416 



University of California Publications. 



[Geology 



heavy accumulations of auriferous gravels mantle the bedrock 

 geology. Along the bluffs of the Mokelumne River and in the 

 region north of the river the greenstone schists are literally 

 charged with long angular fragments of an aphanitic quartz 

 porphyry in which the quartz phenocrysts are small and infre- 

 quent. 



The quartz porphyry flows are the copper bearing members. 

 In the exposures along the river, especially on the north bank, 

 the base of the quartz porphyry is found to interleave with the 

 green glossy slate, and with coarse angular breccias. In the vicin- 

 ity of the new shaft two main flows, separated by 300 feet of 

 tuffs, are discriminable. A few hundred feet to the south they 

 become confluent, but diverge almost immediately, becoming 

 separated by a mass of greenstone schists whose coarsely pyro- 

 clastic character is still obviously patent. 



The quartz porphyries have as a general rule been very thor- 

 oughly foliated, though some unsheared lenses have escaped 

 metamorphism. The schistosity is occasionally crumpled and 

 bent back on itself, and a slip-strain cleavage produced. The 

 unsheared rock is remarkable on account of the abundance of' 

 large quartz phenocrysts, in a light flinty groundmass. In addi- 

 tion to the quartz, the microscope shows some porphyritic plagio- 

 clase (16°, maximum symmetrical extinction), orthoclase in 

 nearly equal amount, and a few chlorite-epidote aggregates, which 

 may possibly represent former ferromagnesian minerals. The 

 groundmass is a microcrystalline aggregate of feldspar, part of 

 which is striated, abundant chloritic matter in small flakes, and 

 some magnetite. A few rather large grains of chalcocite were also 

 noted. Mineralogically and structurally, the rock may be desig- 

 nated a quartz monzonite porphyry.* 



The following is described as a type uncommon at Campo 

 Seco. The rock is a dark colored quartz porphyry upon whose 

 flow banding a rude foliation has been superimposed. Under the 

 microscope is revealed an intricate fluidal structure characteristic 

 of stiffly moving acid volcanies. Quartz phenocrysts are numer- 

 ous, but bipyramidal forms are wanting. Certain minor flow 

 bands consist of magnetite, while others are rendered opaque by 



* W. Lindgren, Mon. 43, U. S. G. S., p. 81. 



