st.joh*.] VOLCANIC ROCKS — CARIBOU RANGE. 501 



locality these deposits are intimately associated with brown basaltic 

 lava, which latter also partakes in the disturbances that tilted the older 

 trachytic flow, and which appear to have been gently folded over a 

 broad, low ridge, whose axis lies in a northerly and southerly direction. 



In the eastern border region of the great plain of the Snake these tra- 

 chytic deposits occur under still more varied relations. Professor Brad- 

 ley describes the general structural features of the extreme northern 

 spur ridge of Mount Putnam, which terminates in the plain between 

 Eoss Fork and Lincoln Creek, as consisting of the light-colored Pliocene 

 sandstones and limestones, interlaminated with sheets of u trachytic 

 porphyries and coarse volcanic sandstones, all dipping about north 54° 

 east, at angles varying from 15° to 30°." These deposits Professor Brad- 

 ley found upraised in a rather sharp anticlinal fold, the southwestern 

 flank of which shows steeply inclined basalts, dipping at an angle of 

 72°, S. 34° AY. It would appear that the trachytic sheets here referred 

 to were deposited by outflows during the period of deposition of the 

 Pliocene beds, with which they have become regularly incorporated, 

 rather than intrusions of igneous matter between the soft strata. Hence 

 their relative age, and some notion of the volcanic phenomena accompany- 

 ing the laying down of the strata during this latest period of the Tertiary, 

 may be readily conceived. 



►Similar exhibitions of these early trachytic flows are also met with in 

 the lower portion of the lower valley of the Snake, although they are, in 

 the latter region, not so clearly associated with Pliocene beds as at the 

 locality above mentioned, while, at the same time, in their intimate rela- 

 tions with alternations of basaltic lava and heavy beds of modern con- 

 glomeritic accumulations, they offer new and still more varied phases, 

 which indicate the long continuance of these eruptions and the variety 

 in their materials, approaching the later stages probably immediately 

 preceding the grand eruptions which overflowed with molten lava and 

 desolated a vast extent of the interior basin of the Pacific slope within 

 a period so comparatively recent that it may truly be termed modern. 

 These deposits have also been described as reclining on the flanks of the 

 mountain borders of the valley, dipping at a moderate angle in the 

 direction of the lower depression, but they appear not to have reached so 

 high on the mountain summits as the before-mentioned and doubtless 

 older deposits. About midway of the lower valley of the Snake, its whole 

 breadth is choked by huge masses of trachyte and laminated porphyritic 

 trachytes, associated with reddish-brown minutely vesicular basalt,- 

 which here seem to form the basis of the deposit, as though it had poured 

 out of a vast fissure in the immediate neighborhood, inundating the 

 valley. But above this point the valley has been so thoroughly swept 

 out as to make it a matter of some doubt whether the volcanics ever ex- 

 tended far above PjTamid Creek, not a vestige of them having been 

 noticed on either side of the valley. 



In the region of the T6ton Bange, especially in the borders of Pierre's 

 Basin, the earlier volcanics exist on a far grander scale. They also pre- 

 sent a marked physical change as compared with the flows holding 

 similar position in the region above briefly noticed, reaching up on the 

 western flank of the Teton Bange, to the north, to an elevation of above 

 8,000 feet, but declining to the south, where, however, they have suf- 

 fered extensive erosion and have been entirely removed from the op- 

 posed flank of the Snake Biver Bange, until reaching the northeastern 

 extremity, where they occur in gently uplifted benches, precisely like 

 the great volcanic foreland on the west flank of the Teton Bange. These 

 flows once filled the entire area of Pierre's Basin, indeed they probably or 



