Names used locally, and sometimes incorrectly from a strictly technical sense, for rocks and minerals will follow the generally accepted names, and both will be duplicated in the index at the back of the pamphlet to facilitate finding either one.
DETERMINATIVE KEY
A rock or mineral specimen which is unfamiliar to the collector may be identified by using the information in this booklet in either of two ways: (1) the reader may turn through the pages and compare his specimen with the photographs of others named there and read their descriptions until he finds a match for his specimen; or (2), the better way, he may classify his specimen first by the use of the determinative key which follows and be directed thereby to the pages in the book for confirmation of the name by the photographs, description, and discussion of the substance. The writer recommends the second method and has prepared this booklet on the assumption that the determinative key will be used.
The simplest and probably the best means of separating specimens of different rocks and minerals is on the basis of hardness, which means resistance to scratching. Crushing strength is different from hardness; therefore, in testing for hardness, do not attempt to pulverize. Merely determine if the specimen can be scratched with the substance indicated.
Determination of the mark or “streak” of a mineral when rubbed on a hard white rock or unglazed porcelain is demonstrated in the photograph on [page 56].
A. Specimens that can be scratched readily with the THUMB NAIL. 1. Become muddy when rubbed with a wet finger. Page [Shale] 20 [Fire Clay] 24 [Flint Fire Clay] 26 [Diaspore Clay] 27 2. Crumble easily into hard sand grains. [Sandstone] 30 3. Chalky, white, porous. [Tripoli] 37 [Weathered Chert] 36 [“Cotton Rock” Dolomite] 15 4. Clear and glassy, or glistening white; may split and show glassy, flat faces. [Gypsum] 63 B. Specimens scratched readily with a POCKET KNIFE, or IRON NAIL but not with the thumb nail. 1. Loose sand grains scratched off. [Sandstone] 30 2. Granular, but grains are tightly interlocked; also “bubbles” or effervesces in dilute muriatic (hydrochloric) acid. [Limestone] 10 [Dolomite] 10 [Marble] 15 Formed in a cave. [Cave Onyx] 16 [Travertine] 16 3. Chalky white, porous. [Weathered Chert] 36 4. Black. [Coal] 49 [Black Shale] 20 5. Pebbles or gravel cemented together. [Conglomerate] 54 6. Powder becomes muddy when wetted. [Hard Shale] 20 [Flint Fire Clay] 26 [Diaspore Clay] 27 7. Intense red; leaves a red mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Hematite] 56 [Iron Band Diaspore] 58 [Paint Ore] 58 8. Yellow, brown, or black and leaves a yellow-to-brown mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Limonite] 57 [Ochre] 57 9. Heavy, black, leaves a black or brownish black mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Manganese Ore] 58 10. Heavy, with bright metallic luster, and lead-colored on a freshly broken surface. [Galena] 59 11. Looks like rosin, or may be ruby-colored or black, but has a high resinous luster on freshly broken surface. [Sphalerite] 60 12. Glassy luster; water-white, milky, honey-colored, pink, gray; may occur in six-sided crystals, sometimes pyramid-shaped; always breaks with flat glistening faces; always reacts in the lump with cold dilute muriatic (hydrochloric) acid. [Calcite] 16 13. Like calcite above but may have a pink, pearly luster and curved crystal faces; reacts with cold dilute acid when powdered but not readily in lump form. [Dolomite] 18 14. Opaque white, glassy or bluish, very heavy, lustrous on freshly broken surface; does not react with acid. [Barite] 61 15. Flaky, micaceous like “isinglass”. [Mica] 44 C. Specimens TOO HARD to be scratched readily on a fresh surface with a pocket knife or iron nail; weathered specimens may be slightly scratched. 1. Very fine-grained throughout, compact; occurs in nodules, pebbles; breaks with a slick, curved, oyster-shell-like (conchoidal) fracture. [Chert], if white, gray or stained yellow or red 34 [Flint], if black 34 [Agate], if banded 37 [Petrified Wood], if it shows the grain or bark of wood 37 2. Granular like sandstone but extremely hard and breaks through the grains as readily as around them. [Quartzite] 41 [Quartzitic Sandstone] 30 3. Fine-grained, dark green to dark gray to greenish black; occurs in boulders north of Missouri River and in the granite and porphyry country or southeastern Missouri. [Basalt] 46 4. Very fine-grained, compact, pink, red, brown, gray; usually “freckled” or sprinkled with grains about 1/16 inch in diameter. [Porphyry] 45 [Rhyolite] 45 [Rhyolite Porphyry] 45 5. Coarse-grained (BB-shot size to considerably larger), glassy luster where freshly broken; pink, red, grey. [Granite] 38 [Gneiss], like granite but banded; occurs in boulders north of the Missouri River 38 6. Coarse-grained, dark green, dark gray, greenish-black. [Gabbro] 54 [Diabase] 48 7. Brassy, metallic, heavy; leaves a black to greenish black mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Pyrite] 51 [Marcasite] 51 8. Glassy fragments breaking with rough fracture, or may occur in six-sided crystals; clear, water-white, milk-white, gray or pink; in sand grains; in granite. [Quartz] 41 9. Intense red; leaves a red mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Hematite] 56 [Iron Band Diaspore] 58 [Paint Ore] 58 10. Yellow, brown or black, but leaves a yellow to brown mark or streak when rubbed on a hard white rock or on unglazed porcelain. [Limonite] 57
ROCK AND MINERAL DESCRIPTIONS
Limestone and Dolomite
Limestone is a bedded or layered rock found abundantly in Missouri in bluffs, creek beds, hill sides, and is known to underlie the soil in most of the south half of the state. It occurs in thin slabs, thick layers, and in massive beds which may make a small cliff in themselves. Limestone is soft enough to be scratched with steel. It is commonly white to grayish, but may be stained tan, yellowish, or reddish by iron oxide, or darkened through shades of gray to black by the presence of very finely-divided, black carbonaceous matter. It may be microscopically fine-grained (and then it can be used in lithographic printing in the reproduction of very fine images), or its grains may vary in size up to one-half inch in cross section.
