THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XXXYII. — An Auriferous Conglomerate of Jurassic 

 Age from the Sierra Nevada / by W. Lindgren. 



Some of the largest quartz veins of the gold belt of the 

 Sierra Nevada intersect the Mariposa beds, the most recent 

 sedimentary member of the bedrock series of that range. The 

 age of these beds has long been in some doubt but the latest 

 investigations* appear to render it almost certain that they should 

 be considered as belonging to the uppermost Jurassic. Hence 

 the quartz veins of the gold belt have generally been con- 

 ceded to have been formed shortly after the post-Mariposa 

 mountain building disturbance ; the time limits would be the 

 end of the Jurassic on the one hand and the beginning of the 

 Chico-Cretaceous on the other. The disintegration of these 

 auriferous veins has furnished the material for the Cretaceous, 

 Tertiary and Pleistocene gravel beds. 



There is evidence that the Sierra Nevada has remained 

 elevated above the sea since the beginning of the Chico Cre- 

 taceous, for beds of that age are found in nearly horizontal 

 position along the western foot of the range. The accumula- 

 tion of the auriferous gravels m,ust thus have begun already 

 during the earlier part of the Chico period ; at Folsom, for 

 instance, a small area of these Cretaceous sandstones is found 

 in the deepest part of a depression occupied by Neocene gravel 

 deposits ; below them, on the granitic bedrock, a thin layer of 

 auriferous gravel occurs. The configuration of the bedrock 



* Alpheus Hyatt, Trias and Jura in the Western States. Bull. Geol. Soc. Am , 

 vol. v, p. 395. James Perrin Smith, Age of the Auriferous Slates of the Sierra 

 Nevada. Bull. Geol. Soc. Am., vol. v, p. 243. 



Am. Jour. Sci. — Third Series, Vol. XLVIII, No. 286.— October, 1894. 

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