stanton.] MONTANA AND COLORADO. 25 



Colorado and Montana— Continued. Feet. 



Thinlylaminated arenaceous shales with Ostrea anomioides 340 



Sandstones 15 



Shales 75 



2, 850 



Dakota: 



Quartzite 30 



Limestone with great numhers of a small fresh-water gasteropod 10 



Sandy shales 150 



Red earthy limestones, magnesias 50 



Conglomerate 40 



Sandstone and shales 95 



Sandstone 151 



52G 



Total Cretaceous 4, 311 



Sections at other localities give a much greater total thickness, but 

 the marine portion, or combined Colorado and Montana, seems not to 

 vary greatly. It is evident that both of these formations are repre- 

 sented in the section, but there seems to be.no lithologic or structural 

 reason for making a division and all of the fossils yet reported from 

 the locality belong to the Colorado fauna and probably all came from 

 near the horizon of Ostrea anomioides. The following species were col- 

 lected there by members of the U. S. Geological Survey of the Terri- 

 tories. 



Ostrea anomioides. Baculites asper? 



Trigonia, related to T. evansana. Scaphites ventricosus. 



Inoceramus sp. ? 



At a locality near the Missouri river below Gallatin, Montana, where 

 the same beds are exposed, the first three of the above-named species 

 occur, and associated with them are Corbicula inflexa, Pharellaf pcalei, 

 and a number of others 1 represented by casts that can not be specifi- 

 cally determined. 



COLORADO AND NEW MEXICO. 



Along the eastern base of the Front range in Colorado the Upper 

 Cretaceous formations are well developed and easily distinguishable 

 both by lithologic features and by an abundance of characteristic fos- 

 sils. From the northern boundary of the state to the Arkansas river 

 and beyond, the first prominent " hogback," or foothill of the moun- 

 tains is composed of the sandstones and conglomerate of the Dakota. 

 It is so constant lithologically that it can be readily recognized without 

 the aid of the plants which are the only fossils that are commonly found 

 in ifc in this region. On the plain immediately east of the Dakota ridge 

 the shales and limestones of the Colorado, the softer clay shales and 



1 See Prof. Moek's lists in Aim. Kept. IJ. S. Geol. Suv. Terr., for 1872, pp. 474, 475. 



