Part 4

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

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_.