448 REPOET UNITED STATES GEOLOGICAL SURVEY. 



face a heavy ledge of dark-brown rock shows a conspicuous band, dip- 

 ping westward at a moderate angle, apparently inclosed in brown earthy 

 deposits. It may be that the indurated dark ledge is an intrusive sheet 

 of volcanic material 5 but the brown inclosing deposits recall similar-look- 

 ing beds in the south spur of the Upper Gros Ventre Butte, while the 

 latter may possibly prove to be identical with extensively exposed brown- 

 ish deposits in the neighboring hills to the northeast of the Snake, and 

 which also occur in the Mount Leidy Group to the southeast. 



A few hundred yards west of Station XLVII, which occupies a low drift- 

 covered ridge on the south side of Elkhorn Valley, a low knoll near the 

 end of the ridge, 300 or 400 feet high, is crowned with an isolated mass of 

 dark-green volcanic rock, weathering reddish like trachyte, though it is 

 porphyritic in character. It shows m ore or less distinct bedded structure, 

 and dips to the northwestward at angles of 15° to 20°. It seems very- 

 probable that this ledge may be identical in character with the before- 

 mentioned dark-brown ledge in the butte south of the outlet of Jackson's 

 Lake, with which it also agrees in the direction and angle of inclina;tion; 

 but at the present locality the character of the inclosing deposits was 

 not ascertained. Its position, however, indicates that it either belongs 

 to a flow overspreading or alternating with sedimentary materials, or 

 a sheet of volcanic matter intruded between the layers of sedimentary 

 deiDOsits. 



THE GROS VENTRE RANGE. 



The belt of highlands known under the several appellations Gros Ven- 

 trej Green Eiver, and Wyoming Mountains forms a well-defined moun- 

 tain range extending westward from a point about opposite Union or 

 Warm Water Pass, in the Wind Eiver Mountains, 40 to 50 miles, when it 

 terminates in an exceedingly rugged mountainous region overlooking 

 Jackson's Hole, above the Grand CaQon of the Snake. Its width here, 

 including the area between the Gros Ventre on the north and Hoback's 

 Eiver on the south, is in the neighborhood of 20 to 25 miles ; but to the 

 east it apparently narrows, and at the same time appears to be less scored 

 with profound gorges such as have been carved out of the western por- 

 tion. This western front presents a variety of topographic and geologi- 

 cal features, such as find ready explanation in the nature of the compo- 

 nent rocks ; almost ideally perfect Archsean peaks and rounded abrupt- 

 terminating spurs, contrasting with the peculiar sweeping foreland and 

 block-lDie craggy- weathered summits, of scarcely inferior altitude, fash- 

 ioned out of uplifted sedimentary formations. This broad western end 

 of the range is deeply penetrated by the Little Gros Ventre, which, with 

 the attendant erosion of numerous lateral ravines, has unlocked a vast 

 extent of the interior x3ortion of the range, revealing for satisfactory 

 study its general structural features. As our own examinations were 

 confined to this part of the range, we may be permitted to repeat with 

 some minuteness the observations there made. Of the southern flank, 

 or that belonging to the drainage of Hoback's Eiver, we can only speak 

 in general terms ; but of the region to the east, on the headwaters of 

 Green Eiver, and also on the ojjposite slopes descending to Gros Ventre 

 Eiver, we have in the report of Dr. Hayden of Capt. W. F. Eeynolds's 

 exiDedition (1860) many and most important details respecting the char- 

 acter and distribution of the geological formations in that quarter. 



Approaching the range south of the debouchure of the Little Gros 

 Ventre, the long foreland slope is flagged with Quebec Group limestones, 

 with here and there remnants of heavy-bedded magnesian limestone, 

 the latter appearing in greater force in the foot-hill benches to the north, 



