wmTE.] STRATA AT BLACK BUTTES STATION. 223 



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

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

 ■svere formed in any other than lacustrine or interior sea waters. In otlier 

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

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

 the meeting of ilmiatile and lacusti^ine or sea waters. This inconstancy 

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

 almost everyAvhere, and indicates a general prevalence of shallow water 

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

 fi?esh-water, brackish-water, and marine types in these and other Lara- 

 mie strata is very j^eriilexing, and will be fuither discussed on follo^A'ing 

 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 iilane of demarkation, or any indication of a 

 division between the Laramie and Wasatch Groups. The thickness 

 therefore that is assigned to JSTo. 1 of the foregoing section is only an 

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

 the Green Eiver Basm, where there is a true miconformity of strata at 

 or about the jimction of the two groui)S, but I have not had the good 

 fortime to examine any such localities iiersonally. Li this statement 

 reference is not made to the great miconformity of the Wasatch uijon. 

 the Laramie at the extreme western x^ortion of that geogTaphical basin, 

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

 discussed on following pages. 



Within 100 feet above the base of I^o. 1 of the foregoing section I 

 found fragments of bones, i)robably those of Agatliaumas sylvestris 

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

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

 kiud above i^o. 2 of the section ; and it will thus be seen that vertebrate 

 remains of Cretaceous type are foimd above all the invertebrate fossils 

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

 the uppermost portion of the Laramie Grouj). 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 imcon- 

 formity of the strata of the ux)per part of the Laramie series ui)on those 

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

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

 terestuig differences among the fossils that respectively characterize the 

 strata above and below the nnconformity. For exami^le, in Bitter Creek 

 Valley the tyi)ical forms of Corhida xnuUfcra are not found above the 

 nnconformity, and those of C. stthimdifera, which I now regard as only 

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

 suhtrigoualis to which Mr. Meek gave the name of 6'. tropldophom is 

 found only below the imconformity, while that wliich he called C. crasm- 

 telUformis is found only above it. MeJania insculpta, which seems to dif- 

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

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

 formity, while tlie last-named species is fomid only above it. Also 

 Corhicula cythcriformis is found only below the unconformity, while the 

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

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



