. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 126 AQUEOUS BOCKS. !FiG. 11.—Microstructure of oolitic limestone. to show that many of the oolitic limestones owe their structure to the lime-seereting power of raieroscopic algse.^ Limestones vary almost indefinitely in structure and color. From the soft tufaceons or highly fossiliferous varieties there is a con- stant gradation to dense compact rocks breakini^ with a conchoidal or splintery fracture the true nature of which is sometimes to be ascer- tained only by chemical tests. There is a like variation in color. White th
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 126 AQUEOUS BOCKS. !FiG. 11.—Microstructure of oolitic limestone. to show that many of the oolitic limestones owe their structure to the lime-seereting power of raieroscopic algse.^ Limestones vary almost indefinitely in structure and color. From the soft tufaceons or highly fossiliferous varieties there is a con- stant gradation to dense compact rocks breakini^ with a conchoidal or splintery fracture the true nature of which is sometimes to be ascer- tained only by chemical tests. There is a like variation in color. White through all shades of gray to black are common, and more rarely occur yellow, brown, pink, or red varieties, the colors depending on organic matter and ferruginous oxides. Owing to the readiness with which calcium carbonate under- goes crystallization, even at ordinary temperatures, few lime- stones are wholly amorphous, but grade insensibly into holo- crystalline varieties such as are classed with the metamorphic rocks. The name marble is given to such limestones as are of sufficiently close texture to take a polish and of such colors as to make them desirable for ornamental work. A large proportion of the marbles belong, however, to the metamorphic group. (See p. 141.) Figure 12 shows the microscopic structure of a dark gray, variegated, highly fossiliferous limestone belonging to the Cincinnati group, near Hamilton, Ohio. It is a natural result of their method of formation that few limestones are of pure cal- cium carbonate. A portion of the calcium is often replaced by magnesium, giving rise to magnesian limestone, or to dolomite. This last can as a rule be distinguished from limestone only by its increased hardness () and specific gravity (). Frequently chemical tests are necessary, limestone effervescing readily when treated with dilute hydrochloric acid, while dolo- mite is unacted upon. ^American Geologist, Vol. X, No. 5, Please note that these ima
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