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 a time 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 canons 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. 





