SILVER AND GOLD, CONTINUED. 



21' 



ous, Jura-Trias, and Cretaceous rocks. There are some igneous 

 intrusions. The principal product of the Black Hills is gold. 

 The lead-silver deposits have already been described (2.08.18), and 

 the tin, mica, etc., will be mentioned later. 1 



2.10.03. The gold occurs in placers of Quaternary and recent 

 age, as well as in Potsdam sandstones, which are old shore beaches 

 now hardened to rock ; in pyritous beds in schistose rocks, and in 

 segregated quartz veins. The Quaternary and recent placers are 

 the usual gravels, which are more fully described under " Cali- 

 fornia." The Potsdam sandstone is an extremely interesting de- 

 posit. It has resulted from the wearing action of the waves of 

 the Potsdam ocean on the Archaean schists. The Potsdam also 

 carries other deposits in the vicinity of porphyry sheets and dikes, 



3 4 



FIG. 54. Geological section of the Black Hills. After Henry Neivton 

 Report on the Black Hills, p. 206. 



1. Schists. 2. Granite. 3. Potsdam sandstone. 4. Carboniferous. 5, 6. Jura-Trias. 



7. Cretaceous. 



which consist of auriferous pyrite, sometimes oxidized. This has 

 replaced the original calcareous cement of the quartzite. The 

 pyritous beds are in a great impregnation zone 2000 feet broad 

 (Carpenter), of slates and schists, with portions especially rich in 

 auriferous pyrites. They occur near the town of Lead City, not 

 far from Deadwood, in the northern hills. The deposits present 

 many analogies with Example 16, and also are like fahlbamls 

 (1.06.10). The ore is not high grade, running $3 to $4 per ton, 

 but it is treated at great profit by mining it in enormous quanti- 

 ties. There are also many so-called segregated quartz veins in the 



1 F. R. Carpenter^ "Ore Deposits in the Black Hills," M. E., XVII. 

 870. Prelim. Rep. on the Geol of the Black Hills, Rapid City, So. Dak., 

 1888. W. O. Crosby, " Geology of the Black Hills," Bost. Soc. Nat. Hist., 

 XXIII., p. 89. Newton and Jenney, Report on the Black Hills, Washing- 

 ton, 1880. C. R. Van Hise, " The Pre-Cambrian Rocks of the Black Hills," 

 Bull. Geol. Soc. Amer., I. 203-244. N. H. Winchell, " Report on the Black 

 Hills," Rep. Chief of U. S. Engineers, 1874, Part II., p. 630. 



