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 imperiectly 
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 Mokelumne 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 are 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 flanks 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 oceupies 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 
