258 GEOLOGY OF THE BLACK HILLS. 



formation, This ledge is remarkably persistent and continuous across 

 the country. It was traced for four miles from Spring Creek in a southeast 

 direction, and was reported by the miners to extend north to Rapid Creek, 

 a distance of eight miles, still preserving its great width undiminished. A 

 small gulch emptying into Spring Creek from the south was excavated its 

 whole length in the outcrop of this immense quartz formation. The gravel 

 deposits near its head are entirely made up of the disintegrated fragments 

 of the ledge. On the south bank of the creek the quartzite is yellow 

 and brown in color, and resembles impure jasper. On breaking, it showed 

 sometimes visible particles of gold, but samples of the rock submitted to 

 Mr. Ricketts, assayer at the School of Mines, proved to contain but traces 

 of the precious metal. The same result was obtained from iron pyrites 

 from the bed of the creek. The oxides of iron from portions of this ledge 

 were bright-yellow in color, resembling plumbic ocher, but on testing with 

 a blow-pipe they were found to contain neither lead nor silver, but were 

 merely a variety of limonite. The most promising ferruginous quartz from 

 this ledge was roughly tested at the time of its discovery by crushing 

 several pounds in an iron mortar and panning carefully, but only a color 

 of gold was obtained; and it is doubtful if these immense quartz formations 

 are of any value except as having furnished gold to the placer gravels. 



In the bluff formed by this ledge, on the bank of the stream, are 

 several small caves, penetrating the rock for 10 to 15 feet, like drifts in 

 a mine, which proved to have been formed by the decomposition and 

 washing away of soft clay-slate inclosed in the quartz. There are not 

 less than three of these quartz formations crossing the valley of Spring 

 Creek. One, two miles below the Mammoth, is characterized by a great 

 development of "ribbon-quartz," thin bands of white quartz, and black 

 quartzite or slate alternating in the ledge, giving it a banded or ribbon 

 appearance. In places the rock became a pure white quartz, and, by its 

 superior hardness resisting denudation, formed the crests of the rocky hills 

 and ridges, and could be traced for a long distance across the country. 



The only minerals besides gold found in the Spring Creek district were 

 iron pyrites and the oxides resulting from its decomposition. A miner (Mr. 

 Blake) brought me samples of a rock which he supposed to be black sul- 



