EXDUCH.J SOUTHERN AREA RESUME. 139 



A determination of the thickness of these sand dunes is somewhat 

 difficult. We may say that about 80 feet would prove to be an average. 

 Some of them were found to be several hundred feet high, while others 

 were not over 20 feet. The largest accumulations occur on the north- 

 ern side of the "belt" and at such points where some immovable obstacle 

 is presented to the general course followed by the wind. 



RESUME OF THE DEPRESSED SOUTHERN AREA. 



Among the formations found in this area, the most interesting one is 

 that composed of the Tertiary groups. In preceding pages I have simply 

 tarnished a description of their geognostic features, omitting to make 

 any allusion to their position in a classificatory system. Many different 

 views are held upon this subject, emanating principally from the fact 

 that the various classifications have been advanced by stratigraphists 

 and palaeontologists following various special lines of inquiry. It is 

 impossible, at the present time, to reconcile the different opinions, and 

 all that can be done is to weigh the evidence for and against each one, 

 eliminating those that are unt enable, and thus arrive at satisfactory 

 conclusions. The material now on hand ought to be sufficiently instruc- 

 tive to enable some one to accomplish this desirable end. Until this 

 work is accomplished, each geologist who has made any examination of 

 the younger formations of the West, will, essentially, have his own pri- 

 vate classification. In the subjoined pages I shall present the groups 

 in such order as I have found them in the field, and shall assign to them 

 positions in classification to which I believe them entitled. 



Six groups of Tertiary deposits are found in our district — Wasatch, 

 Green River, Bridger, Sweetwater, Niobrara, and Wyoming conglomerate. 

 Of these the Wasatch and Green River Groups are certainly conformable. 

 With regard to the Green River and Bridger I cannot speak with cer- 

 tainty, as the latter is too imperfectly represented in our district. Direct 

 connection •was, observed between the Wasatch and Sweetwater, and 

 again between the Sweetwater and Mobrara. Both of these junctions 

 ] aoved to be unconformable. We may regard, therefore, the Sweetwater 

 Group as of local character rather than belonging to a regular and typical 

 succession of Tertiary groups. 



If we study these groups in connection with their vertebrate remains 

 only, and compare them with occurrences of other countries, they will 

 ail be considered older than if their fossil flora is taken as the standard 

 of measurement and identity. Regarding the Cretaceous formation as 

 closed by the Fox Hills, and the Laramie Group as forming an inde- 

 pendent transitional formation, we must necessarily begin the Tertiary 

 with the Wasatch Group. This, then, forms either a part or the whole 

 of the equivalent of Eocene of other countries. Within our district and 

 elsewhere the Green River immediately follows upon the Wasatch. In 

 its evolutional history, and in the class of its component strata, the 

 Green River Group marks the advent of a new era. Fauna and flora, 

 as compared to those of the Wasatch, are changed ; with them the beds 

 in which they are contained. Unconformities between the two are not 

 wanting, although none exist in our district. As the Wasatch essen- 

 tially represents an epoch of definite type, totally differing from that of 

 the Green River period, I do not feel inclined to regard the two as be- 

 longing to the same division of the Tertiary formation, but place the 

 ( Jreen River series at the base of the Miocene. 



As a rule, the Bridger is directly superimposed upon the Green River. 

 Its relative position is fully established, therefore, and it enters the reg- 



