202 IRVING. 



rugged and irregular talus. Indeed one would scarcely realize, 

 in studying the eastern portion of the hills, that in an uplift of 

 such slight comparative elevation, erosion could have cut into 

 the uplifted strata so bold and steep a canon as that of Spear- 

 fish creek. Another very noteworthy feature of the gulches in 

 the limestone is that they are in the majority of cases without 

 water for the greater portion of the year. Those which have 

 cut through into the Cambro-Silurian contain water, but the 

 others do not. This seems to have been the result of the solu- 

 bility of the limestone, which on the west of Spearfish shows 

 many sinkholes, some of which are of no inconsiderable size. 

 The stream of Beaver creek is particularly noticeable in this 

 way, for it flows for some distance as a good sized creek, and 

 then sinks beneath the limestone, to appear again after a sub- 

 terranean passage of some miles. 



That faults of considerable magnitude have occurred in the 

 limestone, in connection with the uplifts of Ragged Top, and 

 the eruptive masses of Squaw creek seems probable. They 

 are suggested by the presence of frequent brecciated zones, 

 which have afforded crevices, through which subsequently stim- 

 ulated, siliceous solutions have made their way, transforming 

 the broken limestone into a hard flintlike mass of cemented 

 fragments. The Ragged Top verticals are of this type. 



B. HYPOTHESES REGARDING HISTORY OF 

 REGION. 



The geological history of the Black Hills uplift has been dis- 

 cussed at some length by Newton,^ and subsequently by Crosby- 

 and Carpenter.^ The hills were probably an island during the 

 period following the metamorphism of the Algonkian series, 

 and upon the shores of this gradually sinking land were depos- 

 ited the materials derived from its degradation. Thus was 

 formed first the coarse, basal conglomerate, which everywhere 



1 The Geology and Mineral Resources of the Black Hills of Dakota, pp. 203-224. 



2Proc. Bost. Soc. Nat. Hist., Vol. XXIII, page 488. 



3 Preliminary report of State School of Mines of South Dakota. Pages II-52. 



