With the above information in mind, one may proceed with certainty to identify a layer of dolomite from a quarry or hillside, or a crystal of it in a hand specimen. First, determine that it is scratched readily with a knife blade or iron nail. Anything too hard to be scratched by steel is neither calcite nor dolomite. Second, scrape a small mound of powder on the lump specimens. Third, apply one or two drops of cold dilute acid to the lump near the powder and allow the acid to run into the powder. If the _lump_ effervesces _freely_ the specimen is _calcite_ mineral or limestone rock. If the _lump_ does _not effervesce freely_ but the _powder does_, it is _dolomite_ mineral or dolomite rock, dolomitic limestone. If neither lump nor powder effervesce it is neither calcite (ordinary limestone) nor dolomite (dolomitic limestone). In the latter case, it may be gypsum, barite, Shale, weathered chert, clay, or fire clay, or other rock. The composition of dolomite is calcium-magnesium carbonate, CaMg(CO₃)₂, and when pure runs about 54½ per cent calcium carbonate and 45½ per cent magnesium carbonate. However, dolomite is _not a mechanical mixture_ of the two carbonates; it is a single crystalline compound wherein the calcium and magnesium are securely interlocked within the arrangement of the atoms. For that reason, the extraction of magnesium metal or other magnesium compounds from dolomite is so difficult and costly that other magnesium minerals, although not nearly so abundant and accessible to industry as dolomite, have been processed to obtain the lightweight metal magnesium. The thick beds of Missouri dolomitic limestone (and some fairly pure dolomite) have been used chiefly as agricultural stone for soil sweetening, for building stone, gravel, and other purposes to which rough stone is put. Shale [Illustration: Shale bluff at a strip mine near Columbia.] Shale is a compressed, and layered or laminated clay or mud rock. Consequently it will return to mud if it is wetted with water and rubbed. This may serve as a test for shale. It may occur in thick layers or formations, five, ten to fifty or more feet in thickness, and it ranges downward to paper-thin partings between beds of limestone. It is also commonly associated with coal beds. The color of shale varies from light gray to black, or it may be tan, yellow, red, rust, purplish, or green. It is platy, and these thin plates or laminae, piled on each other, make up the shale bed. [Illustration: Hand specimen of shale shown in preceding picture. Note the characteristic thin layering or lamination.]