stanton.] BLACK HILLS SECTION. 23 



THE UPPER MISSOURI REGION. 



The immense area of Cretaceous ill Nebraska, North Dakota, South 

 Dakota, and Montana has had little careful study excepting along the 

 course of the Missouri river and around the base of the Black hills. 

 The section quoted at the beginning of this paper gives a general de- 

 scription of the beds as developed in this region, but they are subject 

 to considerable local variation and it is especially worthy of note that 

 the calcareous Niobrara division disappears entirely in Montana, as it 

 does along the eastern base of the Wasatch mountains and elsewhere 

 in Utah, and in southwestern Colorado. 



In the Black hills the strata have been most fully described by Prof. 

 Henry Newton. 1 He was able to recognize there all five of the divi- 

 sions of the Upper Missouri section. The principal differences as com- 

 pared with that section are the presence of several thin beds of sand- 

 stone i nters tratified with the upper portion of the Fort Benton shales, 

 and the fact that nearly all of the divisions are much thinner. The 

 estimated thicknesses are : 



Black hills Cretaceous section. 



1. Dakota. Feet. 



Coarse yellow or red sandstones with discontinuous variegated clays. 

 In some places a part of the sandstone is soft and white, and at other 

 localities it is changed to a dense quartzite 250-400 



2. Fort Benton. 



Dark plastic clays which usually contain large quantities of alkaline 

 salts and selenite. Thin bedded calcareous sandstones, containing 

 numerous fossils, are frequently found in the upper part 200-300 



3. Niobrara. 



Gray calcareous shales and marls with some limestone ; Tnoceramus labia- 



tiis and Ostrea eongesta very numerous. Thickness, roughly estimated 100-200 



4. Fort Pierre. 



Dark gray and bluish plastic clays with many fossiliferous calcareous 



concretions in the upper part. Thickness, estimated 150-250 



-5. Fox Hills. 



Ferruginous sandstone, with Veniella Immilis, Sphazriola transversa, Ideon- 



arca shumardi, etc. Thickness, roughly estimated 100 



Accordiug to these estimates the total thickness of the Colorado for- 

 mation in the Black hills region is only 300 to 500 feet, and the entire 

 marine Cretaceous section is less than 1,000 feet. It should be remem- 

 bered that no accurate measurements of local sections were made. The 

 following species of fossils have been reported from the Colorado beds 

 of this region : 



Ostrea eongesta. Prionotropis woolgari. 



Inoceramus labiatus. Prionocyclus wyomingensis. 



Tnoceramus fragilis. Scaphites warreni. 



Inoceramus perplexus. Scaphites wyomingensis. 



Inoceramus altus. Scaphites larvacformis. 

 Fusus shumardi. 



Newton, Henry.an.l Walt- V P. Jenney, Geol. of the Black Hills of Dakota, Washington, 1880. The 

 Cretaceous is described on pp. 169-183. 



