390 GEORGE B. RICHARDSON 



gypsum among the red sediments, besides the presence of 

 gypsum disseminated throughout the red sediments, as shown 

 by the rock analyses, lead to the conclusion that the gypsum was 

 deposited as a chemical precipitate contemporaneously with the 

 detrital sediments. Such an origin demands a somewhat arid 

 climate.^ 



Now, an arid climate, sufficient to cause the precipitation of 

 beds of gypsum tends to cause the preservation of the color of 

 red sediments. Being unfavorable for the existence of abundant 

 life in inland waters, such a climate minimizes the prevalence 

 of deoxidizing influences incident to the presence of organisms. 

 In this connection the general absence of fossils in the red beds 

 is noteworthy. 



With such favorable conditions for the accumulation of a 

 red soil and for its deposition as the red beds of the Black 

 hills let us look now for direct evidence bearing on the origin 

 of the color furnished by the composition of the rocks under 

 consideration. 



The chemical composition of the red beds of the Black hills 

 is essentially that of a residual red clay, notwithstanding the 

 abundance of carbonates and sulphates. These unusual con- 

 stituents were not of detrital origin, but were caused by con- 

 ditions of sedimentation. 



The gypsum already has been referred to, and considering 

 the general paucity of life in the water in which these red beds 

 were accumulated and the salinity of this water, the calcium and 

 magnesium carbonates, which are disseminated throughout the 

 red beds, appear to have been formed as chemical precipitates 

 instead of having had a more direct organic origin. The pres- 

 ence of crystals of these carbonates means that they were depos- 

 ited from solution. Their widespread and uniform dissemina- 

 tion, and the absence of veins and local accumulations, implies 

 original deposition with the detrital sediments rather than sub- 

 sequent introduction from percolating water. A secondary 

 derivation from decomposition subsequent to sedimentation is 



^ The succession of a genial Carboniferous climate by post-Carboniferous arid 

 conditions is emphasized by Chamberlin, Jour. Geol., Vol. V, p. 678. 



