160 GEOLOGICAL SURVEY OF THE TERRITORIES. 



considered them to be of Upper Cretaceous rather than Tertiary age. 

 He says, " There are some reasons, however, why we should suspect 

 this white mesa to be Upper Cretaceous rather than Tertiary, and these 

 are, first, that all the Tertiary rocks of the Eocky Mountain country, as 

 far as they have been examiued, are of fresh-water or estuary origin, 

 have been usually deposited in basins of less extent and depth than 

 would be indicated by this great plateau, which has evidently been 

 greatly reduced in dimensions by the erosion it has suffered ; second, 

 the materials composing the Tertiary strata found on the great central 

 plateau are generally soft, and yield readily to the action of the elements, 

 presenting rounded and unbroken outlines or pinnacles aud deeply chan- 

 neled surfaces, the results of erosion. On the contrary, the Upper Cre- 

 taceous strata, as they appear in several points on our route, holding 

 precisely the relative position of the white mesa to the Lower Creta- 

 ceous sandstones, consist of a series of shales and limestones which, 

 though dark internally, weather to an almost chalky whiteness, and yet 

 are as resistant to atmospheric erosion as any other sedimentary rocks. 

 Judging from the view we had of it, we regarded the white mesa as 

 continuous with the white mesa bordering the Colorado, which has 

 about the same altitude. If so, the strata composing it must occupy a 

 very large area north and west of our camp 98, one almost too large to 

 accord with the supposition that it is of Tertiary age." 



My reasons for referring the strata of section No. 19 to the Tertiary 

 are, first, the finding of vertebrate organic remains identified by Profes- 

 sor Cope as of Eocene age ; and,' second, the iithological identity to the 

 beds farther north, in which Marsh, Cope, and Leidy have found so 

 many vertebrate remains identified by them as of Eocene age. 



As already noticed in the preceding chapter, the strata included in 

 the Upper Cretaceous weather very white, although they do not show 

 so prominently as the upper portion of those given in section No. 19. 

 It is impossible at present to define the exact limits of the basin in 

 which these layers were deposited. As mentioned in preceding parts of 

 this report, the Sawatch range was probably partially above water in 

 Cretaceous and Pre-Cretaceous times. It probably formed a part of 

 the eastern shore of the ancient lake, although all traces of the beds 

 along its western flanks have been entirely removed. To the northward 

 it was probably connected with the Green River Basin. Prof. T. B. 

 Comstock,* speaking of the Green River group, says, " There are indi- 

 cations that its eastern boundary was outside of the present limits of 

 the Green River Basin, aud there is no room for doubt that the Uintah 

 Mountains and the Wasatch chain, then as now, towered above its sur- 

 face. Northward it is equally clear that the Wind River range formed 

 the shore of the great lake, with probably more or less of gently sloping 

 border, during a portion of the era of Lower Eocene deposition." Be- 

 fore we can decide definitely as to the connection of the strata of the 

 Green River Basin with those noted between the Grand and Gunnison 

 Rivers, the country north of the Grand will have to be examined. 

 What the southern and southwestern limits of the lake were, it is im- 

 possible to say at present. Enough has been said to prove its vast 

 extent in that direction. The investigations of the next season's work 

 will probably throw considerable light upon the subject. 



Professor Marsh discovered evidence of a basin south of the Uintah 

 Mountains, about the mouth of the White River. This is probably the 

 direct extension of the basin I have described. Professor Marsh thinks 



* Report upon the reconnaissance of Northwestern Wyoming, p. 123. 



