312 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



tion of crystals of quartz or of hematite. Growing plants, it 

 is true, take up from the soil or the waters dissolved phosphates, 

 which pass into the skeletons of animals, a process which 

 has been active from very remote periods. I showed, in 1854, 

 that the shells of Lingula and Orbicula, both those from the 

 base of the palaeozoic rocks and those of the present time, have 

 (like Conularia and Serpulites) a chemical composition similar 

 to the skeletons of vertebrate animals.* The relations of both 

 carbonate and phosphate of lime to organized beings are similar 

 to those of silica, which, like them, is held in watery solution, 

 and by processes independent of life, is deposited both in 

 amorphous and crystalline forms, but in certain cases is appro- 

 priated by diatoms and sponges, and made to assume organized 

 shapes. In a word, the assimilation of silica, like that of phos- 

 phate and carbonate of lime, is a purely secondary and acci- 

 dental process ; and where life is absent, all of these substances 

 are deposited in mineral and inorganic forms. 



* American Journal of Science (2), XVII. 236. 



