

■brtWW^Kl 







228 



THE AUEIFEEOUS GRAVELS OF THE SIEEEA NEVADA, 





that here, as farther south on the Sierra Nevada, by far the larger portion of 

 the mass is made up more or less of finely triturated volcanic material. 



The number of localities of infusorial silica along the flanks of the Sierra, as 

 well as in the Cascade Eange and other portions of the Cordilleras, is so great 

 that it would be impossible to enumerate them all. It is sufficient to have 

 mentioned some of the most important, and to have called attention to their 

 mode of occurrence. 



In view of what has been said in the preceding pages, in regard to the 

 former exaggerations of the thickness of the infusorial deposits at various 

 localities in the Cordilleras, it will only be necessary to briefly refer to Ehrcn- 

 berg's endeavor to account for this supposed enormous development of rocks 

 made up of such extremely minute organisms. Observations made by him 

 on the island of Ischia showed that, in that locality at least, an appearance 



■ 



of great thickness of the infusorial masses had been given by the carrying 

 down from above and deposition, at the base of a cliff of volcanic rock, of ma- 

 terial originating in a hot spring, the water of which fell from the edge of 

 the rock to the bottom, thus giving rise to a conical deposit ; which, although 

 really of comparatively small dimensions, might possibly be mistaken for an 

 interstratified mass of considerable extent. In the case of a very high 

 mural face of rock, like those in the Des Chutes Basin, it would no doubt be 

 possible that such deposits should have been formed on terraced edges or 

 shelf-like projections which are often observed in lofty cliffs. This, however, 

 does not seem to have been the case in any of the localities of infusorial ma- 

 terial examined by the writer, or in those described by others as occurring in 

 the Cordilleras. The appearances everywhere in that region are such as to 

 decidedly justify the inference that the microscopic organisms were developed 

 in the position in which they were found, once occupying the surface, and 

 afterwards becoming covered by other layers of similar material, the result 

 of successive growths, with intercalated non-fossiliferous deposits of greater 

 or less number and thickness, according to the varying conditions prevailing 

 at the time the mass was in process of accumulation. 



In concluding this section, a few paragraphs may be added with reference 

 to the cause of that constant association of the infusorial deposits with the 

 volcanic formations, which observation has so clearly established. These re- 

 marks are quoted from the article previously referred to as having been 

 published, in 1867, by the writer of this volume in the Proceedings of the 

 California Academy of Sciences : 





