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GEOLOGY OF THE SIERRA NEVADA. 



31 



The most easily recognized division of the bed-rock is into the granitic 

 and the metamorphic ; for there is so little of that which is not granitic which 

 has not undergone considerable chemical change, that the whole series of 

 non-granitic may, not improperly, be called metamorphic. So large a portion 

 of this metamorphic rock has a slaty structure, that the whole series is often 

 termed the " auriferous slate series " ; or, more briefly still, the " auriferous 

 slates " ; not that the whole formation is gold-bearing, but that these are the 

 rocks with which the gold is associated, this metal, however, occurring almost 

 exclusively — so far as the bed-rock is concerned — in connection with the 

 veins of quartz which are so abundantly distributed through certain portions 

 of the series. When, therefore, in the course of the present volume, we speak 

 of the " bed-rock slates " or the " auriferous slates/' we mean, unless there 

 is some special limitation to the contrary, to include all the non-granitic 

 rocks of the gold region of the Sierra Nevada, underlying the imperfectly 

 consolidated, detrital materials, which, with the accompanying volcanic de- 

 posits, form the superficial covering of so large a portion of the western 

 slope of the 



* 



range. 



By far the greater part of the mass of the Sierra Nevada consists of the gran- 

 ite forming the axis of the chain. This rock constitutes, indeed, nearly the 

 whole of the Southern Sierra, from Tahichipi Pass almost as far north as Mari- 

 posa County. Only just at its western edge, at the base of the foot-hills, 

 is there a narrow belt of rock having either a schistose or laminated struc- 

 ture. As we go north from the point where the San Joaquin River leaves 

 the foot-hills, we find the belt of non-granitic rock rapidly widening, and, at 

 the same time, the line of junction between the granite and the metamorphic 

 becoming more and more irregular. As far as the Mokelmnnc River there 

 appears to be a well-marked separation between the two formations, and few 

 isolated areas of granite lying outside of the main axial mass. Farther north, 

 however, there arc portions of the intrusive rock quite surrounded by the 

 metamorphic. Just north of the American River, granite forms a wide belt 



in the foot-hills of the Sierra, while opposite to it on the Hanks of the range 



a great spur projects nearly down to Placerville, reducing the width of the 

 zone of metamorphic at this point to about twenty-five miles ; while a little 

 farther north it is more than twice as much as that. Through the counties 

 north of the American River the auriferous slate series occupies nearly the 

 whole width of the western slope of the Sierra, with occasional areas of 

 granite enclosed in it, the axial mass being almost entirely limited to the 



