UPPER RED BEDS OF THE BLACK HILLS Z77 



SiOa - O.IO 



AI2O3 - 0.12 



CaO ------ 32.44 



" MgO ------ 0.33 



H2O 20.80 



SO3 ---.-. 45.45 



CO2 ------ 0.85 



100.09 



Gypsum also occurs, forming interlacing networks of veins in 

 the red sediments. The veins can be traced directly to adjacent 

 beds of gypsum, and range from paper thinness to two or three 

 inches. Frequently vein structure, crystals oriented perpendicu- 

 lar to the walls, occurs and sometimes two periods of formation 

 of crystals are evident. 



The red beds rest on the underlying purple limestone every- 

 where conformably. There is no indication of a period of 

 exposure of the limestone to subaerial influences, but there is a 

 sharp contrast in the character of the sediments. Massive lime- 

 stone generally is followed abruptly by red shales ; though in 

 places — sections IV, V, and XVIII — gypsum immediately over- 

 lies the limestone. 



In contrast to the uniformity of the lower contact, the transi- 

 tion from the upper red beds to the Jurassic is variable. The 

 change does not occur in the red beds themselves, but rather 

 marks the beginning of the Jurassic. The upper contact is locally 

 an undulating, gently eroded surface; frequently, however, the 

 contact is plane and apparently conformable. The upper contact 

 is occasionally marked by pebbles of quartz varying in size from 

 pin-heads to peas (section XVII). In one instance (section 

 XIX) gypsum was found at the extreme top of the formation. 

 The most pronounced change, though, is that of color. The 

 uniform chocolate-brown shales of the upper Spearfish are suc- 

 ceeded by green shales or buff sandstones. 



Microscopic characters. — Under the microscope the red shale 

 is seen to be composed of minute white particles with irregular 

 outlines coated by, and frequently including, an amorphous 

 brown-red pigment. Quartz is the chief constituent, besides 



