BT.JOHS.] pieree's basin. 441 



indicating that the mountain courses of the streams lie entirely in the 

 sedimentary formations, which is further proved to be the fact by actual 

 observation. In the northern part of the basin, Leigh's Creek, the 

 sources of which originate in the great Archaean expansion in the north- 

 ern half of the Teton Eange, and flow out through the volcanic fore- 

 land which here flanks the metamorphic nucleus, shows in its bed quan- 

 tities of gneissic, schistose, and volcanic bowlders, and which compose 

 the low bars and bordering terraces. 



In defining the limits of the basin area, mention was made of the 

 low upland barrier across its northern end. It remains to notice the 

 geological character of this barrier, which presents the appearance of a 

 low, gently undulating prairie upland, on the one hand gently descend- 

 ing from the well-defined foot of the Teton Eange, and on the other 

 from the northern terminus of Pierre's Mountains, uniting below Leigh's 

 Creek, where it is broken through at its lowest part by Pierre's Eiver, 

 which here begins its canon in the volcanic rocks. This upland really 

 belongs to an extensive tract, extending northward to Henry's Pork 

 of Snake Eiver, and in which a large part of the courses of Pierre's 

 Eiver and the North Fork and several smaller tributaries lie in nar- 

 row canons, 100 to 500 feet in depth. The general slope is quite gradual, 

 northwestward; on the divide just south of Canon Creek, where it 

 probably attains its maximum height, about 6,600 feet, to the bluffs on 

 Pall Eiver, a few miles above its confluence with Henry's Fork, the de- 

 scent being in the neighborhood of 1,400 feet. 



The entire tract is based upon the older volcanic flows, which show in 

 the deeper canons several hundred feet thickness of laminated porphy- 

 ritic trachytes, overlaid by drab and pink trachytes or trachytic tuffs, 

 upon which rest in turn remnants of firmer dark -brown, somewhat vesic- 

 ular lava, with probably other varieties of trachyte and trachytic lavas, 

 extending to the lower level of the Snake Basin, where they are replaced 

 by the ordinary basaltic flows which everywhere floor the great plain. 

 To the first mentioned varieties belongs the volcanic fringe which rises 

 up on the western foot of the Teton Eange throughout its length be- 

 tween North Pork and West Pass Creek, and which reappears along the 

 foot of Pierre's Mountains, at a point about due west of the mouth of 

 "West Teton Eiver, to the south of which they have been denuded. From 

 either side of the basin these deposits converge in gradually diminishing 

 benches toward the head of Pierre's Canon, presenting an abrupt breast 

 on the basin side and gently inclining with the inclination of the flows 

 in the opposite direction, or northward. There is apparently a series of 

 two or more of these benches, which formed as many barrier levels across 

 the northern end of Pierre's Basin, the gradual drainage of which ex- 

 plains the shelving contour of the older terrace formations with which 

 it is filled, and which probably date subsequent to the volcanic eruptions. 

 The surface of this upland barrier is generally composed of a finely com- 

 minuted light-brown soil, covered with a luxuriant growth of grasses 

 and herbaceous plants. Wherever the wash has bared the steeper slopes, 

 there appear exposures of water- worn drift materials. These loose ma- 

 terials were doubtless spread over the surface of the subjacent volcanic 

 deposits prior to their erosion and removal from the basin area. So it 

 would seem that, within comparatively modern date, the whole Snake 

 Eiver region was submerged, while the sluice-like accumulations filling 

 the debouchures of the basin streams, and which form to-day a by no 

 means inconspicuous feature in its surface relief, are absolutely modern, 

 belonging to a past not very remote even to human comprehension. 



