50 AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
The dip of the auriferous slate series is as difficult to account for as the 
variation in the strike. On the whole, in the central portion of the belt, — 
the inclina- 
that is, central in relation to a transverse section of the range, 
tion of the slates is usually nearly vertical; it may be said to vary from 75° 
to 85°; and it is, in the majority of instances, to the northeast, or towards the 
crest of the range. The narrower the belt of slates, the more regular the 
inclination to the east. As the series widens in going towards the north, the 
dip becomes more irregular, and over extensive areas it is to the west; at 
the same time the strata make a more decided approach towards horizontal- 
ity; and, on the whole, dip less steeply as we proceed towards the crest of the 
range. A carefully constructed section along the line of the Central Pacific 
railroad, on which the inclination of the strata at every outcrop was laid 
down, failed to furnish proof of any regular system of folds, there being fre- 
quent and rapid changes, both in the amount and the direction of the meli- 
nation of the strata, and the whole series presenting phenomena similar to 
that described as existing in the northern portion of the Coast Ranges, where 
the rocks seem to have been lifted up ex masse. The principal apparent 
difference, in the case of the Sierra rocks, is, that here there has been more 
lateral compression, and that consequently the average dip is higher. The 
cause of this compression may be sought for in the presence of two enormous 
masses of granite on this line of section, one of them forming the crest of the 
Sierra, and undoubtedly the mass of the material underlying the stratified 
portion of the bed-rock series, while the other is seen in the foot-hills, consti- 
tuting a belt some twenty miles in width, as visible on the surface, with an 
unknown extension under the Great Valley, where it is covered with recent 
detrital formations. A long and patient study of the specimens collected, 
with the aid of the microscope, and combined with a sufficient number of 
observations in the field, may, it is to be hoped, eventually furnish the neces- 
sary data for explaining the structure of the auriferous belt; at present, it 
does not seem possible to make any more decided statements in regard to 
this difficult question than those which have been given above. On a future 
occasion we may attempt to throw some farther light on the problem here 
presented. 
In view of its importance as bearing somewhat directly on the special 
subject of the present volume, we have now to introduce a few remarks 
in regard to a class of rocks occurring on the west slope of the Sierra, 
and not belonging to the bed-rock nor yet to the volcanic and detrital 
