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GENERAL SKETCH OF THEIR DISTRIBUTION. 
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THE TERTIARY AND RECENT AURIFEROUS DETRITAL AND VOLCANIC FORMATIONS 
OF THE WEST SLOPE OF THE SIERRA NEVADA, 
Section L—A General Sketch of the Distribution and Mode of Occurrence of 
the Volcanic and Gravel Formations of the Sierra. 
Tue 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 are 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 
a 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, are 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 
