THE INFUSORIAL DEPOSITS: THEIR MODE OF OCCURRENCE. 229 
The mode of occurrence of these fresh water infusorial deposits in Cali- 
fornia, and on the Pacific coast in general, is very simple. They are accu- 
mulations of organisms which have been collected at the bottom of the lakes, 
or in the lake-like shallow expansions of rivers, in which they grew. This 
growth took place at atime when volcanic agencies were busily at work, giv- 
ing rise to accumulations of ashes, pumice, and other materials. The rapidity 
with which these infusorial deposits form, at the present time even, the vast 
extent over which they are distributed, and the general importance in the 
geological history of the earth, are now matters which are well understood, 
of the masses thus accumulated and in regard to which the store of facts has 
been rapidly growing in magnitude during the past few years. The mud 
deposits and deltas of rivers, the bottoms of lakes and swamps, and the bed 
of the ocean itself, are the repositories of these forms. Heat and stagnant 
water seem to be what is required for their rapid reproduction and the con- 
sequent rapid accumulation of their remains. 
The infusorial deposits of Central California —I refer now to those of fresh 
water origin, and connected with volcanic masses —are all situated in such 
positions as to show, that they were formed and deposited in shallow water ; 
that, through the various alternations of calm and convulsion in the Sierra, 
they were at one time allowed to accumulate in quiet, then swept over by 
masses of gravel and sand, indicating a furious rush of water, then covered 
with a shower of ashes and pumice from the neighboring volcanoes of the 
Sierra then in active operation; and finally, at the grand finale of the basaltic 
lava overflow of the chain, capped with this indestructible material, which has 
effectually prevented the washing away of the otherwise easily removed in- 
fusorial deposits. This is the connection between the volcanic and the infu- 
sorial masses ; by their absolute indestructibility the former have protected 
the latter from denudation, and consequently we see them always accompany- 
ing each other: for where the cover did not exist there the denuding forces 
have swept away every vestige of the soft and easily yielding material, or 
else it remains concealed under the water. To form an idea of the extent 
of the erosion which has taken place since these infusorial beds were depos- 
ited, and the consequent change in the configuration of the country, we must 
bear in mind that the whole of the present river caiions on the west slope of 
the Sierra have been excavated since that time, and that, in many places, the 
strata have been removed to a vertical depth of between two and three thou- 
sand feet. 
