412 



University of California Publications. [Geology 



The formations of the Sierra Nevada region are according to 

 the United States Geological Survey grouped together as the 

 Bedrock Complex and the Superjacent Series. The Superjacent 

 Series is composed of the Tertiary auriferous gravels of fluviatile 

 origin, and the succeeding heavy accumulations of rhyolites, 

 andesites, and basalts, all resting upon the eroded edges of the 

 Bedrock Complex. This Complex includes a succession of closely 

 appressed rocks in belts paralleling the general trend of the 

 range, and dipping steeply towards its axis. They comprise 

 quartzites and microcrystalline mica schists (Calaveras forma- 

 tion, Carboniferous) , black clay slates (Mariposa formation, 

 Upper Jurassic), meta-andesites and their schistose equivalents. 

 At the close of the Jurassic these were all extensively intruded 

 by plutonic magmas, chiefly granodorites and peridotites. 



The meta-andesites and their associated pyroclastics often 

 attain enormous thicknesses, freqently reaching five miles, with- 

 out doubt, however, due in great part to folding, although as 

 yet unrecognized in the field.* In the earlier folios of the Gold 

 Belt they were mapped as diabases and augite porphyrites, but 

 in the later publications (notably the Mother Lode Folio, F. L. 

 Bansome) they were designated meta-andesites in order to 

 emphasize their essentially extrusive character. Their exact posi- 

 tion in the stratigraphic column is still a moot point in the geol- 

 ogy of the Sierra Nevada, though the evidence, both petro- 

 graphic and stratigraphic, favors a late Jura-Trias age for the 

 bulk of the series. 



We propose to describe the copper deposits in geographic 

 order from north to south. A number of small idle or unworked 

 prospects were not visited. 



SPENCEVILLE, NEVADA COUNTY, AND VICINITY. 



North of Spenceville toward the head of Little Dry Creek 

 numerous copper prospects have been opened up in the meta- 

 andesites. The country rock is usually massive, but in the vicin- 

 ity of the copper deposits has been slickensided and sheared, and 

 belts of strongly foliated chloritic schists up to 4 feet thick have 

 been produced. The strike is N. 45° W. and the dip vertical. 



* H. W. Turner, Jackson Folio. 



