280 GEOLOGY OF THE BLACK HILLS. 



graphical party ; and, for want of time and suitable transportation, my 

 assistants were unable to properly prospect and explore the branches of this 

 stream. 



Whitewood, heading on the east side of Terry Peak, unites with 

 Deadwood, a branch rising on the northwest side, forming a swiftly-run- 

 ning stream flowing nearly 300 miner's inches of water, but sinking a short 

 distance below the forks, and thence continuing as a dry canon through the 

 limestone to the plains. 



Whitewood and its branches flow through narrow and deep gorges 

 excavated in the slates and igneous rocks, with the Potsdam sandstone 

 capping in many places the tops of the narrow ridges between its numer- 

 ous branches. The base of the Potsdam is here the usual conglomerate of 

 round water-worn quartz pebbles and small bowlders. The intrusion of 

 the igneous rocks has penetrated the Potsdam as well as the slates and 

 quartzites, and has often metamorphosed the sandstone so that it is unre- 

 cognizable, except that traces of its stratification are retained, and the strata 

 are observed resting at a low angle unconformably on the upturned edges 

 of the slates. 



The most common form of this altered Potsdam is a brownish granular 

 rock, full of feldspar crystals, irregularly distributed through its mass Lo- 

 cally, this metamorphosed sandstone is seen as a hard granular rock, break- 

 ing with a sharp conchoidal fracture, without any traces of crystalline min- 

 erals, and specimens in their colored and burnt appearance resemble frag- 

 ments of pottery ware. The slates and quartzites are similar to the rocks 

 of the Spring Creek district, and broad strata of quartzite, intermized with 

 limonite and white quartz, resemble closely the Mammoth ledge on that 

 stream, except that here small ramifying veins of white quartz not more 

 than half an inch wide intersect the dark-gray quartzite like a net- work; 

 another evidence that these altered strata are the result of the action of 

 solutions depositing silica permeating the formation. The strike of the slate 

 and quartzite belt is observed to be east and west, at a point a few miles 

 north of Terry Peak. This formation over great areas is concealed by the 

 intrusion of large masses of igneous rocks, forming the more prominent 

 peaks in the district, as Terry, Custer, Crow, Black Butte, &c. 



