IMPURITIES OF PHOSPHATE ROCK. 75 



ditions that are found in marine waters, that their remains have 

 often accumulated to form extensive and nearly pure limestones. 

 Fresh-water limestones of organic origin are also not uncommon, 

 although of lesser extent and thickness than are the marine forma- 

 tions. Moreover, not only is the carbonate taken from the water 

 through the action of organic life, but owing to changed conditions 

 in both fresh and marine waters it may be thrown out of solution, 

 forming limestone by chemical action. Thus by organic and chem- 

 ical processes extensive marine and fresh water limestones are 

 formed. 



Silica (Si02) in its round of circulation in the earth presents 

 some interesting analogies and yet strong contrasts to both cal- 

 cium phosphate and calcium carbonate. In point of abundance 

 silica exceeds both the carbonate and the phosphate, being by far 

 the most abundant constituent of the earth's crust, making up, 

 according to the estimate of Clarke, 59.79 per cent of the litho- 

 sphere.* In point of solubility, on the other hand, silica is much 

 less soluble than calcium phosphate, and under the conditions that 

 ordinarily prevail on and near the surface of the earth, many times 

 less soluble than calcium carbonate. However, by reason of its 

 abundance and the fact that in the form of sand it is ever present 

 in the soil and surface residual materials, it is found in solution 

 in all ground waters, and is present in the waters of the ocean 

 in small although recognizable amounts. Silica is also used to 

 some extent by plants and animals as skeleton-building material, 

 the largest users of silica for this purpose being, among plants, 

 the diatoms, and among animals, the unicellular radiolarians and 

 certain of the sponges. From the skeletons of these organisms a 

 limited amount of silica of organic origin has been included in sedi- 

 mentary rocks. Silica, however, as a skeleton-building material 

 has not been so extensively used as to result in the formation of 

 large deposits, and aside from diatomaceous earth, usually of local 

 extent, large deposits of silica of organic origin are unusual. 

 The massive accumulations of flint, not infrequently found in sedi- 

 mentary rocks, are formed by the replacement of the original rock 

 by silica in solution in the ground waters, presenting in this re- 

 spect an analogy to a similar process which has operated in the 

 formation of certain calcium phosphate deposits. 



*U. S. Geol. Surv. Bull. 330, p. 31, 1908. 



