rtanton.] SOUTHERN COLORADO. 27 



Montana --Continued. Feet. 

 Fox i I ills- 

 Arenaceous clays capped Uy a l>ed of yellow fossil if erous sandstone. 

 Fossils: 1 Cardium speciosum, Mactra alia, Tancredia amcricana. 

 Feniella humilis, Callista deweyi, C. oivenana, SpliacrioJa cordata, 

 Liopistha undata, Anchura americana, Scajyhiies conradi, ete. Thick- 

 ness ...800-1,000 



Total thickness of Montana formation 8, 700 



Laramie: 



Sandstones and coal beds, about 400 



Clays, ironstones, and rarely coal beds 800 



Fossils : rdants and fresh and brackish-water mollusks 



Total thickness 1, 200 



The Fort Pierre shales here have a much greater development than 

 in any other part of the Kocky mountain region. With this exception 

 the section is almost identical with Meek and Hayden's Upper Missouri 

 section. 



Southward from the neighborhood of Denver the Montana formation 

 becomes much thinner, until, on the Arkansas river between Canyon city 

 and Pueblo, its thickness is approximately 3,000 feet. It is also less 

 fossiliferous and more uniform in lithologic character, so that the Fox 

 Hills is not usually recognizable as a distinct division, though the fos- 

 sil iferous zone, with its characteristic species at the top of the Fox Hills, 

 has been observed at Colorado Springs, and the underlying shales at 

 that place and in the Arkansas valley yield some Fort Pierre forms, 

 such as Inoccramus cripsii, I. proxhmis, Liicina occidentalis, Baculites 

 ovatus, Scajyhites nodosus, Heteroceras, etc., but throughout the forma- 

 tion as a whole fossils are not so numerous either in species or indi- 

 viduals as they are in the section last described. 



In the area now under consideration the Colorado formation becomes 

 a. more important feature of the geology on account of the greater area 

 that it covers and of the better development of its fauna. The chief 

 cause of its increased area is a low anticlinal fold extending out into 

 the plain as a prolongation of the axis of the Front range. The erosion 

 that has planed off this fold until it is scarcely noticeable as a topo- 

 graphic feature has removed the Laramie and Montana strata from an 

 area at least 25 miles wide, and at a few places along the crest of the 

 fold has just reached the Dakota sandstone. 2 Consequently the nearly 

 horizontal shales and limestones of the Colorado form the surface over 

 a large area, 3 and are well exposed in long bluffs on the Arkansas, the 

 Cucharas, and other streams, and also in the usual narrow belt along 

 the foothills. 



The characteristic Fort Benton shales resting on Dakota sandstone 

 vary considerably in thickness. Near the mountains at Canyon city 



1 Some of these also occur in the Fort Pierre division. 



2 The Dakota is exposed on the hank of Arkansas river 8 miles above Pueblo and on the plains 10 

 miles cast of Cucharas. 



3 On the geological sheets of Hayden's Alius of Colorado the area assigned to the Colorado formation 

 along the Front range and in the southern central part of the state is too large on account of the fact 

 that the Fort Pierre shales are all mapped as Colorado, although the legend states that they are 

 included in the Fox Uills. 



