40 



GEOLOGY OF THE BLACK HILLS. 



Sijnopsis of the Roclm of the Black HilUi. 





Ages. 



Feet. 



Strata. 



6 



M 

 O 



Miocene. 



200 



White clay, with chalcedony : at base a conglomer- 

 ate. 



N 

 O 

 CO 



Cretaceous. 



600 



Clays and shales; gray, yellow, and black: with 

 some sandstone. 



300 



Coarse yellow sandstone, with sandy shale. 



Jura. 



200 



Gray, white and red, clays and marls, with some im- 

 pure limestone. 



Red Beds. 



340 



Eed clay, sometimes arenaceous, interrupted by a 

 single bed of limestone. 



6 

 o 



N 

 O 



Carboniferous. 



125 



Sandstone, red and variegated, somewhat calcareous 

 and argillaceous. 



175 



Alternating sandstone and magnesian limestone. 



175 



Silicious limestone ; silica in nodules and geodes. 



175 



Gray limestone, massive, pure. 



40 



Shaly limestone, pinkish ; in part arenaceous. 



Silurian. 

 (Potsdam.) 



250 



Silicious sandstones, in part quartzitic. Conglomerate 

 usual at base. Calcareous cement and intermin- 

 gled glauconite in upper jjart. Colors, dark brown 

 and red, rarely white. 



H 



t 



? 



Slates. 



1 



Schists, with lenses of intrusive granite. 



