white.1 TRAP OUTFLOW IN RALSTON CREEK VALLEY. 195 



As is shown in the numerous sections which accompany Dr. Hayden's 

 reports upon this region, all the Mesozoic strata are upturned against 

 the flanks of the Eocky Mountains ; the whole series, including the 

 Laramie Group, thence extending out almost horizontally upon the 

 plains. 



The Laramie strata were originally flexed with the others, but being 

 softer they have mostly, but not in all cases, been removed by denudation 

 from the immediate line of flexure, so that we oltener than otherwise 

 see these strata in a nearly level position, while the others close by are 

 flexed. But they are plainly seen to rest upon the strata of the Fox 

 Hills Group, and no distinct stratigrapkical plane of demarkation has 

 yet been detected between them. 



The outflow of the trap which now caps the two Table Mountains in 

 the immediate vicinity of Golden City, the northern side of the northern 

 one of which is near to the locality under discussion, took place at a 

 comparatively late date, and its outburst must have necessarily ruptured 

 and, at least locally, disturbed the strata at and around the place of 

 exit. The little group of hills immediately upon the north side of Balston 

 Creek, and at the southern base of one of which is the coal-shaft with its 

 fossiliferous layer of sandy shale, here considered, are composed entirely 

 of trap similar to that which caps the adjacent Table Mountain. 



A carefid examination of these hills convinces me that they occupy 

 the site of the rent through which a large part if not all the melted trap 

 came, which doubtless once covered a large portion of this district ad- 

 jacent to the foot-hills, but of which only comparatively small portions 

 have escaped destructive erosion. These small portions now cap the 

 two Table Mountains and form the small group of hills here referred to. 

 This outburst took place directly in line of the upturned edges of the 

 Cretaceous strata against the flanks of the Eocky Mountains, the posi- 

 tion and extent of which have already been explained. This dynamic 

 movement, however, did not materially affect the strata of either the 

 Eed Beds or the Dakota Group, as is shown by the present direct con- 

 tinuity of the crests of their hogbacks. The strata of the Colorado 

 Group are too soft to produce a hogback, and they are so covered 

 by debris that we cannot know to what extent they may have been dis- 

 turbed by the outburst. The hogback of the Fox Hills Group has been 

 disturbed and interrupted. A portion of it standing conspicuously in 

 the valley at a point not far to the southwestward from the coal-shaft in 

 question has its strata nearly vertical, instead of standing at the much 

 less angle of elevation shown by the other hogbacks, when, if undis- 

 turbed, since the mountain uplift tbey should be the steeper, because 

 they are nearer to the foot-hills. The trend of this short hogback of 

 Fox Hills strata is also so deflected that if a line representing its axis 

 were continued in the curve it indicates in a northward and northeast- 

 ward direction it would lie approximately tangent to the southeastern 

 base of the group of trap hills here referred to, near the point where the 

 coal- shaft has been sunk. 



I otter the following explanation of these phenomena : The trap rent 

 was a very large one, not in the form of a dike, but circumscribed, irreg- 

 ular, and several hundred yards across. It came in the line of flexure 

 of the Cretaceous strata where they are upturned against the flank of 

 the mountains, probably breaking through those of the Colorado Group, 

 but evidently lifting, displacing, and overturning at least a portion of 

 the Fox Hills strata, together with a portion of the then and there super- 

 imposed Laramie strata. This is believed to be the exact condition of 

 those adjacent to the trap hills, through which the coal-shaft has been 



