§4 IV. N. LOGAN 



being made on either palaeontological, lithe-logical or economic 

 conditions. The subdivisions are : The Bituminous Shale, the 

 Lincoln Marble, the Flagstone Beds, the Fencepost Beds, and 

 the Inoceramus Beds. The Shale group has two "divisions, the 

 Ostrea Shales and the Blue Hill Shales. The Niobrara series 

 is divided into the Fort Hayes Limestone and the Pteranodon 

 group, the latter being further divided into the Rudistes Beds 

 and the Hesperonis Beds. 



FORT BENTON 



The bituminous shale. — Although I have named these beds as 

 one of the subdivisions of the Limestone group I prefer to 

 discuss them separately for convenience of correlation. The 

 Bituminous shale is a moderately compact argillaceous shale 

 which in some localities is somewhat: calcareous. The prevailing 

 color of the shales at the base of the beds is dark blue, which 

 passes to light gray at the upper limit. The beds contain the 

 remains of a marine fauna, since plesiosaurs' bones, sharks' teeth, 

 and impressions of Inoceramus are found in them. The maxi- 

 mum thickness of the stratum is not more than twenty-five or 

 thirty feet. 



In the Arkansas valley in Colorado its stratigraphical equiva- 

 lent, the Graneros shales, 1 reach a thickness of 200 feet. In 

 the Huerfano area 2 these shales have a thickness of 100 feet. 

 Near Sioux City, Iowa, and Ponca, Nebraska, 3 a bed of shales, 

 having a thickness of forty feet, occupies the same geological 

 horizon and possesses primarily the same characteristics. In 

 the Black Hills area near Buffalo Gap these shales have a thick- 

 ness of more than twice that of the Eastern area, being in the 

 neighborhood of 100 feet in thickness. The stratum appears to 

 be very persistent, occurring in all the known areas of the cen- 



1 Gilbert, G. K., The Underground Waters of the Arkansas Valley in Eastern 

 Colorado, Seventeenth Ann. Rept. U. S. Geol. Surv., 1896. 



2 Stanton, The Colorado formation and its Invertebrate Fauna, Bull. U. S. Geol. 

 Surv., 1893. 



3 Calvin, The Relation of the Cretaceous Deposits of Iowa to the Subdivisions 

 of the Cretaceous proposed by Meek and Hayden, Am. Geologist, Vol. XI. 



