1888.] 509 [Crosby. 



ance gradually dying out or changing to simple elevation farther 

 east, in the Rocky Mountain region and beyond. The Paleozoic 

 ocean thus gradually became, during Carboniferous time, mainly a 

 continental area, the elevation being, at the last, attended by 

 plication, but only in the thick marginal zones of sediments. 



The Black Hills uplift was essentially a restoration, so far as 

 possible, of the conditions obtaining in this region in pre-Potsdam 

 times ; and, as thus defined, the beginning of the uplift certainly 

 dates from the formation of the carboniferous limestone ; and the 

 uplift was, at first, not local but continental. Toward the close 

 of Carboniferous time it had probably progressed sufficiently so 

 that the summits of the main ranges of the Pocky Mountains were 

 above water and afforded, by their degradation, the sands re- 

 quired to build the upper Carboniferous strata of the Black Hills. 



The lower and upper Red Beds, with their impure limestone 

 and beds of gypsum, seem to indicate a nearly stationary condi- 

 tion of the continent after the Appalachian revolution. During 

 this period, the Black Hills were surrounded and probably cov- 

 ered by broad and shallow seas which were gradually silting up. 

 We may well suppose that these Triassic seas exhibited, on a far 

 larger scale, the conditions of the present playa lakes of the 

 Great Basin. 



The composition of the Red Bed series is, throughout, sugges- 

 tive of shallow water. Even the purple limestone is so highly ar- 

 gillaceous as to be simply a consolidated calcareous mud ; and, as I 

 have elsewhere explained, the remarkably perfect shrinkage cracks 

 which it exhibits at nearly all points and, apparently, through a 

 considerable part of its thickness, oblige us to suppose that it was 

 deposited in a shallow basin which was subject to periodic desic- 

 cation. These conditions would naturally depend largely upon an 

 excessively hot and dry climate ; and in the repeated exposure of 

 the Red Bed sediments during their deposition to a hot and arid 

 atmosphere, we have, perhaps, a satisfactory explanation of their 

 prevailing red color, which is believed to be due to the partial de- 

 hydration, as well as the peroxidation, of the contained iron oxide. 1 

 Other important features of the Reel Bed series, such as the beds 

 of gypsum and the almost complete absence of organic remains, 

 evidently demand similar physical conditions. The gray and ash- 

 colored fossiliferous marls and limestones of the Jurassic series 



1 Proc. Boston Soc. Nat. Hist., xxiii, 219-222. 



