68 



ECONOMIC GEOLOGY 



with the sulphides of lead, zinc, antimony and molybdenum spar- 

 ingly present. The ores were deposited from hot alkaline solu- 

 tions. Fluorine was an important mineralizer. The rich tellu- 

 ride ore is shipped to Pueblo for smelting, while the lower grades 

 are chlorinated or cyanided at the mines (Fig. 50) . The banner 

 production of this small area was reached in 1900 when an out- 

 put of more than $18,000,000 was credited to the district, but 

 the output has since declined to nearly $10,000,000 per annum. 

 San Juan District: The San Juan mining belt covers a large 



FIG. 49. Section in the Victor, 

 Smuggler Lee, and Buena Vista mines, 

 Cripple Creek district, Colorado, showing 

 the parallel ore bodies A . (After Penrose.) 



FIG. 50. Section 

 showing the forms of the 

 veins in the Blue Bird 

 mine, Cripple Creek dis- 

 trict, Colorado, a, Ore; 

 b, country rock. (After 

 Penrose.) 



area of mountainous territory in the southwestern part of Colo- 

 rado. It embraces the counties of Dolores, Hinsdale, La Plata, 

 Ouray, San Juan and San Miguel. The continental divide 

 traverses the area with several peaks surpassing 14,000 ft. in 

 altitude. The base of the geological series is Archean. The over- 

 lying Tertiary sedimentaries are capped with andesites, diabases, 

 diorites, etc. Masses of rock composed of volcanic ejectamenta 

 are not infrequent. Many V-shaped valleys of incision traverse 

 the area. The whole field presents the appearance of a deeply 

 cut volcanic plateau. 



