388 GEORGE B. RICHARDSON 



extensive land area which may have been such from Cambrian 

 down to Cretaceous time, but during red-bed deposition the 

 respective limits of water and land are unknown. At Sioux 

 Falls, S. D., there is an exposure of the Algonkian, the Sioux 

 quartzite, which artesian-well borings show to have a considerable 

 extent below the Cretaceous. The Sioux quartzite is a red rock 

 which could have furnished red sediments. Microscopic study, 

 however, renders it unlikely that this formation contributed to 

 any considerable extent to the Spearfish formation. A character- 

 istic feature of the Sioux quartzite is that it is composed of 

 rounded quartz grains, the outlines of which are delicately 

 traced by circlets of iron oxide imbedded in a matrix of inter- 

 stitial silica crystallized in conformity with the nucleal quartz.^ 

 The red beds under consideration show no trace of this siliceous 

 rim, which would be expected were the rocks derived from the 

 Sioux quartzite. 



The Rocky mountain region, however, was an available source 

 of sediments for the red beds of the Black hills. During red- 

 bed time this area was flanked by the deposition of red sedi- 

 ments whose constituents can be directly traced to such an origin.^ 

 And although the eastward extent of these red beds toward the 

 Black hills is now deeply hidden by overlying rocks, so that actual 

 stratigraphic connection has not been traced between the red 

 beds contiguous to the Rocky mountains and those of the Black 

 hills, yet such connection seems probable. Wells that have been 

 put down deep enough east of the Rockies invariably have pene- 

 trated these red rocks. And the diminution in thickness of red- 

 bed sediments from about three thousand feet adjacent to the 

 mountains to five hundred feet in the Black hills, with an 

 accompanying decrease in fineness of materials strongly suggests 

 that the red beds of the Black hills are continuous with those of 

 the eastern slope of the Rocky mountains. 



The unusually favorable conditions, referred to above, for the 



'S. W. Beyer, Iowa GeoL, Vol. VI (1897), p. 102. 



" A. C. Spencer, " Geology of the Rico Mountains," Twenty-first Annual Report, 

 U. S. Geological Survey (1900), Part II, p. 68; G. K. Gilbert, Pueblo Folio, U. S. 

 Geological Survey, 1897. 



