WHITE. I MESOZOIC AGE. 29 



the Cretaceous rocks of Wyoming, Colorado, and Utab, by uniting the 

 equivalents of certain of those groups under one designation, while pre- 

 serving essentially the main features of the original classification. There 

 is comparatively little difficulty in recognizing among the Cretaceous 

 strata of the regions mentioned, the principal features of the clossifica- 

 tion which was originally made by Meek and Hayden for those of the Up- 

 per Missouri River region. That which I have followed consists mainly 

 in reducing the number of groups by placing together those that have 

 the nearest paleontological affinities. I have thus placed the equivalents 

 of the Fort Benton and Niobrara groups together under the name of Col- 

 orado Group ; and those of the Fort Pierre and Fox Hills groups together 

 under the single name of Fox Hills Group, leaving, as all others have 

 done, the Dakota, as everywhere, a single, separate group. Such a 

 grouping of the Cretaceous strata is not only quite sufficient for all re- 

 quired purposes within the region to which it is applied, but it is as 

 natural as the original grouping is for the Upper Missouri Eiver region. 

 The modification I have adopted is, for this district at least, quite as 

 natural, stratigraphically, as that proposed by Mr. King (who joins the 

 equivalent of the Fort Pierre Group to that of the Niobrara and Fort 

 Benton groups to make up the Colorado Group), but I find it more 

 natural, paleontologically. 



All the groups of strata that are referred to the Cretaceous period in 

 this report are, within this district, not only strictly conformable with 

 each other as regards their stratification, but I have never been able to 

 fix upon a j)lane of demarcation between any of them with entire pre- 

 cision. The aspect of the strata throughout the vertical range of all 

 three of these groups, is that of continuous sedimentation. The general 

 characteristics by which the groups are separated from each, other will 

 be given under the head of each. 



THE DAKOTA GROUP. 



The strata of this group are exposed as a narrow b^nd among the up- 

 turned strata that flank the uplifts in the northwestern portion of this 

 district, and also as a small but somewhat broader exposure, which is 

 found to cap a portion of Midland Eidge, as is shown upon the geologi- 

 cal map accompanying this report, and also upon one of the sheets of 

 the great atlas of Colorado, soon to be published. 



A very small exposure is also to be seen near the southwestern flank 

 of Yampa Mountain, and another small one, of somewhat greater ex- 

 tent than the last, in Agency Park, near the right bank of White Eiver. 

 The latter exposure is brought up by a slight flexure that is apparently 

 the vanishing northern end of the Blk Mountain Uplift, the main eleva- 

 tion of which is far to the southward. 



The general lithological characteristics of this group are somewhat 

 variable in different portions of the great Eocky Mountain region, but 

 this variability is not such as to prevent its ready identification wherever 

 it is sufficiently exposed, even in very widely-separated localities. In 

 this district the group reaches an aggregate thickness of between 500 

 and 600 feet, and consists of two divisions, which, in some places, are 

 more clearly defined than in others. The lower portion, having a thick- 

 ness of some 300 feet, consists of a dark-colored, coarse, silicious peb- 

 ble-conglomerate, which is somewhat irregularly bedded and easily dis- 

 integrated. The upper portion, having a thickness of from 150 to 200 

 feet, consists of a yellowish or brownish, rough, heavy-bedded sand- 

 stone, between which and the conglomerate some variegated bad-laud 



