kt.johs.] TETON PASS MOUNTAINS. 433 



8.530 feet. The immediately neighboring ridges attain altitudes less 

 than a thousand feet higher, sinking away in opposite directions to the 

 northwest and southeast, where they are crowned with innumerable 

 peaks 7,000 to 9,000 feet elevation above the sea. This region is very 

 generally clothed with coniferous forests ; the northern slopes especially 

 are often densely wooded with beautiful growths of slender-tipped spruce 

 and pine, the presence of which offered serious obstacles. in the way of 

 tracing out the geological structure in distant parts of the range beyond 

 our reach. 



The Teton Pass hills, as seen from the plain in passing along the 

 southwest border of Pierre's Basin, present a succession of ridges grad- 

 ually diminishing in height as they approach the Low Pass sag, their 

 northerly face falling abruptly over the edges of the upraised strata, 

 which in like manner determine the gentler slope of their opposite de- 

 clivities. In connection with the Pierre Mountains, mention was made 

 of the occurrence of a heavy ledge of conglomerate overlying the drab 

 shaly horizon of the Jurassic, the relative position of which is further 

 shown in the section B, bed Xo. 37. The strike of the strata maintains 

 this ledge in the crest of the southeast continuation of Station XL ridge, 

 where it is lost in the debouchure of Low Pass Creek ; but to the south- 

 east it reappears in one of the low ridges above referred to, where it is 

 observed dipping south-westward, overlaid by drab limestone and hard 

 sandstone, offering the same succession of deposits which compose the 

 uppermost series in the section through Station XL in the Pierre's 

 Mountains. These deposits rise in the crests of the ridges to the south- 

 east, in which direction the strike of the strata carry them farther and 

 farther into the mountains, so that, as we approach Teton Pass, the foot- 

 hill ridges reveal successively older deposits, until, at the head of the 

 basin, the upper measures of the Carboniferous appear in the foot of the 

 spur which afforded access to Station XXXIX — a low mountain sum- 

 mit commanding a comprehensive view of the beautiful plain of Pierre's 

 Basin, bordered on the one hand by the great foreland slopes of the 

 Teton Bange, and on the other by the comparatively low but more broken 

 foot-hills of the Pierre's and Teton Pass Mountains. 



The vicinity of Station XXXIX afforded instructive geological study, 

 showing the intimate relations in the stratigraphy and dynamics which 

 exist in common between this mountain group and that briefly noticed 

 in the preceding pages. The summit of the mountain is capped by a heavy 

 ledge of rather hard, even-bedded, pale brick-red sandstone, dipping. 

 46°, W. 10° S. The rock is complicated by joint structure which might 

 easily be mistaken for the bedding but for the lamination, obscure traces 

 of which .the rock sometimes retains. The deceptive joint or cleavage 

 planes incline north at an angle of 40°. The only vestiges of organic 

 remains contained in the rock are small algse-like markings preserved in 

 a variety of curious shapes weathered in relief on the exposed surfaces. 

 of the slabs. The rock is also perforated by slender tubes and cavi- 

 ties which may have connection with original organic nuclei. To the' 

 southeastward, just beyond an intervening, deep, densely wooded gulch, 

 the same ledge is traced in the crest of a corresponding ridge, the south 

 slope of which is covered with the rusty debris. On the south slope of 

 Station XXXIX a considerable deposit of dark drab, spar-seamed, frag- 

 mentary limestone, with associated shaly beds, occurs, which doubtless 

 overlies the above-mentioned sandstone ; it contains a few poorly pre- 

 served fossils, small crinoidal discs, the flat valve of an Avicirfopecten ? 

 a small lamellibranch resembling Myacites, and a little gibbous gastero- 

 pod. Although none of these forms are positively identifiable with 

 28 as 



