6T.J0IIN.] TETON PASS MOUNTAINS. 433 



8,530 feet. The iimncdiiitely neigliboring ridges attain altitudes less 

 than a thousand feet higher, sinkinj^- away in opposite directions to the 

 northwest and southeast, where thej^ are crowned with inimmerable 

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

 generally clothed ^\^th coniferous forests ; the northern slopes especially 

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

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

 tracing out the geological structure in distant j)arts of the range beyond 

 our leach. 



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

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

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

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

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

 cli\'ities. In connection with the Pierre JMountains, mention was made 

 of the occiuTence of a heavj^ ledge of conglomerate overijing the drab 

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

 shown in the section B, bed IN'o. 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, w^here it is 

 observed dipping southwestward, 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 i)lain of Pierre's 

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

 Teton Eange, and on the other by the comi)aratively low but more broken 

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



The vicinity of Station XXXIX afforded instructive geological study, 

 sho\ving the intimate relations in the stratigraphy and dynamics which 

 exist in common between this mountain group and that brietly noticed 

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

 ledge of rather hard, even-bedded, i)ale brick-red sandstone, dipping 

 40"^, W. 10° S. The rock is coraiilicated by joint structure which might 

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

 of Avhich 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 algre-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 dobris. 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 fevr poorly pre- 

 served fossils, small crinoidal discs, the flat valve of an AvicidoxKcten f 

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

 pod. Although none of these forms arc positively identifiable with 

 28 GS 



