222 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
who also included in his paper a general review of the progress of microscop- 
ical discovery in this department over the region of the Cordilleras, with a 
description, accompanied by plates of forms observed at various localities in 
the Humboldt Valley, on the Truckee River, and in the neighborhood of 
Great Salt Lake. In this paper of Ehrenberg’s a number of questions were 
proposed relating to topics connected with the investigation of the infusorial 
deposits. These questions were answered by the writer, so far as it was in 
his power to do so, in a manuscript communication to the eminent Berlin 
microscopist, and by him published, with comments, in the Proceedings of 
the Academy.* 
From all the various papers and cominunications above cited, a clear idea 
can be obtained of the relations of the infusorial strata to the voleanic forma- 
tions with which they are associated, and of the great variety and interest 
of the organic forms which are found therein. All that it seems necessary 
to do, in connection with the present volume, is to give a brief réswmé of the 
facts, and to state a little more clearly and connectedly what has been 
especially observed in the region of the auriferous gravels, — that is, in the 
central portions of the Sierra Nevada. 
From the various local details incorporated in the preceding chapter, it 
cannot fail to have been noticed that among the volcanic materials associated 
with the gravels there are large quantities of fine-grained rock, usually white 
or grayish-white in color, and known to the miners by various names ; as, for 
instance, “ volcanic ash,” “cement,” “white lava,” “chocolate,” “ pipe-clay.” 
The bulk of the material composing these various deposits is almost invari- 
ably of volcanic origin, as is readily discovered by microscopic investigation. 
Sometimes the different mineral ingredients have been so finely triturated 
that the result is simply a mud, which has become consolidated, and in which 
little of the original structure can be discerned. Other portions of these depos- 
its retain their distinctly crystalline character, and the component minerals 
can be easily recognized. Most of the white fine-grained lavas seem to be 
of a rhyolitic character. It is not unreasonable to assume that the original 
form of much of this material was that of an ash, having been thrown in that 
condition from the various volcanic vents which must have crowned the sum- 
mit of the Sierra during the later Tertiary times. This ash was carried 
down the slope of the Sierra by the streams which were the agents in the 
* Monatsbericht der Kénigl Akademie der Wissenschaften zu Berlin ; Sitzung der phys, — math. 
Klasse, 19 Feb. 1872. 
