UPPER RED BEDS OF THE BLACK HILLS 393 



igneous rocks. Moreover, if the igneous rocks of the Black hills 

 exerted a dehydrating influence sufficient to change the red beds 

 from a possibly mottled previous condition to their present uni- 

 form color, such influence surely would have dehydrated the vari- 

 colored iron pigments in the underlying Carboniferous sand- 

 stone. This, however, did not occur, and the suggestion of Dana 

 is not applicable in the Black hills. 



Green variations. — The occurrence of green variations in red 

 beds has caused some^ discussion. In the case of true green beds 

 among sediments derived in general from red soils, it seems 

 likely that such green beds were either deposited from a locally 

 different source than the red material, or that they represent red 

 sediments which were deoxidized in the area of deposition. But 

 small green spots and streaks, which constitute the general 

 occurrence of green material in the Spearfish formation, can best 

 be explained by considering them to have been developed sub- 

 sequent to deposition. 



These variations can be accounted for by the influence of 

 occasional bits of organic matter present in the sediments. 

 Such decomposing organisms reduced the ferric iron of the red 

 pigment to a soluble form that was removed in solution, and 

 green spots and streaks remained. In spite of the lack of more 

 evidence of organic remains in the red beds, these green varia- 

 tions are difficult to explain in any other way. The irregular 

 distribution of the green patches, their occasionally following 

 cracks in the rocks, and their similarity in composition to the 

 adjacent red clay, from which they differ only in containing less 

 iron, point to this explanation. 



George B. Richardson, 



' George Maw, Quarterly Journal Geological Society of London, Vol. XXIV, 

 p. 35 1; T. N. Dale and W. T. Hildebrand, Nineteenth Annual Report United 

 States Geological Survey, Part III, p. 255. 



