26 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA. [bum.. 106. 



the sandstones of the Montana, and the coal-bearing sandstones and 

 shales of the Laramie are exposed in the order in which they are named. 

 There are numerous fossiliferous zones throughout the marine portion 

 of the strata, and it is usually not difficult to trace the characteristic 

 species of the Colorado and Montana formations, respectively, almost 

 to the dividing line between the two formations. 



The section given below is based on descriptions of the Cretaceous 

 strata in the Denver region by Mr. Geo. H. Eldridge. 1 Although orig- 

 inally intended only for a limited area surrounding Denver, Mr. 

 Eldridge's description with very slight modifications will apply equally 

 well to any part of the Cretaceous belt between Wyoming on the north 

 and Pike's peak on the south. Perhaps the Laramie portion of the sec- 

 tion should be excepted in this statement, as it is not so well developed 

 in the northern and southern portions of this area as in the Denver 

 basin. 



Cretaceous section in central Colorado. 



Dakota : Feet. 

 A thin bed of conglomerate eomposedof well-rounded quartzose pebbles 

 united by strong siliceous cement. Hard, usually gray, sandstone 

 in two prominent benches separated by a band of lire clay. Fossil 

 plants are abundant at some localities. Total thickness of the forma- 

 tion about 300 



Colorado: 



Fort Benton — 



Dark shales with, frequent intercalations of fossiliferous drab lime- 

 stone in the upper third. Inoeeramus labiatus is the most abun- 

 dant and characteristic species and Prionotropis woolgari is occa- 

 sionally found 400-500 



Niobrara — 



Drab-white limestone containing Inoeeramus deformis, I. labiatus, 



Ostrea congesta, etc . 40 



Gray clays and buff silioeo-calcareous shales with Ostrea congesta 



and numerous scales of fishes 260 



Total thickness of Colorado formation 700-800 



Montana : 



Fort Pierre — 



Drab shaly clays with numerous highly fossiliferous concretions 

 of drab limestone and occasional unimportant bands of sandstone. 

 Characteristic fossils:-' Inoeeramus cripsii, I. proximus, Avicula 

 linguiformis, A. nebrascana, Lucina occid entails, Anisomyon (sev- 

 eral species), Ptyckoceras mortoni, P. crassum Hcleroceras (several 

 species), Baculites ovatus, B. compressus, riaventiecras placenta, 

 Sphenodiscus lenticular e, Scajrfiitcs nodosus. Nautilus deJcayi, etc. 

 Thickness 7, 700 



1 On some Strati graphical and Structural Features of the Country about Denver, Colorado. Proc 

 Colo. Sci. Soc, vol. in. pp. 90-07. On certain peculiar Structural Features in the Foot hill Region of 

 the Rocky mountains near Denver, Colorado. Bull. Philos. Soc. of Washington, vol. xi, p. 2 in. 



8 The fauna contains a large number of species the most of which are figured by Meek, U. S. 

 Geol. Sur. Terr., vol. ix, pis. 11-27. 



