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 (3.5-4.5) and specific gravity (2.8-2.95). 

 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, 1892. 



