84 



GEOLOGICAL SURVEY OF THE TERRITOEIES. 



There is oue feature in the geological structure of the mountains 



of Montana, observed 

 by the survey of the 

 past seasons for the first 

 time and not noticed in 

 such a marked degree 

 in any other portion of 

 the West, and that is 

 the inversion of the sed- 

 imentary beds, so that 

 the oldest incline at a 

 greater or less angle on 

 those of more modern 

 ages. This phenomenon 

 occurs at a number of 

 places, but the most 

 marked are in the mid- 

 dle canon of the Madi- 

 son, at the head of 

 Jackass Creek, and in 

 the East Gallatin range. 

 Several illustrations 

 are given in this report 

 which will render this 

 matter clear. The fol- 

 lowing illustration rep- 

 resents a section from 

 east to west through 

 the East Gallatin range. 

 (Fig. 24.) The Silurian 

 strata incline past a 

 vertical 30O to 50°. The 

 central portions of the 

 range are Carbonifer- 

 ous limestones and are 

 nearly or quite vertical, 

 while eastward the Ju- 

 rassic, Cretaceous, and 

 ligiiitic strata incline 

 at various angles and 

 pass down in step-like 

 ridges to the valley of 

 Shields's Eiver, as they 

 were gradually elevated 

 by the uplift of the 

 range. We will not 

 enter into a discussion 

 here of the origin of 

 the forces that brought 

 about these results, but 

 it would appear that 

 there were two in ope- 

 ration, one which raised 

 the mass vertically, and 

 a side or tangential 

 force which crowded the Silurian beds over past a vertical. In some 



