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ORIGIN OF CRYSTALLINE ROCKS. 509 
of animals, a process which has been active from very remote pe- 
riods. I showed in 1854 that the shells of Lingula and Orbicula, 
both those from the base of the paleozoic rocks and those of the 
present time, have (like Conularia and Serpulites) a chemical com- 
position similar to the skeletons of vertebrate animals.* Th 
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 
appropriated by diatoms and sponges, and made to assume organ- 
ized shapes. In a word, the assimilation of silica, like that of 
phosphate 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. 
I have thus endeavored to sketch, in a concise and rapid man- 
ner, the history of the earlier rock-formations of eastern North 
America, and of our progress in the knowledge of them; while 
I have, at the same time, dwelt upon some of the geognostical 
and chemical questions which their study suggests. With the 
record of the last thirty years before them, American geologists 
have cause for congratulation that their investigations have been 
so fruitful in great results. They see, however, at the same time, 
how much yet remains to be done in the study of the Appalachians 
and of our northeastern coast, before the history of these ancient 
rock-formations can be satisfactorily written. Meanwhile our ad- 
yenturous students are directing their labors to the vast regions of 
western America, where the results which have already been ob- 
tained are of profound interest. The progress of these investiga- 
tions will doubtless lead us to modify many of the views now 
accepted in science, and cannot fail greatly to enlarge the bound 
of geological knowledge. 
* Amer. Jour. Sci., II, xvii, 236. 
