

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, — 

 that is, central in relation to a transverse section of the range, — the inclina- 

 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 incli- 

 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 en 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 



