210 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



of tlie Tampa, the horizon being practically the same at both localities. 

 It was first described in Powell's Eeport on the Geology of the Uinta 

 Mountains, p. 133. 



11. Campeloma vetula Meek & Hayden. 



A couple of imperfect specimens only of this species were discovered 

 in the valley of the Yampa, but they are doubtless identical with those 

 at Black Buttes Station , which I have identified with this species. It was 

 originally discovered in the Judith Eiver beds of the Upper Missouri 

 Uiver region, and is described and figured in vol. ix of the United 

 States Geological Survey of the Territories. 



As has before been stated, with the exception of the greater range of 

 the Ostrea, the Laramie fossils found in the valley of the Yampa have 

 there a very limited vertical range, probably not exceeding ten or fifteen 

 feet. It was also stated that this horizon is within four or five hundred 

 feet of the top of the Laramie Grou-i?. As shown by its fossils and po- 

 sition it is plainly equivalent Avith the i)rincipal fossiliferous horizon at 

 J31ack Buttes, Hallville, and Point of Eocks Stations in the valley of Bit- 

 ter Creek, about 100 miles to the northwestward of the Yampa Valley 

 locality ; which horizon is also known to be comparatively near the top 

 •of the Laramie Grouj) in that region. 



With a view to learning all that can be known concerning the junction 

 of this group with the Wasatch Group above it, I spent two days in the 

 vicinity of the north side of Yampa Valley, searching for a plane of 

 demarkation betvv^een the two groups, but wholly without success. As 

 a rule, one is usually able to recognize their respective identity with- 

 out difficulty, by general lithological characteristics 5 and the fossils of 

 each, when found, leave no doubt as to which of the two groups the 

 strata containing them belong. The differences of lithological charac- 

 ter, however, are so little anywhere within a limited vertical range as to 

 offer no suggestion of a boundary plane between formations where I 

 '€xamined them in this region 5 and, so far as I could discover, all the 

 strata between those wliich contain characteristic Laramie fossils and 

 those that contain characteristic Wasatch fossils are strictly conforma- 

 "ble. It is in view of these facts that I reached the conclusion that what- 

 ever of catastrohpal or secular changes may have taken place elsewhere 

 to interrupt sedimentation and mark a boundary between the strata of 

 the Laramie and those of the Wasatch Group or their equivalents (and 

 such are known to have taken place), in this region at least, sedimenta- 

 tion was continuous from the one epoch to the other. This fact, if it be 

 :such, has a most important bearing upon the geological history of the 

 !IN"orth American continent^ and which will be discussed on following 

 pages. 



The region of the valleys of the Yampa and White Elvers is an im- 

 portant one as regards the development of the Laramie and Fox Hills 

 'Groups west of the Eocky Mountains, and for an account of its geologi- 

 cal structure I refer to my report for 1876, together with maps published 

 by this survey, and to King's geological map of the Green Eiver Basin, 

 pubhshed in advance of his report. 



Proceeding down the valley of Yampa Eiver to the vicinity of Yampa 

 Mountain, I made some examinations of the Fox Hills strata, and ob- 

 servations concerning their connection with the Laramie Group. The 

 only invertebrate fossils I found in the neighborhood of that mountain 

 were some fragments of Inoceramus harabini Morton, I. vanuxemi Meek 

 .& Hayden, and Baculites ovatus Say. They were found in the neighbor- 

 hood of the south end of the mountain ia upturned strata of the Fox 



