214 KEMP'S ORE DEPOSITS. 



2.09.22. Gilpin County has already been mentioned under 

 " Copper " (2.04.08). The general geology of the veins is much like 

 that of Clear Creek, although the ores are quite different. R. 

 Pearce has shown the existence of bismuth in the ore, and gives 

 reasons for believing that the gold is in combination with it. 

 Clear Creek County contains veins on a great series of jointing 

 planes in gneiss (granite), and in large part replacements of the 

 wall. Others are replacements of porphyry dikes or of pegmatite 

 segregations. The ores are chiefly galena, tetrahedrite, zincblende, 

 and pyrite, and the gangue is the wall rock. The curious decrease 

 of value in depth of a series of parallel veins in Mount Marshall was 

 referred to (1.05.05). Georgetown is the principal town and mining 

 center. Others of importance are Idaho Springs and Silver Plume. 1 



2.08.23. Boulder County contains veins along joints or faulting 

 planes in gneiss, or granite, or associated with porphyry dikes, or 

 pegmatite segregations, and carrying tellurides of the precious 

 metals more or less as impregnations of the country rock. The 

 prevalent country rock is called by Emmons a granite-gneiss. 

 Van Diest distinguishes four successive terranes of massive and 

 schistose rocks along three principal axes and two side ones, and 

 states that the mines are on the sides of the folds. The country 

 is very generally pierced by porphyry dikes, with which the ore 

 bodies are often associated. A large number of species of tellu- 

 ride minerals have been determined from the region, especially by 

 the late Dr. Genth of Philadelphia. The mines afford very rich 

 ores, somewhat irregularly distributed. 2 



ver Cliff, Colorado," Engineering and Mining Journal, Nov. 2, 1878, p. 

 314. W. Cross, " Geology of the Rosita Hills," Proc. Colo. Sci. Soc., 1890, 

 p. 269. Rec. S. F. Emmons, "The Genesis of Certain Ore Deposits," 

 M. E., XV. 146. Tenth Census, Vol. XIII., p. 80. L. C. Gray bill, " On the 

 Peculiar Features of the Bassick Mine," M. E., XL, p. 110; Engineering 

 and Mining Journal, Oct. 28, 1882, p. 226. Rec. O. Loew and A. R. Conk- 

 ling, "Rosita and Vicinity," Wheeler's Survey, 1876, p. 48. See also Ste- 

 venson in the Report for 1873. 



1 S. F. Emmons, Tenth Census, Vol. XIII. , p. 70. Rec. F. M. End- 

 lich, Hayden's Survey, 1873, p. 293 ; 1876, p. 117. P. Fraser, Hayden's 

 Survey, 1869, p. 201. J. D. Hague, Fortieth Parallel Survey, Vol. III., p. 

 589. Rec. R. Pearce, Proc. Colo. Sci. Soc., Vol. III., pp. 71, 210. " The 

 Association of Gold with Other Metals," M. E., 1890. J. J. Stevenson, 

 Wheeler's Survey, Vol. III., p. 351. F L. Vinton, " The Georgetown (Colo.) 

 Mines," Engineering and Mining Journal, Sept. 13, 1879, p. 184. 



8 A. A. Eilers, " A New Occurrence of the Telluride of Gold and Sil- 



