ACADEMY OF NATURAL SCIENCES. 323 



many places, the strata have been removed to a vertical depth of between two 

 and three thousand feet. 



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 ex- 

 tent 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 coun- 

 try from Walker's Lake to the Pes Chutes River, a distance of about four hun- 

 dred miles, and extending over a breadth of not less than one hundred. 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. Observations 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 

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

 infinitesimally small for each individual, but in reality enormous for the number 

 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 diatomncea, 

 is extremely probable. We know that silicification 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 con- 

 tain 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 rapidity as to form what may 

 be called, without exaggeration, mountain masses. It is also possible that tem- 

 perature may have something to do with this rapid development, and that vol- 

 canic regions may on this accouut 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 diatomacea, if closely examined, 

 would present" any such difficulties as to make the assumption necessary that 

 they have been ejected from the interior of the earth. Iu cases where infusoria 

 seem to have been actually ejected from craters, as is said to have been the case 

 in some of the South American volcanoes, it is not difficult to understand that 

 an ancient crater may have become filled up and temporarily converted into a 

 lake ; and that, after the growth and deposition of an infusorial deposit at the 

 bottom, a new eruption may have broken out in the same place as a previous 

 one, or in its immediate neighborhood. In such a case, among the ejected ma- 

 terial, a large quantity of the infusoria would be found mingled with the ashes, 



