BLACK HILLS GEOLOGY. ' 203 



lies at the base of the Cambrian formation. Tiiat this island 

 was never entirely beneath the waters has been proved by 

 Crosby/ who shows that the Cambrian series sinks to a thick- 

 ness of barely 50 feet in the southern hills, and there dips away 

 from the central, granitic area at too slight an angle to have ex- 

 tended up over the higher peaks. Following the deposition of 

 the Cambrian there is a great, unrecorded, geological interval 

 during which the Ordovician, Silurian and Devonian strata were 

 deposited in other parts of the continent. Since the studies of 

 Newton, strata of probable Silurian age have been identified and 

 we have a partial filling of the gap. The absence of Devonian 

 strata is, however, still to be explained. If present, these must 

 have but a sHght development, because there is only a short 

 space between the Silurian and those rocks which are of positive 

 Carboniferous age. 



For this break two explanations have been advanced : 

 I St. That the Black Hills area was elevated at the close of 

 the Cambrian, so as to be covered by a very shallow sea. Dur- 

 ing this time little or no sedimentation occurred, nor was there 

 any marked erosion. Subsequently a gradual subsidence took 

 place and the Carboniferous series was deposited with perfect 

 apparent conformity. Against this may be advanced the argu- 

 ment that no conglomerates exist at the top of the Cambrian to 

 indicate shore conditions. If, however, the area of the hills 

 formed the bottom of a shallow sea, without itself projecting 

 above the surface, the absence of conglomerates presents no dif- 

 ficulties. Other land areas were too remote to have supplied them. 

 2d. The other view is that the absence of these formations in- 

 dicates the subsidence of the region of the hills to abyssmal 

 depths, during which time little or no sedimentation occurred, 

 and that the deep-sea deposits, such as have been shown to accu- 

 mulate with extreme slowness in all known localities, are so 

 thin that they have not yet been identified. If this view be 

 accepted it is necessary to suppose the occurrence of a very sud- 

 den subsidence of the entire area to vast depths, for the Cam- 

 brian series is throughout a deposit characteristic of compara- 



^ Proceedings of Boston Society of Natural History, Vol. XXIII, 507. 



