

t 



4 



4 



286 jBt GEOLOGY OF TUE BLACK HILLS. * 



limestone suqpeeds the Potsdam, ap})arently conformable to it, and forms 

 an en<#ircling rtyige of low white hills around Warreu Peaks, at a distance 

 of two or three miles. The whole thickness of tliis formation is at least 

 600 feet, including tlie ove^^ing massive sandstones. 



The limestone shows but little ( \ idcnce of metamorphic action, though 

 in places the trachyte has broken through this formation and appears at 

 the surface, altering slightly the upper sandstone. Beyond the belt of 

 Carboniferous, which is nearly two miles wide, the Triassic limestone and 

 the Red Beds occupy a narrow ring, succeeded by an expanse of sandstone 

 and clays of Jurassic age, extending for miles across the country. The 

 tops of the ridges are formed by a ferruginous massive sandstone, probably 

 the Dakota sandstone, and where it has been removed by erosion the harder 

 layers of the Jurassic cap the hills and the mesa divides between the 

 stream. 



In the interior area of Warren Peaks no Archaean strata were found, 

 and the slates, quartzites, and schists so largely developed in the main 

 range of the Black Hills were here entirely wanting. The rocks of this 

 district seem to be singularly free from quartz in any form. About five 

 miles southeast of Warren Peaks, near the head of a branch of the Red- 

 water, a fault in the strata exposes a fine section of the Potsdam sandstone, 

 with its lower layers full of slate and quartzite pebbles, but unfortunately 

 the base of the formation w^as covered by a talus, and the rock on which 

 it rested could not be seen. On the north side of the area of igneous 

 rocks a trachyte composed of feldspar and black mica was observed, and 

 near by several large loose bowlders of white quartz were embedded in 

 the surface of the ground, but the ledge from which they were derived was 

 not found. Black hornblende also appeared as a constituent mineral, asso- 

 ciated with feldspar, in the rocks of this district. 



Half a mile northeast of the central peaks a gold-bearing ledge was 

 discovered and traced in a northwest and southeast direction for several 

 hundred yards. On examination it was found to be irregular in shape, 

 without any well-defined walls or boundaries, and merging on all sides 

 into the adjacent trach3'te rocks. The rocks composing this peculiar forma- 

 tion were exceedingly varied in character and appearance, and a score of 



