26 KEMP'S ORE DEPOSITS. 



stock Lode ; l and, by the same investigator, antimony, arsenic, 

 lead, copper, gold, and silver were proved to be contained in the 

 granite near Steamboat Springs, Nev. 2 S. F. Emmons has also 

 shown that the porphyries at Leadville contain appreciable, though 

 small, amounts of silver. 3 Of forty-two specimens tested, thirty- 

 two afforded it ; of seventeen tested for lead, fourteen yielded re- 

 sults. Undoubtedly the multiplication of tests will show similar 

 metallic contents in other regions. Thus the augite of the eastern 

 Triassic diabase will probably yield copper, for this metal is abun- 

 dant in connection with the outflows. 



1.03.07. That the metals are so generally combined with sul- 

 phur in ore deposits seems to be due to the extended distribution 

 of this element, and to its being a vigorous precipitating agent of 

 nearly all the metals at the temperatures and pressures near the 

 surface. Sulphur is widespread as pyrite, an original mineral in 

 many igneous rocks, and one much subject to alteration ; while 

 sulphuretted hydrogen is Qomnion in waters from sedimentary 

 rocks, and is a very general result of organic decomposition. Xat- 

 ural gas and petroleum from limestone receptacles almost always 

 contain it. (See, in this connection, J. F. Kemp, " The Precipita- 

 tion of Metallic Sulphides by Natural Gas," Engineering and 

 Mining Journal, Dec. 13, 1890.) Many sulphides, too, are soluble 

 under the pressures and temperatures prevailing at great depths, 

 but are deposited spontaneously at the pressures and temperatures 

 prevailing at or near the surface. 



1.03.08. Where veins occur in igneous rocks the bases for 

 gangue minerals have been obtained from the rock-making silicates. 

 Calcium is afforded by nearly all the important ones ; silicon is 

 everywhere present ; barium has been proved in many feldspars, 

 in small amount ; and magnesia is present in many pyroxenes and 

 amphiboles. Of the sedimentary rocks, limestone of course affords 

 unlimited calcium, and recently Sandberger reports that he has 

 identified microscopic crystals of barite in the insoluble residues of 

 one. (Sitzungsberichte d. Math. phys. Classe d. k. bayer, Akad. d. 

 Wiss., 1891, xxi. 291.) This is of interest, as barite is such a com- 

 mon gangue in limestone. 



1.03.09. It may be remarked that the natural formation of both 



1 Monograph III., U. S. Geol. Survey. 

 8 Monograph XIII. , U. S. Geol. Survey. 

 8 Monograph XII. , U. S. Geol. Survey. 



