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230 



THE AUEIFEEOUS GRAVELS OF THE SIEEEA NEVADA. 









Everything shows that the surface covered by fresh water in the region east 

 of the crest of the Sierra was, at a not very distant epoch, much greater in 

 extent than it now is. There existed, probably during or immediately after 

 the glacial epoch, a chain of great lakes occupying a large portion of the 

 country from Walker's Lake to the Des Chutes River, a distance of about 

 four hundred miles, and extending over a breadth of not less than one hun- 

 dred. A large portion of this region is now a volcanic plateau; and, where 

 cut into by the force of running water, the deposits of infusorial strata may 

 be seen, sometimes thin and unimportant, but often of great thickness. Ob- 

 servations and measurements of terraces and determination of the altitude of 

 all these old lake deposits will enable us at some future time to indicate on 

 the map the area once occupied by this great chain of inland seas. The vast 

 extent of the lacustrine infusorial formations on the east side of the Sierra is 

 thus accounted for, as well as the comparatively small area which they cover 

 on the western slope. 



In addition to the stratigraphical reason given above why the infusorial 

 strata should occur connected with eruptive masses, there may be a chemical 

 one which shall, in part, account for the apparent great development of the 

 diatomacece in volcanic regions. These organisms require an amount of silica, 

 infinitesimally small for each individual, but in reality enormous for the num- 

 ber of organisms required to develop themselves over the vast area and with 

 the thickness which they occupy. That a volcanic region should supply a 

 larger amount of silica in the state in which it can be appropriated by the 

 diatomacece is extremely probable. We know that siliciheation of all organic 

 matters occurring in these volcanic regions of our coast proceeds with the 

 greatest rapidity, and has taken place on an extensive scale. The thermal 

 springs contain a great amount of free silica, and it is in the vicinity of such 

 springs that large infusorial deposits are frequently found. It seems that it 

 could only be in regions particularly favorable for the secretion of their 

 silicious coverings, that these infusoria could be accumulated with such ra- 

 pidity as to form what may be called, without exaggeration, mountain masses. 

 It is also possible that temperature may have something to do with this rapid 

 development, and that volcanic regions may on this account be favorable 

 to it. 



To my apprehension, the phenomena of infusorial deposits in connection 

 with volcanic masses admit of an easy explanation on this coast, at least; and 

 I can hardly believe that any of the localities of diatomacew, if closely exam- 





