90 abstracts: geology 



gold veins of Farncomb Hill, (4) veins in the pre-Cambrian rocks, (5) 

 metasomatic replacements along bedding planes, and (6) gold-silver 

 deposits in Dakota quartzite. The Breckenridge ores were deposited 

 through the agency of thermal waters and gases given off from a solidi- 

 fying mon^onitic magma and perhaps cooperating with water of atmos- 

 pheric or of less definitely assignable origin. Whether the metals in 

 the ores came from the magma or from the rocks invaded by it is not 

 known. The principal gaseous constituents of the magmatic waters 

 were hydrogen sulphide and carbon dioxide. 



The ores were probably deposited in early Tertiary time and were 

 enriched throughout the later Tertiary. The character of the deposits 

 is notably dependent on the kind of country rock. 



Most of the mines show a decrease in the proportion of galena with 

 augmented depth, which means in general a depreciation of the ore. 

 Lead ores of shipping grade probably nowhere in the district extend to 

 a depth of much more than 300 feet. The downward change in the 

 character of the ores indicates enrichment. A large part of the galena 

 in the district is believed to have been concentrated by downward- 

 moving atmospheric water. 



There are three general classes of gold placers — (1) the bench or high- 

 level placers; (2) the deep or low-level placers; and (3) the gulch wash- 

 ings. The gulch washings were the first worked and yielded much gold 

 to the pioneers in the district. The most noted placers of this group 

 were on the slope of Farncomb Hill. The bench placers are in the 

 terrace gravels and older hillside wash. They have been extensively 

 worked in the past by hydraulic methods, but were not being exploited 

 in 1909. They are generally of low grade. The deep placers, in the 

 low-grade gravels, occupy the bottoms of the present valleys and are 

 now being worked by dredging. The pay channels are from 180 to 400 

 feet wide and average locally up to about 50 cents a cubic yard; but 

 much of the material does not yield 20 cents a yard. F. L. R. 



