P2 C.KOT-OC.V OF THE BLA<'K MILLS 



sc-liivsts, lilliny up irregularities in their surfaces, and its basal member is 

 generally formed of coarse materials derived from the erosion of the subja- 

 cent rocks 



Consisting mainly of coarse and friable sandstones, with conglomerates 

 easily eroded, it covers supei-ficially very limited areas, and its exposures, if 



unexaggerated, would be represented on a geological 

 map by a line rather than by a belt or zone. The 

 cliffs or bluffs of greater or less prominence that have 

 already been noticed as surrounding the Archaean 

 area of the Hills on all sides like a wall, and which 

 Fio. 9.— Se(ti<m ou Spring are brokcR tlirougli here and there by the draining 



Cu'ok, show iiig uncoil tbnn- , , , _ 



ity of the Potsdam siiiui- strcams as they escape through narrow canons, are 

 schists. composed of Carboniferous rocks, and at their base, 



1. Coarse Potsdam sandstone 



and tongionioratf. betwecu tlicm and the metamorphic slates and schists, 



'X. Schistose rocks, a. Joint- 

 ing plane in the schists, lies the Potsdam sandstone. The Potsdam often out- 

 crops in a bluff or in a series of steps of sandstone with intervening slopes, 

 but generally the entire exposure is concealed under a talus composed of 

 the debris of the Carboniferous limestone and of the Potsdam itself. In 

 many places, however, clean and good sections are presented, especially 

 in the walls of the canons. Occasionally, near the cliff of fossiliferous rocks, 

 isolated buttes or hills of Carboniferous limestone, with the underlying 

 Potsdam, have been left by denudation in the midst of the Archaean; but 

 on account of the friable and easily eroded character of the. Potsdam, out- 

 liers of that formation alone are not frequent. In the northern part of the 

 Hills, however, in the region north and northeast of Custer Peak, large ai'eas 

 of the Potsdam are exposed. 



West, northwest, and north of Terry Peak, in the wilderness of deep 

 and intricate canons traversed by the headwaters of Spearfish, and Red- 

 water Creeks and their branches, many of the canons cut down through the 

 Carboniferous into the Potsdam, and a few go still deeper, penetrating the 

 schists beneath. The Potsdam has been examined in several of these canons, 

 and found to preserve its usual character and its usual relations to the asso- 

 ciated rocks, but it was not thoroughly studied and no attempt has been 

 made to maj) its sinuous outcrop. 



