68 GEOLOGY OF THE DENVER BASIN. 



presence, in some of the lower beds, of small limonite concretions; and in 

 a variable amount of alumina, especially noticeable in a comparison 



between the shalv and noiishalv varieties. 



Middle division. — This consists of light-gray marls, here and there streaked 

 with darker-gray and huff bands. It is usually a little over loo feet thick. 



upper division. — This lias an average thickness of "_'40 feet and is an 

 accumulation of buff and yellow shales of a peculiar earthy and more or 

 less calcareous composition. Several narrow layers of yellow, impure, 

 saccharoidal limestone occur, together with beds of line, sandy material. 

 The whole series is fossiliferous. There is a remarkable distribution of 

 alkaline salts throughout the series, weathering out in small white spots. 

 Gypsum is frequently met with, and iron occurs in small concretions. 



The contrast in composition and color between the yellow shales and 

 the sediments of the overlying Pierre affords a clear line of demarcation 

 between the two formations. 



LIFE. 



The fossils of the lower limestone member of the Niobrara include 

 Inoceramus problematicus, Tnoceramus deformis, Ostrea, undetermined fish 

 remains, ami sharks' teeth. The last are coated with a heavy, shiny-black, 

 carbonaceous material, much resembling bitumen in appearance, and in 

 the limestone containing them the odor of this substance is frequently 



detected. 



The fossils of the vellow series embrace the forms Ostrea congesta, 



a species of Inocerainus. one of a Baculites, several long, black spines of 

 elliptical cross-section, probably derived from the shells of some molluscan, 

 and innumerable remnants of fish integuments, occurring as small In-own 

 or blue membranous fragments. 



Tlie vertebrate remains of animals which constitute the Pteranodon 

 fauna of Professor Marsh occur further eastward, in Kansas, in chalky 

 beds which are supposed to correspond in horizon with the Niobrara 

 formation. 



MONTANA GROUP. 



The Molilalia group occupies a highly inclined position alone' the 

 foothills, and, with the exception of an area in the northwest portion ot 



