white.] STRATA AT BLACK BUTTES STATION. 223 



might be examined in vain for fossils of any kind. Tet there is no evi- 

 dence that any of these layers are of truly estuary origin, or that they 

 were formed in any other than lacustrine or interior sea waters. In other 

 words, the variation referred to seems to have been the result of shifting 

 conditions of limited extent hi a large body of water, rather than that of 

 the meeting of fluviatile and lacustrine or sea waters. This inconstancy 

 of the layers composing a large part of the Laramie Group is common 

 almost everywhere, and indicates a general prevalence of shallow water 

 during the Laramie period. The frequent mixture in a single layer of 

 fresh-water, brackish- water, and marine types in these and other Lara- 

 mie strata is very perplexing, and will be further discussed on following 

 pages. 



The dip of all the strata at Black Buttes Station is gently to the east- 

 ward, and going in that direction a few miles one finds himself upon the 

 strata of the Wasatch Group. I crossed and recrossed this space at dif- 

 ferent places in the neighborhood, and failed, as I had done before at 

 other localities, to find any plane of demarkation, or any indication of a 

 division between the Laramie and Wasatch Groups. The thickness 

 therefore that is assigned to No. 1 of the foregoing section is only an 

 approximate estimate. I do not doubt that there are places, even within 

 the Green River Basin, where there is a true unconformity of strata at 

 or about the junction of the two groups, but I have not had the good 

 fortune to examine any such localities personally. In this statement 

 reference is not made to the great unconformity of the Wasatch upon 

 the Laramie at the extreme western portion of that geographical basin, 

 or rather upon the eastern borders of the Salt Lake Basin, which will be 

 discussed on following pages. 



Within 100 feet above the base of No. 1 of the foregoing section I 

 found fragments of bones, probably those of Agathaumas sylvestris 

 Cope, which is probably the identical locality at which Professor Cope 

 obtained his type specimens. I found no invertebrate fossils of any 

 kind above No. 2 of the section; and it will thus be seen that vertebrate 

 remains of Cretaceous type are found above all the invertebrate fossils 

 of the Black Buttes locality, and that the former were obtained from 

 the uppermost portion of the Laramie Group. Some reference to the 

 discussions that have taken place as to the geological age of these strata 

 will be made on a following page. 



Before taking leave of the Bitter Creek series there is an interesting 

 matter to be considered in relation to it. The existence of an uncon- 

 formity of the strata of the upper part of the Laramie series upon those 

 of the lower part, especially observable in the neighborhood of Point of 

 Bocks, has already been noticed. As already hinted, there are some in- 

 teresting differences among the fossils that respectively characterize the 

 strata above and below the unconformity. For example, in Bitter Creek 

 Yalley the typical forms of Corbula undifera are not found above the 

 unconformity, and those of C. subundifera, which I now regard as only 

 a variety of the first, are not found below it. The variety of Corbula 

 subtrigonalis to which Mr. Meek gave the name of C. tropidoplwra is 

 found only below the unconformity, while that which he called C. crassa- 

 telllformis is found only above it. Melania insculpta, which seems to dif- 

 fer from M. tvyomingensis only in the absence of the nodes or short 

 spines which characterize the latter, is found only below the uncon- 

 formity, while the last-named species is found only above it. Also 

 Corbieula cytlieriformis is found only below the unconformity, while the 

 closely related form C. occidentalis is found only above it. It would be 

 too much to assume that all the upper forms here named are the lineal 



