EM'ucii.J SOUTHEKN AREA RESUME. 139 



A dcteim illation of the tLiokness of these sand dunes is somcAvhat 

 diliicult. We may say tiiat about 80 feet would prove to be an avert'.jjfe. 

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

 were not over 20 feet. The largest aceuniulations oeeur on the north- 

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

 is presented to the general course followed by the wind. 



EfiSUMfi OF THE DEPRESSED SOUTHERN AREA. 



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

 that c()iii])Osed of <he Tertiary groups. In preceding ^lages I have simply 

 furnished a descrii)tion of their geognostic features, omitting to make 

 any allusion to their position in a classiticatory system. ]\fany different 

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

 that the various elassiiications have been advanced by stratigraphists 

 and paheontologists following various special lines of inquiry. It is 

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

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

 eJiminating those that are untenable, and thus arrives at satisfactory 

 conclusions. The material now on hand ought to be suiliciently 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 })ri- 

 vate classiOcation. In the subjoined ])ages I shall i)resent the groui)8 

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

 iwsitions in classiH cation to which I believe them entitled. 



Six groups of Tertiary deposits are found in our district — W^asatch, 

 Green River, Bridger, Sv/eetwater, Niobrara, a.ud Wyomingconglomerate. 

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

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

 tainty, as the latter is too imi^erfectly represented in our district. Direct 

 connection was observed between the Wasatch and Sweetwater, and 

 again between the Sweetwater and Mobrara. Both of these junctions 

 ])ro\ed to be unconformable. We may regard, therefore, the S\Aeetvv'ater 

 Group jis of local character rather than belonging to a regular and tyi>ical 

 succc'ssion of Tertiary groups. 



If we study these groups in connection with their vertebrate remains 

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

 all 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 Hdls, and the Laramie Group as forming an inde- 

 l)cnd(Mit transitional formation, we must necessarily begin the Tertiary 

 with the AVasatch Group. This, then, forms either a part or the wiiole 

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

 elsewhere the Green River immediately folloA\^s 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 florji, 

 as comi)ared to those of the Wasatch, are changed ; with them the beds 

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

 wanting, although none exist in our district. As the A¥asatch 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 Teii:iary formation, but place th« 

 Green River series at the base of the Miocene. 



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

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



