XVIII REPOET UNITED STATES GEOLOGICAL SURVEY. 



little doubt that the "Lake Beds" of Dr. Hay den, as seen in Middle 

 Park, the "Brown's Park group" of Professor Powell, and the "Uinta 

 group" of Mr. King, all belong to one and the same epoch, later than 

 and distinctly separate from the Bridger group. 



In that portion of the region which lies adjacent to the southern base 

 of the Uinta Mountain Eange, and which is traversed by Lake Fork 

 and the Du Ohesne Eiver, not only the Uintah group, but both the 

 Green Eiver and Bridger groups also, are well developed, each possess- 

 ing all its peculiar and usual characteristics as seen at the typical local- 

 ities in the great Green Eiver Basin, north of the Uintah Mountains. 

 This, added to the known existence of Bridger strata in White Eiver 

 Valley, and the extensive area occupied by the Green Eiver group be- 

 tween White and Grand Elvers, has added very largely to our knowledge 

 of the southward extension of those formations. 



In all the comparative examin^jtions of the formations or groups of 

 strata that have just been indicated, he has paid especial attention to 

 their boundaries or planes of demarkation, crossing and recrossing them 

 wherever opportunity offered, noting carefully every change of both 

 lithological and paleoutological characters. While he has been able to 

 recognize with satisfactory clearness the three principal groups of Cre- 

 taceous strata, namely, the Dakota, Colorado, and Fox Hills, on both 

 sides of the Eocky and Uinta Mountains, respectively, they evidently 

 constitute an unbroken series, so far as their origin by continuous sedi- 

 mentation is concerned. While each of the groups possesses its own 

 peculiar paleoutological characteristics, it is also true that certain spe- 

 cies pass beyond the recognized boundaries of each within the series. 



The stratigraphical plane of demarkation between the Fox Hills, the 

 uppermost of the undoubted Cretaceous groups, and the Laramie group, 

 the so-called Post-Cretaceous, is equally obscure ; but the two groups are 

 paleontologically very distinct, inasmuch as the former is of marine origin, 

 while the latter, so far as is now known, contains only brackish- water and 

 fresh- water invertebrate forms. He reports a similar obscurity, or ab- 

 sence of a stratigraphical plane of demarkation, between the Laramie 

 and Wasatch groups, although it is there that the final change from 

 brackish to entirely fresh water took place over that great region. Fur- 

 thermore, he finds that while the three i)rincipal groups of the fresh- 

 water Tertiary series west of the Eoclcy Mountains, namely, the Wasatch, 

 Green Eiver, and Bridger groups, have each peculiar characteristics, 

 and are recognizable with satisfactory^ distinctness as general divisions, 

 they really constitute a continuous series of strata, not separated by 

 sharply-defined planes of demarkation, either stratigraphical or palean- 

 tological. 



During the progress of the field-work, as above indicated, large and 

 very valuable collections of fossils have been made, all of which will 

 constitute standards of reference in the future progress of the work, 

 and quite a large number of the species are new to science. These are 



