30 REPORT UNITED STATES GEOLOGICAL SURVEY. 



sandstones usually exist. These bad-land sandstones are similar in color 

 and general character to those of the Jurassic Group ; and where the 

 conglomerate member of the Dakota Group is not brought up to view 

 with the upper portion, as is the case in Agency Park, the bad-laud 

 strata of this group may be easily mistaken for those of the Jurassic. 



This group seldom furnishes invertebrate fossil remains of any kind, 

 especially in this district and the adjoining region; but very important 

 floral and vertebrate remains have been obtained from it east of the 

 Eocky Mountains. Its lithological characteristics, as well as its strati- 

 graphical position in relation to the other groups, leave no doubt, how- 

 ever, of its identity. Professor Powell reports the existence of coal in the 

 Dakota Group at some localities in Utah, but I observed no indications 

 of its existence among the strata of the group in this district. 



THE COLORADO GROUP. 



As already shown, the limits of the Colorado Group are restricted in 

 this report so as to intentionally embrace only the equivalents of the Fort 

 Benton and Niobrara groups of the Upper Missouri Eiver section, estab- 

 lished by Meek and Btayden, and it is also understood to be equivalent 

 with the Sulphur Creek Group of Professor Powell, as thus restricted. 



It is doubtless true that in some places the strata which are regarded 

 as equivalent with those of the Fort Pierre Group of the Upper Missouri 

 Eiver section, accord more nearly in lithological characters with those 

 beneath, than with those above them. Mr. Meek, who studied the paleon- 

 tology of these groups so carefully, has shown in his works that while the 

 paleontological affinities between the Fort Benton and Niobrara groups, 

 and the Fort Pierre and Fox Hills groups, respectively, are very close 

 they are comparatively very slight between the two former and two 

 latter groups respectively. Although the grouping of the strata of all 

 geological ages in the western part of North America has been largely 

 done by the field geologist, mainly upon stratigraphical grounds, and 

 much of it properly so done, no person will justly question the necessity 

 of giving preference to paleontological evidence in the proper geological 

 grouping of strata. It is for this reason that, while adoi)ting the name 

 "Colorado" for the group under discussion, as having priority over 

 that of " Sulphur Creek," 1 restrict its application to the strata that I 

 understand to be the equivalent of the latter, and also the equivalent of 

 both the Fort Benton and Niobrara groups, instead of including under 

 the name thus adopted, the equivalent of the Fort Pierre Group also, 

 as Mr. King has done. 



The Colorado Group is perhaps more persistent in the uniformity of 

 its lithological characteristics over a great region than any of the other 

 Cretaceous groups, with the possible exception of the Dakota Group. 

 It consists largely of that indurated clayey material, more or less dis- 

 tinctly laminated, which geologists generally, but somewhat loosely, de- 

 nominate " shales"; but occasional layers of sandstone, usually soft, are 

 sometimes met with. The strata of this, compared with those of the 

 associated groups, are easily eroded. Consequently they seldom appear 

 in escarpment exposures ; and the stratification of the grouD is very 

 often much obscured by its own debris. The lower portion of the group 

 is more clayey, less sandy, and darker colored than the upper, but both 

 being disintegrated and eroded with almost equal facility, a distinct 

 line of separation between the two portions is seldom or never observ- 

 able. 



The comparative ease with which the strata of this group are disiu- 



