DISTRIBUTION OP CHALCOPYRITE FISSURES. 443 



near Campo Seco ; and between the Calaveras and Stanislaus in the val- 

 ley between Gopher hill and Bear mountain, and also near the western 

 foot of Gopher hill at Quail hill; and such deposits are reported as far 

 south as in Mariposa county, south of the Merced. At Copperopolis, 

 between Gopher hill and Bear mountain, they are worked on a consid- 

 erable scale for copper, and on a smaller scale at two or three other 

 points. Why this series of fissures along the western foot of the range 

 should differ from the fissures in the same rocks on the western slope of 

 the range generally in containing so much larger proportion of copper 

 pyrites with the quartz and iron pyrites is not clear, but the fact is of 

 geological significance. 



4 



Age of the mineral Veins. 



The fissures are younger than the rocks they traverse, and consequently 

 those that traverse Mesozoic rocks were made or extended after these 

 rocks were deposited. The period of their deposition was one of prevail- 

 ing regional subsidence, as already stated, but it was a period of great 

 eruptive activity, as shown by the miles of thickness of diabases (or 

 greenstones) and serpentines. It is hardly probable that all this eruptive 

 activity took place without dislocation as well as Assuring. Moreover, 

 there are strong indications of faulting at that time, especially at or near 

 the boundary of the two Mesozoic subgroups, although no unconformity 

 among the Mesozoic rocks has been certainly established. 



At the end of the deposition of the metamorphic Mesozoic rooks there 

 followed great uplifting, tilting and metamorphism, and certainly great 

 Assuring. A prominent part of the metamorphism was the quartzitic 

 alteration, which resulted in the production of quartz with pyrite and 

 gold, like that in the fissures. It is practically certain, therefore, that a 

 Large part of the Assuring and Idling of fissures in the Mesozoic rocks 

 occurred with the tilting and metamorphism at the time when the depo- 

 sition of these rocks ceased and they were raised above sea-level. A 

 Long period of subsidence; followed, with little if any dislocation, con- 

 tinuing through the later Cretaceous (Chico), the Eocene (Tejon i, and the 

 early Miocene. Then followed the Tertiary and Quaternary uplifting, 

 to which is due the relief of the present range. En these Tertiary and 

 Quaternary movements there has been great faulting along lines of old 

 fissures, and probably new lissnring; but we have gravels deposited by 

 stream- at the time of the early Miocene movements, and (hey are made 

 up Largely of quartzite and quartz with gold from Mesozoic as well as 

 pre-Mesozoic rocks, and much of the quartz and gold is from fissures. 

 It is therefore certain thai a Large part, at least, of the Assuring of Meso- 

 zoic roclcs and the Idling of fissures with quartz, pyrite and gold took 



