176 GEOLOGY OF THE BLACK UILLS. 



Takin{T up the groups, now, in order, the first to describe is the Dakota. 

 Typically it is a coarse sandstone, generally conglomeratic, yellowish in 

 color and stained red in jjlaces by the oxidation of the iron contained in its 

 nodules. Sometimes the sandstone is white in color and uniform and fine 

 in texture, and in several places large portions of the formation consi.st of 

 intensely hard, ghissy, and compact quartzite, white or brownish in color, 

 and having the density, toughness, sharpness, and conchoidal fracture of 

 typical flint. The quartzitic development was especially observed at the 

 southern end of the Hills, where the Dakota expands into a plateau, and 

 in the region north of Warren Peaks, but it is not confined to those localities. 



The quartzitic sandstones are far removed from the possibility of any 

 igneous action and are to be classed with the quartzites of the Potsdam 

 already referred to. They consist, like them, of grains of sand cemented 

 into a homogeneous mass b}' a silicious cement, but in many cases they are 

 more flint}' in their appearance and fracture. Just as with the Potsdam 

 quartzites, there appear to be but two solutions to the problem of their 

 formation. First, the silicious cement may be derived from the remains of 

 infusoria, sponges, and other silicious organisms, as it is in the case of the 

 Coal Measures and of the European Chalk; or, second, it may be derived 

 from waters containing silica in a soluble condition, either at the time of 

 the formation of the sandstone or after its deposition. 



In several places on the northeastern side of the Hills the usual coarse 

 yellow sandstone, with a thickness of 300 feet, is overlaid by 25 or 30 feet 

 of red, yellow, and green variegated clays, 25 feet of white or gray cla}', 

 and 20 feet of yellowish sandstone. On one of the creeks running from the 

 Hills near Bear Butte 200 feet of the usual coarse sandstones are separated 

 by a series of light colored clays from another and thinner stratum of sand- 

 stone, above wliicli come the dark clays of the Fort Benton group; and on 

 Bear Butte Creek a similar section was seen. These variegated clays are 

 not a constant feature, for in most sections on the flanks of the Hills they 

 do not appear; neither are they, in the broad view, exceptional, for such 

 clays are mentioned by Meek and Hayden in their general section. 



Frequent allusion has already been made to the topographical promi- 

 nence of the Dakota outcrop, and it was impossible in describing the Jura 



