428 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



limestone members, whicli apparently possess these distinctive features 

 over large areas in this region. 



The "red beds" here, also, present what may be regarded as their full 

 development, reaching a thickness of about 2,500 feet, beds No. 19 and 

 No. 26, inclusive, of the foregoing section. They are made up almost 

 entirely of red arenaceous shales and standstones, including just above 

 the middle a stratum of limestone. The Jurassic is introduced by the 

 limestone bed No. 27, in which were observed a few fossils, none of 

 which, however, were suliiciently perfect for identification, although they 

 certainly j)0ssess Jurassic /ac^e6'. But in the superimposed indurated 

 calcareous shales the abundant prevalence of a little Gryphwa, allied to 

 G. Galceolcij plainly identifies the horizon with the Jurassic as elsewhere 

 characterized in the northwest. The vertical limit of the Jurassic de- 

 posits at this locality was not definitely determined. In the neighbor- 

 hood of 1,200 feet above the base, a series of hard sandstone beds are 

 met with, capped by a heavy bed of conglomerate, attaining a thick- 

 ness of about 500 feet, beds No. 35 to No. 3*7 inclusive. These are suc- 

 ceeded above by a heavy deposit several hundred feet in thickness, 

 consisting of alternations of pale red and chocolate-colored shales and 

 drab limestones with drab shaly partings, which afforded no organic 

 evidence by which their age might be determined. The section ceases 

 Avith a heavy deposit of sandstone, No. 44, which, together with the 

 subjacent strata, is apparently conformable with the whole rock series 

 included in the section to the lowest Carboniferous limestones there 

 shown. If non-conformity exists, it is so shght as to readily fail of 

 recognition. 



Looking south and southwest from Station XL, quite the entire area 

 of the Low Pass basin is commanded, limited on the west by a rather 

 high and rugged mountain ridge, which shows in its east face escarp- 

 ments and amj)hitheatres recalling the magnificent scenery of the Teton 

 western foreland. The intervening country is filled with low hills, and 

 traversed by a net-work of narrow valleys and caQons, and clothed with 

 coniferous forests. Here and there dull buft-colored exposures were 

 observed iu the hiU-sides, but nothing affording familiar lithological 

 characters sufficient for the determination of the relative age of the for- 

 mations which they represent. But in the steep mountain wall which rises 

 into the ridge immediately defining the eastern border of the lower 

 valley of the Snake, a heavy series of drab deposits seem to indicate 

 the Carboniferous. It may be that the before-mentioned basin-area of 

 the Low Pass has been eroded out of the comparatively soft deposits 

 of the Laramie Group, which may either occupy a broad synclinal de- 

 pression or a series of narrow folds lying between the two main ridges 

 which constitute the principal topograi)liic feature of this mountain 

 gi'oup. 



The section B, of accompanying plate, gives a profile of these mount- 

 ains, along a northeasterly and southwesterly line, passing through Sta- 

 tion XL, which is based upon data secured by Mr. Bechler. The fore- 

 going section, embracing the belt actually examined at this locality, is 

 introduced to show its relation to the topography of the range along 

 this line. The profile crosses the barrier ridge bounding the lower 

 valley of the Snake a little to the north of the debouchure of the Low Pass 

 drainage, to the north of which the ridge rises 1,500 feet higher than at 

 the point where it is crossed by the profile. In the opposite direction 

 the eastern border of the mountains 'is gained in a distance of only two 

 or three miles beyond the northeast limit of the section, and from data 



