UPPER RED BEDS OF THE BLACK HILLS 389 



accumulation of a parent residual red soil and for its deposition 

 as red sediment in the Black hills area were climatic. 



It is generally believed that the Carboniferous climate in the 

 present temperate zone was warm and moist. Under such influ- 

 ences the rocks of the Rocky mountain region, which general 

 region is believed to have been land since the Cambrian,' were 

 subjected to very favorable conditions for extensive decomposi- 

 tion and for the formation of a mantle of residual red soil. 

 And because of the considerable decomposition the continued 

 formation of red soil coincident with the removal of surface 

 accumulations to supply red-bed sediments was facilitated. 



It has been noted, however, that many regions which are 

 now covered with residual red soil do not contribute red material 

 to areas of sedimentation, because the red color is destroyed by 

 deoxidation before or during deposition. But in the case of the 

 red beds under consideration there is reason to believe that this 

 deoxidizing influence was unimportant. 



A relatively arid climate in many regions is known to have 

 followed the warm and moist Carboniferous. That this was true 

 in the red-bed region of the central West is testified to by the 

 presence of beds of rock salt and gypsum. In the Black hills, 

 though no beds of salt have been found in the red beds, yet 

 the salt spring near Cambria suggests a local deposit ; and 

 interbedded gypsum is abundant. 



There can be little doubt that the gypsum of these red beds 

 was accumulated by precipitation from concentrated water con- 

 taining calcium sulphate in solution. The " bar theory " of 

 Ochsenius^ clears the difficulty of conceiving how thick beds of 

 •chemically precipitated matter can be accumulated ; and all the 

 field relations of the gypsum point to such an origin. There is 

 no indication that the gypsum is the result of the action of sul- 

 phuric acid on limestone. The bedded character of the gypsum 

 interstratified with detrital sediments, the general occurrence of 

 the gypsum in lenses, the frequent presence of layers of red 

 .sand and clay in beds of impure gypsum and of thin layers of 



' Emmons and Hills, op. cit. 



^ Zeitschrift fiir praktische Geologic, 1893, P- 189. 



