICIAN-SILURIAN</dt> -<dt>ORDOVICIAN-SILURIAN UNDER DRIFT</dt> -<dt>DEVONIAN-MISSISSIPPIAN</dt> -<dt>PENNSYLVANIAN UNDER DRIFT</dt> -<dt>CRETACEOUS-TERTIARY</dt> -<dt>KEY</dt> -<dd>Wisconsin glacial drift</dd> -<dd>Illinoian glacial drift</dd> -<dd>Pennsylvanian boundary</dd></dl> -<div class="pb" id="Page_3">3</div> -<p>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.</p> -<p>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.</p> -<p>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 (<a href="#fig1">fig. 1</a>).</p> -<p>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.</p> -<p>The glacial drift is the youngest and uppermost of the divisions -of the rock column (<a href="#fig2">fig. 2</a>). 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.</p> -<p>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.</p> -<div class="pb" id="Page_4">4</div> -<div class="img" id="fig2"> -<img src="images/p03.jpg" alt="" width="624" height="999" /> -<p class="pcap">Figure 2—Diagram of layers of rocks in Illinois.</p> -</div> -<table class="center"> -<tr class="th"><th class="l" colspan="2">Era </th><th> </th><th>General Types of Rocks</th></tr> -<tr class="th"><th> </th><th class="l" colspan="4">Period or System and Thickness</th></tr> -<tr class="th"><th> </th><th> </th><th class="l" colspan="2">Epoch</th></tr> -<tr><td colspan="4" class="l">CENOZOIC “Recent Life”</td></tr> -<tr><td colspan="4" class="l">Age of Mammals</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Quaternary<br />0-500′</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td colspan="2" class="l">Pleistocene or Glacial Age</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l"> </td><td class="l">Recent—alluvium in river valleys</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l"> </td><td class="l">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</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Tertiary<br />0-500′</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Pliocene </td><td class="l">Chert gravel; present in northern, southern, and western Illinois</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Eocene </td><td class="l">Mostly micaceous sand with some silt and clay; present only in southern Illinois</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l">Paleocene </td><td class="l">Mostly clay, little sand; present only in southern Illinois</td></tr> -<tr><td colspan="4" class="l">MESOZOIC “Middle Life”</td></tr> -<tr><td colspan="4" class="l">Age of Reptiles</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Cretaceous<br />0-300′ </td><td class="l">Mostly sand, some thin beds of clay and, locally, gravel; present only in southern Illinois</td></tr> -<tr><td colspan="4" class="l">PALEOZOIC “Ancient Life”</td></tr> -<tr><td colspan="4" class="l">Age of Amphibians and Early Plants</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Pennsylvanian<br />0-3,000′<br />(“Coal Measures”) </td><td class="l">Largely shale and sandstone with beds of coal, limestone, and clay</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Mississippian<br />0-3,500′ </td><td class="l">Black and gray shale at base; middle zone of thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone</td></tr> -<tr><td class="l"> </td><td colspan="3" class="l">Age of Fishes</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Devonian<br />0-1,500′ </td><td class="l">Thick limestone, minor sandstones and shales; largely chert and cherty limestone in southern Illinois</td></tr> -<tr><td class="l"> </td><td colspan="3" class="l">Age of Invertebrates</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Silurian<br />0-1,000′ </td><td class="l">Principally dolomite and limestone</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Ordovician<br />500-2,000′ </td><td class="l">Largely dolomite and limestone but contains sandstone, shale, and siltstone formations</td></tr> -<tr><td class="l"> </td><td colspan="2" class="l">Cambrian<br />1,500-3,000′ </td><td class="l">Chiefly sandstones with some dolomite and shale; exposed only in small areas in north-central Illinois</td></tr> -<tr><td colspan="4" class="l">ARCHEOZOIC and PROTEROZOIC</td></tr> -<tr><td class="l"> </td><td class="l"> </td><td class="l"> </td><td class="l">Igneous and metamorphic rocks; known in Illinois only from deep wells</td></tr> -</table> -<div class="pb" id="Page_5">5</div> -<p>As shown by the diagrammatic rock column (<a href="#fig2">fig. 2</a>), rocks -placed in the divisions called early Cenozoic and Mesozoic are -next in age to the Pleistocene. The map (<a href="#fig1">fig. 1</a>) 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.</p> -<p>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.</p> -<p>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 (<a href="#fig1">fig. 1</a>).</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>On the generalized rock column (<a href="#fig2">fig. 2</a>) 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.</p> -<div class="pb" id="Page_6">6</div> -<h2 id="c1"><span class="small">DESCRIPTION OF ROCKS AND MINERALS</span></h2> -<p>The terms <b>mineral</b> and <b>rock</b> 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.</p> -<p>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.</p> -<h2 id="c2"><span class="small">MINERALS</span></h2> -<p>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.</p> -<p>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 <b>habits</b>, and the flat surfaces -that develop are called <b>crystal faces</b>, the angles of which may -be used to identify the mineral.</p> -<p>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 <b>cleavage</b> -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 <b>fracture</b>, -and this too may give a clue to the identification of the mineral.</p> -<p>There are other physical features useful in identifying minerals. -Some of them, such as <b>color</b>, 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 -<b>streak</b>. <b>Luster</b> refers to the brightness of light reflected from the -<span class="pb" id="Page_7">7</span> -mineral’s surface. <b>Transparency</b> and <b>translucency</b> refer to the -mineral’s ability to transmit light, and <b>tenacity</b> is a measure of -its toughness.</p> -<p>Two special physical characteristics of minerals are important -to their identification—specific gravity and hardness. -<b>Specific gravity</b> 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.</p> -<p><b>Hardness</b> 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.</p> -<div class="verse"> -<p class="t0">1—Talc</p> -<p class="t0">2—Gypsum</p> -<p class="t0">3—Calcite</p> -<p class="t0">4—Fluorite</p> -<p class="t0">5—Apatite</p> -<p class="t0">6—Orthoclase</p> -<p class="t0">7—Quartz</p> -<p class="t0">8—Topaz</p> -<p class="t0">9—Corundum</p> -<p class="t0">10—Diamond</p> -</div> -<p>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.</p> -<h2 id="c3"><span class="small">ROCKS</span></h2> -<p>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, <b>igneous</b>, <b>sedimentary</b>, -and <b>metamorphic</b>.</p> -<p>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 <b>magmas</b>. When they cool, the elements of the individual -minerals come together and crystallize, as water crystallizes -into ice on a winter day.</p> -<p>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 -<span class="pb" id="Page_8">8</span> -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.</p> -<p>The glacial drift in Illinois contains many pieces of igneous -and metamorphic rocks, but most of the rocks native to Illinois -are sedimentary.</p> -<p>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 <b>clastic</b> (meaning -broken pieces) rocks, as opposed to those formed by chemical -precipitation from water and those that consist of fossil -remains.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_9">9</div> -<h2 id="c4"><span class="small">GRANITE <span id="m1">(1)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c5"><span class="small">GABBRO <span id="m2">(2)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Like other igneous rocks found near the surface in Illinois, -gabbro was carried into Illinois by the glaciers and deposited as -glacial debris.</p> -<h2 id="c6"><span class="small">PORPHYRY <span id="m3">(3)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Porphyry is found in Illinois only in glacial drift.</p> -<div class="pb" id="Page_10">10</div> -<h2 id="c7"><span class="small">BASALT <span id="m4">(4)</span></span></h2> -<p>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.</p> -<p>The glaciers brought basalt into Illinois along with other -igneous rocks.</p> -<h2 id="c8"><span class="small">PERIDOTITE <span id="m5">(5)</span></span></h2> -<p>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.</p> -<p>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.</p> -<h2 id="c9"><span class="small">GNEISS AND SCHIST <span id="m6">(6)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Gneiss and schist are not native to Illinois but are found in -the glacial drift.</p> -<div class="pb" id="Page_11">11</div> -<h2 id="c10"><span class="small">CONGLOMERATE <span id="m7">(7)</span></span></h2> -<p>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.</p> -<div class="img"> -<img src="images/p04.jpg" id="ncfig2" alt="uncaptioned" width="800" height="562" /> -</div> -<p>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.</p> -<p>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.</p> -<h2 id="c11"><span class="small">SANDSTONE <span id="m8">(8)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>Durability of sandstones depends largely on the character -of the cement. Some sandstones crumble easily, but others, especially -<span class="pb" id="Page_12">12</span> -those cemented by iron oxides or silica, are tough and -durable. Sandstones break around the grains, giving the broken -surface a granular appearance.</p> -<p>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.</p> -<h2 id="c12"><span class="small">QUARTZITE <span id="m9">(9)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Quartzite is abundant as boulders and pebbles in glacial -drift of Illinois, having been brought into the state during the -“Ice Age.”</p> -<h2 id="c13"><span class="small">SHALE <span id="m10">(10)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>The particles of most clay minerals are thin and flat and -overlap each other.</p> -<div class="pb" id="Page_13">13</div> -<p>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.</p> -<p>Shale is widely distributed in Illinois, especially in “Coal -Measures” rocks, and is used in manufacturing bricks, drain -tile, building tile, and lightweight aggregate.</p> -<h2 id="c14"><span class="small">CLAY <span id="m11">(11)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c15"><span class="small">LIMESTONE <span id="m12">(12)</span></span></h2> -<p>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).</p> -<p>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.</p> -<p>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. -<span class="pb" id="Page_14">14</span> -Limestone composed of tiny, rounded concretions is called <b>oolite</b> -or oolitic limestone.</p> -<p>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.</p> -<p>Limestone outcrops are abundant in Illinois, especially along -the bluffs of the Mississippi, Ohio, and Illinois Rivers.</p> -<p>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.</p> -<h2 id="c16"><span class="small">PEAT <span id="m13">(13)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>Peat is found in many places in Illinois but the largest deposits -are in northeastern Illinois.</p> -<div class="pb" id="Page_15">15</div> -<h2 id="c17"><span class="small">COAL <span id="m14">(14)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>The amount of such change that has taken place determines -the rank of the coal. The lowest ranks are called <b>lignite</b>, the -intermediate group is called <b>bituminous</b> (soft) coal, and coals of -the highest rank, with the highest carbon content, are called -<b>anthracite</b> (hard coal). Mineral matter, such as shale, clay, or -pyrite, generally is present in the coal and becomes ash when -the coal is burned.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_16">16</div> -<h2 id="c18"><span class="small">PETROLEUM <span id="m15">(15)</span></span></h2> -<p>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.</p> -<div class="img"> -<img src="images/p05.jpg" id="ncfig3" alt="uncaptioned" width="483" height="1001" /> -</div> -<dl class="undent pcap"><dt>Arch (anticline)</dt> -<dd>Gas</dd> -<dd>Porous rock</dd> -<dd>Nonporous rock</dd> -<dd>Oil saturated rock</dd> -<dd>Water</dd> -<dt>Break (fault)</dt> -<dd>Nonporous rock</dd> -<dd>Oil saturated porous rock</dd> -<dd>Water saturated porous rock</dd> -<dt>Trap formed by change in character of rock</dt> -<dd>Water</dd> -<dd>Nonporous rock</dd> -<dd>Porous rock</dd></dl> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_17">17</div> -<h2 id="c19"><span class="small">GLACIATED PEBBLES <span id="m16">(16)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c20"><span class="small">SILICA SAND <span id="m17">(17)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_18">18</div> -<h2 id="c21"><span class="small">MOLDING SAND <span id="m18">(18)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Natural bonded molding sand is produced in Fayette, Bond, -Bureau, Carroll, Kankakee, and Rock Island Counties, Illinois.</p> -<h2 id="c22"><span class="small">TRIPOLI <span id="m19">(19)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>Tripoli occurs in Alexander and Union Counties and is milled -at Elco and Tamms in Alexander County.</p> -<h2 id="c23"><span class="small">FULLER’S EARTH <span id="m20">(20)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_19">19</div> -<h2 id="c24"><span class="small">QUARTZ <span id="m21">(21)</span></span></h2> -<p>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.</p> -<div class="img"> -<img src="images/p06.jpg" id="ncfig4" alt="uncaptioned" width="669" height="800" /> -</div> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_20">20</div> -<h2 id="c25"><span class="small">FELDSPAR <span id="m22">(22)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c26"><span class="small">MICA <span id="m23">(23)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_21">21</div> -<h2 id="c27"><span class="small">CALCITE <span id="m24">(24)</span></span></h2> -<p>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.</p> -<p>Transparent calcite possesses the property of double refraction; -an image appears double when viewed through a calcite -cleavage block.</p> -<p>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.</p> -<div class="img"> -<img src="images/p07.jpg" id="ncfig5" alt="uncaptioned" width="800" height="660" /> -</div> -<p>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.</p> -<p>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.</p> -<p>Calcite is the principal mineral in limestones and occurs as -a component of many concretions.</p> -<div class="pb" id="Page_22">22</div> -<h2 id="c28"><span class="small">FLUORITE <span id="m25">(25)</span></span></h2> -<p>FLUORITE, or fluorspar, is made up of the elements calcium -and fluorine. The mineral is easily identified by its perfect -cleavage, color, and hardness.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c29"><span class="small">GYPSUM <span id="m26">(26)</span></span></h2> -<p>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.</p> -<div class="img"> -<img src="images/p08.jpg" id="ncfig6" alt="uncaptioned" width="328" height="800" /> -</div> -<p>Gypsum occurs in several forms. <b>Selenite</b> -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.</p> -<p><b>Satin spar</b> 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. <b>Massive gypsum</b> -is granular.</p> -<p>Gypsum deposits occur deep underground -in Illinois but thus far have not been mined.</p> -<div class="pb" id="Page_23">23</div> -<h2 id="c30"><span class="small">CHERT <span id="m27">(27)</span></span></h2> -<p>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.</p> -<p><b>Flint</b>, 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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>White and gray chert occur as massive bedrock deposits -several hundred feet thick in Union and Alexander Counties.</p> -<h2 id="c31"><span class="small">PYRITE AND MARCASITE <span id="m28">(28)</span></span></h2> -<p>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.</p> -<p>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, -<span class="pb" id="Page_24">24</span> -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.</p> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c32"><span class="small">LIMONITE <span id="m29">(29)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<h2 id="c33"><span class="small">SPHALERITE <span id="m30">(30)</span></span></h2> -<p>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, -<span class="pb" id="Page_25">25</span> -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.</p> -<p>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.</p> -<h2 id="c34"><span class="small">GALENA <span id="m31">(31)</span></span></h2> -<p>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.</p> -<div class="img"> -<img src="images/p09.jpg" id="ncfig7" alt="uncaptioned" width="800" height="615" /> -</div> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_26">26</div> -<h2 id="c35"><span class="small">CONCRETIONS <span id="m32">(32)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p>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).</p> -<p>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.</p> -<p>Limonite concretions, generally with a high content of clay, -silt, or sand, occur in loess, shale, and sandstone.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_27">27</div> -<h2 id="c36"><span class="small">GEODES <span id="m33">(33)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="img"> -<img src="images/p10.jpg" id="ncfig8" alt="uncaptioned" width="659" height="799" /> -</div> -<p>Quartz is the most common -mineral deposited in geodes, but -calcite, aragonite, dolomite, -siderite, pyrite, galena, fluorite, -and sphalerite also are -found.</p> -<p>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.</p> -<p>Hollow geodes are the most -desirable because they have better crystals. They can be distinguished -from solid ones by their comparative lightness of -weight.</p> -<p>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.</p> -<div class="pb" id="Page_28">28</div> -<h2 id="c37"><span class="small">ANIMAL FOSSILS <span id="m34">(34)</span></span></h2> -<p>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.</p> -<p>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.</p> -<div class="img"> -<img src="images/p11.jpg" id="ncfig9" alt="uncaptioned" width="516" height="800" /> -</div> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="pb" id="Page_29">29</div> -<h2 id="c38"><span class="small">PLANT FOSSILS <span id="m35">(35)</span></span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<div class="img"> -<img src="images/p11a.jpg" id="ncfig10" alt="uncaptioned" width="487" height="800" /> -</div> -<div class="pb" id="Page_30">30</div> -<h2 id="c39"><span class="small">KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS</span></h2> -<p>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.</p> -<p>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 -<i>Manual of Mineralogy</i>) when identifying rocks and minerals that are either -from other states or are difficult to identify.</p> -<p>The minerals (p. <a href="#Page_30">30</a>-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.</p> -<p>The rocks (p. <a href="#Page_36">36</a>-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.</p> -<h3 id="c40">MINERAL IDENTIFICATION KEY</h3> -<table class="center"> -<tr class="th"><th colspan="4">I. METALLIC LUSTER, STREAK COLORED</th></tr> -<tr class="th"><th>C—color<br />S—streak </th><th>H—hardness<br />Cl—cleavage<br />F—fracture </th><th>Remarks </th><th>Name and composition</th></tr> -<tr class="th"><th colspan="4">A. Hardness not more than 2.5</th></tr> -<tr><td class="l">C—lead gray<br />S—black </td><td class="l">H—2.5<br />Cl—cubic; perfect in 3 directions<br />F—subconchoidal or even </td><td class="l">Very heavy; occurs as crystals, grains, or masses; easily identified by color and cleavage </td><td class="l">Galena (<a href="#m31">31</a>)<br />PbS</td></tr> -<tr><td class="l">C—copper red<br />S—metallic, shiny </td><td class="l">H—2.5<br />Cl—none<br />F—jagged </td><td class="l">Very heavy; apt to have green coating; distorted or wirelike forms; malleable </td><td class="l">Native copper<br />Cu</td></tr> -<tr class="pbtr"><td colspan="4"> -<span class="pb" id="Page_31">31</span> -</td></tr> -<tr class="th"><th colspan="4">B. Hardness greater than 2.5 but not greater than 6.5</th></tr> -<tr><td class="l">C—yellow-brown to black<br />S—yellow-brown </td><td class="l">H—5.5 (may be as low as 1)<br />Cl—none<br />F—uneven </td><td class="l">In earthy masses; coloring material in many sandstones, conglomerates, and soils; often mixed with and difficult to distinguish from goethite and other iron minerals </td><td class="l">Limonite (<a href="#m29">29</a>)<br />FeO(OH)·H₂O</td></tr> -<tr><td class="l">C—brassy yellow<br />S—greenish black </td><td class="l">H—6<br />Cl—poor<br />F—conchoidal to uneven </td><td class="l">As compact masses, grains, cubes, and in 8- and 12-sided crystals; commonly associated with coal, and with lead-zinc ores of northwestern Illinois </td><td class="l">Pyrite (<a href="#m28">28</a>)<br />FeS₂ </td><td></td></tr> -<tr><td class="l">C—pale brassy yellow to silver white<br />S—greenish gray </td><td class="l">H—6<br />Cl—poor<br />F—uneven </td><td class="l">As fibrous, radiating, tabular, and cocks-comb crystals or compact masses; usually lighter colored than pyrite, but difficult to distinguish from pyrite; associated with coal, and with lead-zinc ores of northwestern Illinois </td><td class="l">Marcasite<br />FeS₂ (<a href="#m28">28</a>)</td></tr> -<tr class="th"><th colspan="4">II. NONMETALLIC LUSTER, STREAK WHITE</th></tr> -<tr class="th"><th colspan="4">A. Hardness not greater than 2 (can be scratched by fingernail)</th></tr> -<tr><td class="l">C—usually white but may be almost any color </td><td class="l">H—2<br />Cl—perfect in one direction, less perfect in two others </td><td class="l">Commonly found in Illinois as twinned or needle-shaped crystals in weathered shales containing pyrite and calcium carbonate </td><td class="l">Gypsum (<a href="#m26">26</a>)<br />CaSO₄·2H₂O</td></tr> -<tr class="pbtr"><td colspan="4"> -<span class="pb" id="Page_32">32</span> -</td></tr> -<tr><td class="l">C—white or a shade of green </td><td class="l">H—2 </td><td class="l">As needle-shaped crystals or powdery coating on pyrite or marcasite; has an astringent taste </td><td class="l">Melanterite<br />FeSO₄·7H₂O</td></tr> -<tr class="th"><th colspan="4">B. Hardness greater than 2 but not greater than 3 (Can be scratched by a penny)</th></tr> -<tr><td class="l">C—colorless, silver white, gray, brown </td><td class="l">H—2-2.5<br />Cl—perfect in one direction </td><td class="l">In scales or “books”; in splits into thin sheets; common in sandstones, shales, and in igneous and metamorphic rocks </td><td class="l">Muscovite (white mica) (<a href="#m23">23</a>)<br />(OH)₂KAl₂ AlSi₃O₈</td></tr> -<tr><td class="l">C—brown or black </td><td class="l">H—2.5-3<br />Cl—perfect in one direction </td><td class="l">As scales or “books”; splits into thin sheets; common in igneous and metamorphic rocks but not in sedimentary rocks such as sandstone or shale </td><td class="l">Biotite (<a href="#m23">23</a>) (black mica)<br />(OH)₂K(Mg,Fe)₃ AlSi₃O₈</td></tr> -<tr><td class="l">C—colorless, white, gray, and various tints </td><td class="l">H—3<br />Cl—perfect in three directions, not at right angles (rhombohedral) </td><td class="l">Common mineral; effervesces vigorously in cold acid; occurs in many crystal forms and as fibrous, banded, and compact masses; chief mineral in limestones </td><td class="l">Calcite (<a href="#m24">24</a>)<br />CaCO₃</td></tr> -<tr><td class="l">C—white, gray, red, or almost any color </td><td class="l">H—3<br />Cl—perfect in one direction, less perfect in two other directions </td><td class="l">Very heavy; commonly in tabular crystals united in diverging groups, as laminated or granular masses; associated with fluorite in southern Illinois </td><td class="l">Barite<br />BaSO₄</td></tr> -<tr class="pbtr"><td colspan="4"> -<span class="pb" id="Page_33">33</span> -</td></tr> -<tr class="th"><th colspan="4">C. Hardness greater than 3 but not greater than 5</th></tr> -<tr class="th"><th colspan="4">(Cannot be scratched by penny; can be scratched by knife)</th></tr> -<tr><td class="l">C—white, gray, light yellow </td><td class="l">H—3.5<br />Cl—in one direction F—uneven </td><td class="l">Relatively heavy; effervesces in acid; associated with fluorite and barite in southern Illinois but is not abundant </td><td class="l">Witherite<br />BaCO₃</td></tr> -<tr><td class="l">C—white, pink, gray, or light brown </td><td class="l">H—3.5<br />Cl—perfect in three directions, not at right angles (rhombohedral) </td><td class="l">In grains, rhombohedral crystals and cleavable or granular masses; effervesces slowly in cold acid when powdered, more vigorously in warm acid; principal mineral in rock called dolomite </td><td class="l">Dolomite<br />CaMg(CO₃)₂</td></tr> -<tr><td class="l">C—colorless, white, gray, grayish black </td><td class="l">H—3.5 </td><td class="l">In fibrous or compact masses or may be in orthorhombic crystals as a coating on galena; very heavy; effervesces in acid; formed by alteration of galena </td><td class="l">Cerussite<br />PbCO₃</td></tr> -<tr><td class="l">C—brown to gray S—usually white but may tend toward brown when weathered </td><td class="l">H—3.5<br />Cl—in three directions not at right angles (rhombohedral) slightly curved surfaces </td><td class="l">In fibrous or botryoidal masses or rhombohedral crystals; effervesces in hot acid </td><td class="l">Siderite<br />FeCO₃</td></tr> -<tr><td class="l">C—yellow, yellow-brown to almost black<br />S—light yellow to brown </td><td class="l">H—3.5<br />Cl—parallel to dodecahedral faces; in six directions </td><td class="l">In crystals, in fibrous or layered masses; associated with galena in northwestern Illinois, with fluorite and galena in southern Illinois </td><td class="l">Sphalerite<br />ZnS (<a href="#m30">30</a>)</td></tr> -<tr class="pbtr"><td colspan="4"> -<span class="pb" id="Page_34">34</span> -</td></tr> -<tr><td class="l">C—colorless, white, yellow, purple, green, blue </td><td class="l">H—4<br />Cl—perfect, parallel to octahedral faces; in four directions </td><td class="l">In cubes and cleavable masses; many colors; mined in Hardin and Pope counties </td><td class="l">Fluorite (<a href="#m25">25</a>) (Fluorspar)<br />CaF₂</td></tr> -<tr><td class="l">C—white, tinted yellow, blue, or green </td><td class="l">H—5 </td><td class="l">As crystalline incrustations or in earthy or compact masses; associated with fluorite-sphalerite ores in southern Illinois, with galena and sphalerite in northwestern Illinois </td><td class="l">Smithsonite<br />ZnCO₃</td></tr> -<tr class="th"><th colspan="4">D. Hardness greater than 5 but not greater than 7</th></tr> -<tr><td class="l">C—white, green, brown, black </td><td class="l">H—5-6<br />Cl—in two directions intersecting at about 60° and 120° </td><td class="l">In long, slender 6-sided crystals; cleavage angle important in differentiating from pyroxenes; common in metamorphic and some igneous rocks </td><td class="l">Amphibole Group (Mg,Fe,Ca)₇​(Si₈O₂₂)(OH)₂ (may also contain Na or Al)</td></tr> -<tr><td class="l">C—gray, dark green, black, dark brown, bronze </td><td class="l">H—5-6<br />Cl—in two directions intersecting at about 90° </td><td class="l">Crystals short, stout, and 8-sided; cleavage angle important in differentiating from amphiboles; common in igneous and some metamorphic rocks </td><td class="l">Pyroxene Group<br />(Mg,Ca,Fe)₂​(Si₂O₆)</td></tr> -<tr><td class="l">C—white, gray, pink, light blue, green </td><td class="l">H—6<br />Cl—in two directions nearly at right angles </td><td class="l">As crystals, cleavable masses and grains; common in igneous and metamorphic rocks, also in stream gravel and sand; many varieties </td><td class="l">Feldspar Group (<a href="#m22">22</a>)<br />K, Na, Ca, Ba (Al, Si)₄O₈</td></tr> -<tr><td class="l">C—white when pure; may be colored by impurities </td><td class="l">H—7<br />Cl—none F—conchoidal </td><td class="l">Finely crystalline variety of quartz; botryoidal or concretionary masses; lining in geodes </td><td class="l">Chalcedony<br />SiO₂</td></tr> -<tr class="pbtr"><td colspan="4"> -<span class="pb" id="Page_35">35</span> -</td></tr> -<tr><td class="l">C—colorless, white, or almost any color </td><td class="l">H—7<br />F—conchoidal </td><td class="l">Most abundant mineral; occurs in 6-sided crystals capped by pyramids, in grains or masses; principal mineral in sandstone, also abundant in igneous and metamorphic rocks; is a variety of silica </td><td class="l">Quartz (<a href="#m21">21</a>)<br />SiO₂</td></tr> -<tr><td class="l">C—red </td><td class="l">H—7<br />F—conchoidal </td><td class="l">A variety of quartz usually colored red by hematite inclusions; common in glacial and river sand and gravel found along Lake Michigan shores and in the Mississippi River </td><td class="l">Jasper<br />SiO₂</td></tr> -<tr><td class="l">C—many; arranged in bands </td><td class="l">H—7<br />F—conchoidal </td><td class="l">Cloudy banded variety of silica; widely used as semi-precious stones. Onyx and silicified wood are forms of agate; found in glacial gravels and upper Mesozoic sediments in southern Illinois </td><td class="l">Agate<br />SiO₂</td></tr> -<tr class="th"><th colspan="4">E. Hardness greater than 7 (cannot be scratched by quartz)</th></tr> -<tr><td class="l">C—red, brown, yellow, green, black, white </td><td class="l">H—7.5<br />Cl—poor<br />F—even </td><td class="l">Irregular grains or masses; sometimes as 12-, 24-, and 36-sided crystals; abundant in glacial sands and Lake Michigan beach sands; common in metamorphic rocks </td><td class="l">Garnet Group<br />(Ca,Mn,Fe,Mg)₃​(Al,Cr)₂(SiO₄)₃</td></tr> -</table> -<div class="pb" id="Page_36">36</div> -<h2 id="c41"><span class="small">ROCK IDENTIFICATION KEY</span></h2> -<dl class="undent"><dt>SAMPLE</dt> -<dd>Scratch with a knife and apply dilute acid (HCl)</dd> -<dd>If rock does not scratch, go directly to I, II, or III</dd></dl> -<dl class="undent"><dt>No effervescence or very slight effervescence</dt> -<dd>I Coarse-grained (<a href="#Page_37">p. 37</a>)</dd> -<dd>II Fine-grained (<a href="#Page_38">p. 38</a>)</dd> -<dd>III Organic (<a href="#Page_39">p. 39</a>)</dd> -<dt>Slight effervescence</dt> -<dd>gray, light gray, white, or brown: Dolomite</dd> -<dt>Vigorous effervescence</dt> -<dd>Composed of pebbles that effervesce</dd> -<dd class="t">Rounded pebbles: Limestone conglomerate</dd> -<dd class="t">Angular pebbles: Limestone breccia</dd> -<dd>Composed of crystals of calcite, fossil shells, or oolites: Limestone</dd> -<dd>Composed of banded layers of crystalline calcite; commonly found in caves, forming stalactites and stalagmites: Travertine</dd> -<dd>Large amount of insoluble residue left on acid-treated surface</dd> -<dd class="t">Individual grains seen with unaided eye: Calcareous sandstone</dd> -<dd class="t">Individual grains not seen with unaided eye: Calcareous shale</dd> -<dd>Composed of porous or cellular mass of calcite; commonly found near springs and waterfalls: Tufa</dd></dl> -<div class="pb" id="Page_37">37</div> -<h3 id="c42">I COARSE-GRAINED ROCKS</h3> -<dl class="undent"><dt>A. Rock consists of interlocking grains or crystals, easily seen; too hard to scratch with a knife</dt> -<dd>1. Crystals aligned in one direction</dd> -<dd class="t">a) Crystals in parallel bands with layers of quartz and feldspar separated by mica and other minerals</dd> -<dt class="jr">Gneiss (<a href="#m6">6</a>)</dt> -<dd class="t">b) Crystals in thin parallel bands; tends to split into thin sheets parallel to banding; some varieties may be scratched with a knife</dd> -<dt class="jr">Schist (<a href="#m6">6</a>)</dt> -<dd>2. Crystals not aligned in any particular direction</dd> -<dd class="t">a) Light gray, pink, red, or tan with only a few dark minerals; feldspar and quartz principal minerals</dd> -<dt class="jr">Granite (<a href="#m1">1</a>)</dt> -<dd class="t">b) Dark to medium gray; composed of feldspar and dark minerals with little quartz</dd> -<dt class="jr">Gabbro (<a href="#m2">2</a>)</dt> -<dd class="t">c) Dark green to black; essentially dark minerals, may have some feldspar; quartz generally lacking</dd> -<dt class="jr">Peridotite (<a href="#m5">5</a>)</dt> -<dd class="t">d) Light color; similar to granite in texture but lacks quartz; composed of feldspar and some dark minerals</dd> -<dt class="jr">Syenite</dt> -<dd class="t">e) Large, easily seen crystals set in a fine- to extremely fine-grained background; any color</dd> -<dt class="jr">Porphyry (<a href="#m3">3</a>)</dt> -<dd class="t">f) Essentially quartz; grains may be identifiable; specimens break through rather than around grains</dd> -<dt class="jr">Quartzite (<a href="#m9">9</a>)</dt> -<dt>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</dt> -<dd>1. Particles or fragments not uniform in size; a mixture of pebbles, sand, and smaller materials</dd> -<dd class="t">a) Solid rock consisting of particles or fragments generally rounded and cemented together</dd> -<dt class="jr">Conglomerate (<a href="#m7">7</a>)</dt> -<dt class="pb" id="Page_38">38</dt> -<dd class="t">b) Solid rock consisting of particles or fragments, generally angular and cemented together</dd> -<dt class="jr">Breccia</dt> -<dd class="t">c) Fragments ranging in size from clay to large boulders; may be compacted, but not cemented; much clay generally present; may effervesce</dd> -<dt class="jr">Glacial till</dt> -<dd class="t">d) Loose particles of many sizes, not cemented together; some particles may effervesce</dd> -<dt class="jr">Gravel</dt> -<dd>2. Rock particles or fragments, about the size of grains of sugar (2 to .05 mm)</dd> -<dd class="t">a) Loose particles consisting largely of quartz</dd> -<dt class="jr">Sand</dt> -<dd class="t">b) Solid rock consisting largely of quartz; can be separated easily into individual particles; granular; breaks around rather than through grains</dd> -<dt class="jr">Sandstone (<a href="#m8">8</a>)</dt></dl> -<h3 id="c43">II FINE-GRAINED ROCKS</h3> -<dl class="undent"><dt>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</dt> -<dd>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</dd> -<dt class="jr">Chert (<a href="#m27">27</a>)</dt> -<dd>2) Light gray, pink, red, or tan varieties common; boulders or fragments in the glacial drift</dd> -<dt class="jr">Felsite</dt> -<dd>3) Dark gray, greenish, black, or maroon varieties common; may have small mineral-filled cavities; occurs as boulders or fragments in the glacial drift</dd> -<dt class="jr">Basalt (<a href="#m4">4</a>)</dt> -<dd>4) Essentially quartz; grains may be identifiable; specimens break through rather than around grains</dd> -<dt class="jr">Quartzite (<a href="#m9">9</a>)</dt></dl> -<div class="pb" id="Page_39">39</div> -<dl class="undent"><dt>B. May or may not be scratched with a knife; fairly uniformly fine grained</dt> -<dd>1) Soft; feels slippery or soapy when wet; may disintegrate in water; gives off an earthy odor when breathed upon</dd> -<dt class="jr">Clay</dt> -<dd>2) Loose; gritty; particles smaller than table salt</dd> -<dt class="jr">Silt</dt> -<dd>3) Solid rock; often in thin beds or sheets; separates into silt; mica flakes may be present; may contain fossils; may effervesce slightly</dd> -<dt class="jr">Siltstone</dt> -<dd>4) Solid rock; breaks into thin platy sheets; may feel slippery when wet; black to gray; may contain fossils; shows thin laminations; may effervesce</dd> -<dt class="jr">Shale (<a href="#m10">10</a>)</dt> -<dd>5) Solid rock: does not break into thin platy fragments; may effervesce slightly</dd> -<dt class="jr">Mudstone</dt> -<dd>6) Solid rock; usually gray or black; splits into platy sheets or slabs; harder than shale</dd> -<dt class="jr">Slate</dt> -<dd>7) Powdery; white or light brown; commonly associated with chert and quartz from which it forms</dd> -<dt class="jr">Tripoli (<a href="#m19">19</a>)</dt></dl> -<h3 id="c44">III ORGANIC ROCKS (DARK COLORED)</h3> -<dl class="undent"><dt>A. Soft; spongy when wet; very lightweight when dry; forms in swampy places</dt> -<dd>1) Fine mass with coarse plant fragments; dark gray to black</dd> -<dt class="jr">Peat (<a href="#m13">13</a>)</dt> -<dd>2) Plant fragments small and not easily recognized; fine-grained; black to dark gray; earthy</dd> -<dt class="jr">Muck</dt> -<dt>B. Hard but can be scratched with a knife</dt> -<dd>1) Black; contains bands of shiny and dull material; burns well</dd> -<dt class="jr">Coal (<a href="#m14">14</a>)</dt> -<dd>2) Dark gray to black; does not contain shiny bands; splits into thin sheets; burns poorly or not at all</dd> -<dt class="jr">Bituminous shale</dt></dl> -<div class="pb" id="Page_40">40</div> -<h2 id="c45"><span class="small">EQUIPMENT FOR COLLECTING</span></h2> -<p class="revint">1. Hammer (bricklayer’s) with one chisel or pick head.</p> -<p class="revint">2. Cold chisel about 6 inches long with an edge about ½-inch -wide.</p> -<p class="revint">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.</p> -<p class="revint">4. Magnifying glass or hand lens—10 power is probably most -useful.</p> -<p class="revint">5. Hardness testers—penny, square of window glass, pocket -knife, or nail.</p> -<p class="revint">6. Streak plate—piece of unglazed white porcelain (such as the -back of a tile) for testing the color of the streak of minerals.</p> -<p class="revint">7. Notebook and pencil for keeping records of the locality and -bed from which specimens are collected.</p> -<p class="revint">8. Collecting bag—a musette bag, a knapsack, or similar bag -of strong material.</p> -<p class="revint">9. Heavy gloves and goggles to protect hands and eyes.</p> -<p class="revint">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).</p> -<p class="revint">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.</p> -<div class="pb" id="Page_41">41</div> -<h2 id="c46"><span class="small">EDUCATIONAL EXTENSION PROGRAM</span></h2> -<p>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.</p> -<p>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.</p> -<p>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.</p> -<p><span class="lr">Illinois State Geological Survey</span> -<span class="lr">Urbana, Illinois</span></p> -<div class="pb" id="Page_42">42</div> -<div class="img"> -<img src="images/p20.jpg" id="ncfig11" alt="ILLINOIS _Land of Lincoln_" width="800" height="142" /> -</div> -<h2 id="trnotes">Transcriber’s Notes</h2> -<ul> -<li>Silently corrected a few typos.</li> -<li>Retained publication information from the printed edition: this eBook is public-domain in the country of publication.</li> -<li>In the text versions only, text in italics is delimited by _underscores_.</li> -</ul> -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK GUIDE TO ROCKS AND MINERALS OF ILLINOIS ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -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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