42 



AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 





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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 j but, thus far, the 

 most careful search for proof of the Palaeozoic 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 palae- 

 ontological grounds can be made, we may add some remarks on the litho- 

 logical character of the formation. In the course of the succeeding chapter 

 we shall have frequent occasion to allude to the different kinds of rock occur- 

 in g 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 prepare! for this purpose. The full results of these investigations will, 

 it is hoped, be published hereafter. 



