4 PROCEEDINGS OF THE NEW YORK MEETING 



sulphides. The shales contain large quantities of iron oxides and perhaps small 

 quantities of calcium carbonate, although not in the form of limestone. 



CHEMICAL REACTIONS 



We can write out chemical equations showing how metallic copper might have 

 been formed from such associations, thus : 



I. 3CuS0 + FeoOa = 3CnO + Fe2(S04)6. 



II. SCuaSO + FeoOs = SCiioO + Fe2(S04)3. 



III. CUSO4 + CaC03 = CuO + CaS04 + COo. 



The copper oxides produced by one or more of the above equations could readily 

 be reduced to metallic copper by the action of ferrous sulphate in the presence of 

 traces of free sulphuric acid, thus:. 



IV. CuO + 2FeS04 + H0SO4 = Cu + FeoC 804)3 + H2O. 

 V. CugO + 2FeS0 + H2SO4 = 2Cu + Fe2( 804)3 + HgO * 



ORIGINAL SOURCE OF THE COPPER 



The original source of the copper is difficult to determine. The nearest known 

 copper beds are to the south more than a hundred miles. If the source were the 

 Red beds themselv^es, then the copper must have been gathered from the imme- 

 diate surface, or possibly from material already removed by erosion, for it could 

 not have been leached from the surrounding shales without the leaching process 

 having made a greater change than has occurred. At present the surface waters, 

 of course, are charged with organic matter, the same as other surface waters where 

 vegetation is abundant, but the iron oxide of the shales oxidize such matter while 

 yet near the surface, as is shown by the red color for 20 feet above the copper bed. 



Remarks upon the subject of the paper were made by W. H. Hobbs, 

 J. F. Kemp, and the President. 



The third i)aper was read by title: 



ANDESITIC ROCKS NEAR SILVERTON, COLORADO 

 BY FRANK R. VAN HORN 



Contents 



Page 



Introduction 4 



Megascopic description of tlie rock 5 



Microscopic observations 5 



Chemical composition 7 



Conclusion 8 



Introduction 



In the summer of 1898, while spending a few weeks at Silverton, San Juan 

 county, Colorado, in the study of the mines and economic geology of the region, 

 the rocks which are the subject of this paper were collected. The fundamental 

 rocks of the neighborhood seem to be granites and granito-diorites associated with 

 gneisses and schists. Over these follow several lava flows, probably of Tertiary 

 age, in succession, accompanied in some localities by considerable tuff, which has 



