THE UPPER RED BEDS OF THE BLACK HILLS. 



contents. 

 Introduction. 



Description. 



General description of the red beds of the Black hills. 



Details of stratigraphy. , 



Microscopic characters. 



Chemical analysis. 

 Discussion. 



Theories for the origin of the color of red beds. 



Application of theories to the red beds of the Black hills. 

 Green variations. 



INTRODUCTION. 



This paper describes the upper red beds of the Black hills 

 and inquires into the origin of their color. 



The red beds of the Black hills are composed of from five to 

 six hundred feet of red sandstones and shales with gypsum in 

 the upper part and a bed of limestone toward the base. This lime- 

 stone enables a threefold division of the rocks. ^ The Opeche 

 formation at the base of the series consists of a hundred feet of 

 unfossiliferous red shaly sandstones. These lower red beds lie 

 apparently conformably upon vari-colored calcareous sandstones 

 of Carboniferous age, the line of division being an abrupt change 

 in color. Separating the upper and lower red beds, and lying 

 conformably between them, is the Minnekahta limestone, which is 

 purplish-gray in color, about fort}' feet thick, persistent in its occur- 

 rence, and contains Permian fossils. The upper red beds average 

 four hundred feet in thickness, and are composed of unfossil- 

 iferous red sandy shales and interbedded gypsum unconformably 

 overlain by more somber-colored rocks of Jurassic age. These 

 upper red beds, named the Spearfish formation, are the subject 

 of this paper. 



'N. H. Darton, " Geology and Water Resources of the Southern Black Hills," 

 Twenty -first Aruinal Report U. S. Geological Survey, Part IV (1901), pp. 513-19. 



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