66 SEDIMENTAEY GROUPS OF THE PLATEAU PROVINCE. 



ever beheld. The lower portions are of bright red, which color predom- 

 inates toward the west, where the bluffs descend to a lower elevation. I 

 found on them the remains of a turtle (Emys eutlmethus Cope), and. some 

 borings of a worm in a hard layer. On top of these are clay and slate- 

 rocks of a muddy-yellow color, with their various ledges rising to perhaps 

 five hundred feet" (vide United States Geological and Geographical Survey 

 of Colorado, page 437). 



These latter beds are Lower Green River limestones. 



In the same article, and immediately preceding this quotation, Pro- 

 fessor Cope says : " At a short distance to the southward another line of 

 white bluffs extends across the line of travel. This is not more elevated 

 than the preceding one ; I only found remains of tortoises in it," 



On either flank of the Aspen Mountain fold this group of beds weath- 

 ering white is seen, and I have several times at first confounded them with 

 the Lower Green River, but the shales of these beds are carbonaceous, and 

 often contain more or less lignitic coal ; those of the Lower Green River 

 are bituminous and yield oil. The limestones of the former are aggrega- 

 tions of shells or shell-marls. 



THE CRETACEOUS GROUPS. 



Planes of demarkation in the Cretaceous groups are not easily drawn. 

 The three great, massive sandstones of the Point of Rocks Group are in 

 many places broken into thinner beds, and then it becomes impossible, with 

 our present knowledge at least, to say to which of these members particular 

 beds may belong. The group below usually is very thinly bedded ; some- 

 times, however, these beds are thicker and more indurated, and when this 

 is the case and the Point of Rocks Group is broken up it is difficult to draw 

 a line between the two groups. The same difficulty arises in separating 

 the yellow arenaceous shales and the black argillaceous shales. Wherever 

 the two groups are exposed side by side, above are seen thinly bedded sand- 

 stones and shales and below are black, minutely laminated shales, but it is 

 very rare indeed that an exact line can be drawn. In the southern portion 

 of the province the Salt Wells beds are massive, and there the separation is 

 more easily made. The black shales of the Sulphur Creek Group are 



