230 THE AURIFEROUS GRAVELS OF THE SIERRA 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 
diatomacee 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 im which it can be appropriated by the 
diutomacee is extremely probable. We know that silicification of all organic 
matters occurring in these voleanic 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 datomacee, if closely exam- 
