612 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45. 



grains to bowlders a foot or two in diameter, and consolidated by 

 siliceous cement. Its lowest member is a typical quartzite, and 

 other quartzite beds are intercalated through the series. The forma- 

 tion is unmineralized except in proximity to its schist contact, where 

 it shows pyrite, chalcopyrite (in places some bornite and chalcocite), 

 and hematite, all in unimportant amounts. Specmiens near the 

 fault plane show results of the fault pressure by a schistose tendency, 

 and under the microscope by undulatory extinction and shattered 

 condition of some of the quartz grains. 



The schist series forms a belt three-fourths to 1^ miles wide, with 

 north-south course. The strike is N. 50° W. to N. 60° W., wath dip of 

 60-80° to the southwest. The rocks range in color from light grayish 

 green to dark blue and in structure run from moderately schistose 

 to highly schistose. The light-colored members are typical seri- 

 cite schists, resolvable under the highest power of the microscope 

 into a dense quartz mosaic, knit with shreds of sericite in parallel align- 

 ment. The darker members, which have the greater distribution, are 

 chloritic schists showing under the microscope predominant chlorite, 

 accompanied at times with sericite, enwrapping grains and mosaic 

 masses of quartz. All sections are mineralized with pyrite in minute 

 grains and crystals, which are scattered through the quartz, chlorite, 

 and sericite alike. As shown by the lack of pressure effects due to 

 crystal growth and the frequent presence of pyrite crystals inter- 

 cepting otherwise continuous laminse, the pjTite is judged to have 

 been deposited mainly through replacement. Gregory, who has 

 made a detailed petrographic study of the schists, finds that they 

 grade from those showing no original structure to those definitely of 

 igneous origin, "formed by the alteration of quartz-porphyrites and 

 probably also of acid volcanic tuffs." From a study of several thin 

 sections, comparison wdth similar rocks of Virgmia and North Caro- 

 lina,^ and careful consideration of the chemical analyses given by 

 Gregory, the writers believe that it can be affirmed with considerable 

 certainty that the schist series represents the mashed equivalent of 

 volcanic rocks of acid to intermediate character. 



Faulting has been profound in the region, but is largely localized in 

 the major overthrust already referred to and attendant cross faults 

 of minor development. The ore deposits are confined to the fault 

 zone, and the principal deposition has taken place in the angles formed 

 by the intersection of the cross faults with the major fault. The 

 latter may be traced along the surface and is marked b}" outcrops of 

 hematite which, however, according to Sticht, bear no relation to 

 the pyrite and is not its gossan. 



1 The sericite and chlorite schists of Mount Lyell are strikingly similar, both in appearance and in micro- 

 scopic detail, to analogous copper-bearing schists of the Virgilina, Gold Hill, and Cid districts in Virginia 

 and North Carolina, which have resulted from the dynamic metamorphism of volcanic rocks. 



