GENERAL SKETCH OF TIIEIE DISTRIBUTION. 







CHAPTER II. 



THE TERTIARY AND RECENT AURIFEROUS DETRITAL AND VOLCANIC FORMATIONS 



OF THE WEST SLOPE OF THE SIERRA NEVADA. 



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Section I. — A General Sketch of the Distribution and Mode of Occurrence of 



the Volcanic and Gravel 'Formations of the Sierra. 



The present chapter will be devoted to a detailed description of the 

 auriferous gravels and the associated volcanic rocks, which in the preceding 

 pages have been pointed out as forming a kind of mantle over the "bed- 

 rock " formations, and which are of so much interest from many points of 

 view, and especially from that of their value as containing gold in sufficient 

 quantity to be worked with profit at a great number of localities. Before 

 entering on this detailed description, however, it will be necessary to give 

 the reader a general idea of the mode of occurrence and distribution of the 

 formations in question; as, without this, he would not readily understand the 

 bearing of the facts presented on the problems involved in these remarkable 

 geological phenomena. It will also be desirable to give a brief sketch of the 

 methods by which these deposits arc worked for the gold which they con- 

 tain, since the exposition of the facts depends so much on the nature and 

 extent of the mining-operations through the aid of which these facts have 

 chiefly been brought to light. After the general introductory sketch, of 

 which the object has thus been indicated, the detailed description of the 

 deposits will be given to such an extent as the materials in our possession 

 will allow, and with a view not to occupy too much space with that which is 

 of purely local importance. The reader will then have been prepared for 

 «i general discussion of the phenomena, in which the scientific aspect of the 

 problems will be those chiefly presented. In following the order thus indi- 

 cated, there will necessarily be some repetition, although this will be as far 



as possible avoided. 



Deposits of gravel, or rolled and water-worn fragments of rock, arc of 

 common occurrence the world over, and they are especially abundant and 

 important in our own country, in New England and the region of the Great 



