42, AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
or Devonian fossils should be found on the western slope of the Sierra, it will 
probably be in the foot-hills, below the auriferous belt; but, thus far, the 
most careful search for proof of the Paleozoic age of those rocks has not 
been rewarded with success. 
Having in the preceding pages given a synopsis of what is known of the 
geological age of the auriferous slate belt of the Sierra, from which it will 
have been abundantly evident that no subdivision of the series based on pale- 
ontological grounds can be made, we may add some remarks on the litho- 
logical character of the formation. In the cowrse of the succeeding chapter 
we shall have frequent occasion to allude to the different kinds of rock oceur- 
ing in the various mining districts. It is, indeed, no part of the object of this 
volume to enter into any exhaustive account of the bed-rock; that would 
naturally be left for another occasion.* At present we are not prepared to 
do this, nor is it necessary for the understanding of the problems involved 
in an inquiry into the mode of occurrence of the gravel deposits. What we 
have to do with the bed-rock in connection with the subject here under dis- 
cussion is chiefly the character and form of its eroded surface, as will be seen 
farther on. It will be necessary, however, frequently to allude to the litho- 
logical character of the material thus eroded, and some general statement of 
the character and distribution of the different kinds of rock which make up 
the bed-rock series will be desirable. This, however, may be preceded by a 
few remarks, additional to those given when describing the granitic axis of 
the Sierra, in regard to the range and extent of the auriferous slate series. 
The most important fact in this connection is this: that the non-granitic bed- 
rock occupies but a narrow strip at the southern end of the Sierra Nevada, 
and that this widens as we proceed north, until it becomes lost under the 
overlying volcanic masses, which cover everything when we reach the north- 
ern boundary of Plumas. At the same time it will be noticed that the prin- 
cipal mining region is that in which the auriferous slate series is most exten- 
sively developed. This is natural enough; for, although the granitic rocks 
are not entirely destitute of metalliferous veins, these are comparatively 
* Mr. M. E. Wadsworth is engaged, under the direction of the writer, in making a microscopical ex- 
amination of both the volcanic and the metamorphic rocks of the Sierra. The investigation, however, is 
not yet completed ; but in most cases in the course of the present volume the names given to the different 
members of the bed-rock and gravel series are those furnished by Mr. Wadsworth after examination of 
the thin sections which have been prepared for this purpose. The full results of these investigations will, 
it is hoped, be published hereafter. 
