~e) 
228 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
b 
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 Range 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 Ehren- 
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 : — 
