436 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



details are effectually obliterated. Hence, it is with, a degree of hesi- 

 tancy that the belt of buff-colored deposits is identified with the Lara- 

 mie formation, and the more distant red-colored strata with the Triassic 

 " red beds," or the variegated red shales overlying the former deposits. 

 But the drab beds in Promontory Peak and corresponding heights along 

 the lower valley of Snake Elver may with greater confidence be referred 

 to the Carboniferous, the latter deposits having been observed in that 

 quarter by Professor Bradley in 1872. 



A brief review of the facts and their bearings elicited by the exam- 

 inations in the northern portion of the range, will, it is hoped, convey 

 something like an adequate knowledge of the identity of the geologic 

 history of at least the northeastern if not the whole of this mountain 

 region. 



In the extreme northeast, a limited area of Jurassic and, perhaps, 

 Laramie deposits occupies the northeast flank of the Pierre's Mountains, 

 where the strata have been involved in extraordinary disturbances, which 

 have indeed tilted the Jurassic beds into nearly vertical position in the 

 immediate vicinity of the present eastern border of the range, and it 

 seems probable that a complete overturn of the beds at this point was 

 effected. The narrow belt of extreme disturbance soon passes beyond 

 the limits of the range to the southeastward, where, in Spring Point, the 

 Carboniferous deposits jut out into the edge of the basin. The western 

 border of the disturbed Mesozoic area, as we have already seen, is defined 

 by the high barrier ridge of Carboniferous strata, which may in places 

 retain the condition of a fold with steep easterly pitch, but which at 

 other points seem to have been faulted with downthrow in the same 

 direction. 



At Station XLII the erosive agencies have reduced this ridge to the 

 condition of a monoclinal, but in the immediate vicinity evidence exists 

 which strongly goes to show that the uplift began either as an anticlinal 

 fold, or in the severance of the strata, the edges of the upraised portion 

 dragged, producing the appearances above referred to in comparing 

 the ridge to a faulted anticlinal fold. The downthrow of the fault has 

 brought the Jurassic or Post-Jurassic beds down to the level of the 

 Carboniferous limestones, against which they apparently impinge, dip- 

 ping in the same direction as the monoclinal portion of the latter deposits, 

 though at a steeper angle, southwesterly. The appearances here alluded 

 to are indicated in the section through Station XLII. With the time at 

 our disposal it was difficult to work out the details of the disturbance 

 the Mesozoic deposits have undergone in this northeastern area. The 

 two dominant ridges, as determined by the fossil contents of their 

 respective limestones, are apparently made up of stratigraphically widely 

 separated deposits. The considerable space intervening and represent- 

 ing a vertical extent of probably near 8,000 feet, is filled with arenaceous 

 deposits which certainly bear strong resemblance to Post-Jurassic or 

 Laramie deposits, and so also in regard to the soft sandstones in the out- 

 lying flank, where they appear to form a low fold, on the one hand in- 

 clining northeastward and on the other dipping in the direction of the 

 sharp outer ridge, whose crest is composed of steeply tilted Jurassic beds. 

 From such evidence it seems perfectly natural to attribute the appear- 

 ances here met with to violent disturbances which resulted in one or 

 more sharp folds, and perhaps accompanied by faulting, of which the 

 area of their exhibition as seen to-day is but a remnant of a wide and 

 much involved belt in the region now occupied by the plain of Pierre's 

 Basin. 



The faulted or monoclinal ridge of Station XLII pursues a course 



