Crosby.l 490 [March 7, 



complete absence of the levelling action of glaciation, a singularly 

 rough and mountainous topography, which culminates in the gran- 

 itic region about Harney Peak. 



The margin of the Archaean area is bordered continuously by an 

 escarpment more or less abrupt, and usually several hundred feet 

 high, formed of the Potsdam sandstone and the Carboniferous lime- 

 stones. This escarpment faces inward or toward the Archaean 

 area, and in the canons which cut into it the slates and schists are 

 seen beneath the sandstone of the Potsdam which rests unconform- 

 ably upon them. With the exception of Harney and a few of the 

 neighboring peaks, the average elevation of the schist and slate 

 area is considerably below that of the encompassing sedimentary 

 rocks (Newton). 



Although the streams in the Archaean area have in many cases 

 sunk their channels below the base of the Potsdam, yet the gen- 

 eral level of the intervening ridges is fully as high as, and often 

 higher than, that geological horizon. Hence, since it is certain 

 that the Potsdam formerly covered a large part, and the Carbon- 

 iferous the whole, of the present Archaean surface, which is being 

 steadily enlarged by the recession of the encircling Potsdam-Car- 

 boniferous escarpment, I cannot follow Newton in ascribing the 

 fact that the receding edge of the Carboniferous limestone over- 

 looks the Archaean surface to the superior hardness of the lime- 

 stone. The difference in level of the two topographies is mainly 

 due to the fact that the normal position of the Paleozoic rocks is 

 topographically, as well as geologically, above the Archaean. 



Arch^an Formations. 

 Newton has divided the Archaean rocks of the Black Hills into 

 two groups — a western, more crystalline and probably older 

 group composed chiefly of schists ; and an eastern, less ciys- 

 talline and probably newer group composed chiefly of slates. 

 The boundary between the two groups appears to be quite sharply 

 defined, and is probably indicated with substantial accuracy on 

 Newton's map. But no sections have yet been studied which ex- 

 hibit clearly the stratigraphic relations of the two groups. The 

 greater age of the western group is inferred, however, from its more 

 crystalline character, and from the fact that the granitic masses 

 about Harney Peak, although traversing the western series of 

 schists up to the very boundary of the eastern series, have not 



