210 REPORT UNITED STATES GEOLOGICAL SURVEY. 



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

 It was first described in Powell's Beport 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, bnt they are doubtless identical with those 

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

 •originally discovered in the Judith Biver beds of the Upper Missouri 

 Biver 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 

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

 feet of the top of the Laramie Group. As shown by its fossils and po- 

 sition it is plainly equivalent with the principal fossiliferous horizon at 

 Pdack Buttes, Hallville, and Point of Bocks 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 Group 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 

 deinarkation between the two groups, but wholly without success. As 

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

 out difficulty, by general Uthological characteristics ; 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 

 loffer no suggestion of a boundary plane between formations where I 

 'examined them in this region ; and, so far as I could discover, all the 

 strata between those which contain characteristic Laramie fossils and 

 those that contain characteristic Wasatch fossils are strictly conforma- 

 L>le. 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 

 3sTorth American continent, and which will be discussed on following 

 ;pages. 



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

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

 Groups west of the Bocky 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 Biver Basin, 

 published in advance of his report. 



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

 Mountain, I made some examinations of the Box 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 bambini Morton, I. vcmuxemi Meek 

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

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



