Part 9

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.

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.

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