UPPER RED BEDS OF THE BLACK HILLS 387 



agreed that during the accumulation of the red beds of the cen- 

 tral West a shallow mediterranean sea, whose outlines are very 

 imperfectly known, existed west of the Mississippi and east of 

 the great basin extending northward from Texas almost into 

 Canada. In the midst of this sea the Rocky mountain province 

 formed a group of islands.' Stratigraphic evidence renders it 

 probable that different conditions prevailed simultaneously in 

 different parts of the sea and that different conditions prevailed 

 at different times in the same area. But insufficient facts have 

 been accumulated to warrant a detailed statement of conditions 

 that existed during the deposition of the red beds. 



The Black hills area was covered by this body of water. The 

 red beds there everywhere succeed the underlying Carboniferous 

 rocks, with no signs of an interval of erosion. There is no evi- 

 dence of thinning as the red beds approach the center of the hills, 

 nor of off-shore conditions. Moreover, the dips carry these 

 rocks over the highest points of the hills. The Black hills did 

 not supply the sediments under consideration. 



For the source of these red beds we must look to the land 

 masses that were contiguous to the Black hills in red-bed times. 

 These were an area of Algonkian rocks to the north and north- 

 east, the lately uplifted Carboniferous limestone to the southeast, 

 and the Rocky mountain area to the southwest and west. 



That the limestone furnished sediments to the accumulating 

 red beds in the Black hills cannot be denied, for the relatively 

 insoluble constituents of limestone often form a residual red 

 clay. Yet it is not likely that this was a prominent source. Of 

 the areas named probably the limestone was the farthest away 

 from the Black hills. Furthermore, the abundant quartz and 

 mica in the red beds, and the presence of feldspar, magnetite, 

 and ilmenite, point to a source from crystalline rocks rather than 

 from limestone. 



The extent of the Algonkian rock area is very indefinitely 

 known, and it is doubtful whether this area contributed to the 

 red beds of the Black hills. In this general region there was an 



'S. F. Emmons, Bull. Geol. Soc. Atner., Vol. I (1890), p. 245 ; R. C. Hills, Proc. 

 Colorado Scientific Soc, Vol. Ill (1888-90), p. 362. 



