94 RE VIE IV S 



AUTHOR'S ABSTRACTS. 

 THE GEOLOGICAL SOCIETY OF AMERICA. CORDILLERAN SECTION. 



FOURTH ANNUAL MEETING. 



T*HE fourth annual meeting of the Cordilleran Section of the 

 Geological Society of America, was held partly in the rooms of the 

 Academy of Sciences, San Francisco, and partly in the rooms of the 

 Department of Geology of the University of California, Berkeley, 

 December 30 and 3:, 1902. The officers elected for the ensuing year 

 were H. W. Fairbanks, chairman ; Andrew C. Lawson, secretary; and 

 W. C. Knight, councilor. The following papers were read and dis- 

 cussed : 



The Syfithesis of Chalcocite ayid its Ge?iesis at Butte, Mont. By 

 Horace V. Winchell, Butte, Mont. 



It was stated that chalcocite is the principal ore of copper at 

 Butte, and that it is one of the latest minerals to be formed in the 

 veins. It is believed that the mineral was formed by a chemical reac- 

 tion between copper sulphate in solution in descending waters and the 

 iron pyrites and other sulphides below. Some synthetical experiments 

 were described to prove this. The active agent in the reduction of 

 the copper salt to the cuprous state seems to have been sulphurous 

 anhydride, SO^, as the experiments showed that when pyrite was 

 digested in a solution of copper sulphate containing SO 2, it became 

 coated with a layer of chalcocite, while in the same solution without 

 the SO 2 the pyrite remained just as bright and yellow as before 

 immersion. 



A Geological Recon?iaissa?ice of the Region of the Upper Main Walker 

 River, Nevada. By D. T. Smith, Yerington, Nev., pre- 

 sented by Andrew C. Lawson. 



This region is in the western part of the state of Nevada, between 

 latitudes 38° 50' and 39° 10'. It comprises the greater portion of a 

 valley of general north-south trend. Nearly ten miles of the eastern 

 range and sixteen of the western have been mapped geologically. 



The eastern range is made up of granitic rocks — andesite, rhyo- 

 lite, and limestone. The western range is also made up of these, 

 together with basalt, schists, garnetiferous rocks, and intrusives. In 

 both ranges are some later Tertiary strata. The metamorphics are 



