BT.JOHN.] VOLCANIC EOCKS JACKSON'S BASIN, &C. 505 



southern border of our district in the vicinity of Station XXX. North 

 of the Blackfoot, these basaltic flows are gently upraised in the long 

 grassy foreland flanking the Blackfoot Bange, forming low benches re- 

 markable for their uniformity of slope. The entire area of the plains- 

 portion of the Willow Creek-Blaekfoot Basin is floored with these de- 

 posits. To the south they do not appear ever to have reached very high 

 on the flanks of the Blackfoot Bange, where their elevation is near 7,000' 

 feet, or 1,400 feet higher than the bluff faces of the sloping benches at 

 the northern end of the range. ■ Hence, it would appear that the basalts 

 have been little if at all disturbed by elevatory forces situate in this 

 range, from which we may infer that in the latter mountain belt no 

 movement has taken place subsequent to the later outpouring of the 

 igneous matter which envelopes their base, and whose apparent inclina- 

 tion was merely the result of the partly fluid mass flowing over pre-ex- 

 isting gentle slopes toward the lower levels of the Snake Basin. But in 

 the basin region east of the Blackfoot Kange, the basalts rise up into a 

 prominent ridge between Willow and John Gray's Creeks, in the rugged 

 crest of which they occur a thousand feet above the adjacent plain. At 

 Station XIY, which occupies one of the culminating points of this ridge, 

 7,400 feet, the basalts gently rise up into the summit, the north and east 

 face of which has been broken down by denudation in precipitous walls. 

 The broad, rounded summit of the mountain is strewn with scoriaceous 

 lava, while in the immediate vicinity much of the compacter and slag- 

 like material has a very recent appearance, merging into the ordinary 

 dark steel-gray and brown basalt of the slopes and basin plain. Al- 

 though it might be difficult at the present day to define its limits, it 

 seems very probable that this locality marks the site of a crater-vent 

 from which issued the matter covering the declivities. This ridge is 

 based upon Mesozoic or Tertiary formations, whose folding undoubtedly 

 took place prior to the earliest of the volcanic flows, and which to the 

 north, in the vicinity of Stations XV and XVI, have been entirely de- 

 nuded of the igneous mantle, a remnant of which, howeA r er, forms the 

 cap of XV, dipping gently to the westward into the basin level. 



•As has previously been mentioned, in the northern extremity of this 

 ridge the sedimentary beds pass beneath a heavy sheet of trachyte, 

 which dips at an angle of 40° to the northeast. The question arises, 

 What are the relations of the various volcanic flows here met with 

 to the dynamical agencies concerned in the uplifting of the sediment- 

 ary deposits which constitute the nucleus of the ridge "? Subsequent 

 to the flow of the trachtye the ridge was extensively denuded, baring 

 the sedimentaries upon which the later flow of basaltic lava rests 

 in immediate contact. Hence the inclined position of the trachyte, if 

 not attributable to late-continued elevatory forces seated within the 

 ridge, might be explained by referring its source to the same volcanic 

 vent from which was poured the basaltic material ; but between the 

 two flows there must have elapsed an interval of time sufficient to allow 

 the almost total degradation of the earlier flow, of which the tilted 

 ledge in the northern slope of the ridge is evidently but a remnant of a 

 formerly extensive deposit, which may indeed have covered the whole 

 country roundabout with a sheet of trachyte continuous with remnants 

 elsewhere occurring. But this explanation may not satisfactorily accord 

 with all the facts, meagre as they are. In a former page reference was 

 made to the occurrence of an upthrust of hornblendic trachyte which 

 marks, if it did not determine, the axis of a lower parallel ridge to the 

 east between theforks of John Gray's Creek. This upthrust trends about 

 W. 40° X. and E. 40° S., and although the effects of displacement at- 



