et. jobs.] VOLCANIC EOCKS — CARIBOU RANGE. 499 



burst out in local upheavals or the violent fracturing of the sedimentary 

 deposits, which denudation has uncovered within a comparatively recent 

 date. South of Mount Bainbridge the intrusive matter was not de- 

 tected, while in Station XYII ridge it appears to have resulted in a local 

 uplift accompanied by fracturing of the sedimentaries, which latter 

 appear to fold round the northwest and southeast extremities of the 

 uplift without break in their continuity. 



The early flowed trachytic lavas. — We next come to a series of flowed 

 volcanics which exhibit in their constitution, perhaps, the most variable 

 features of any of the divisions belonging to this class of rocks. From 

 their position, often high up on the mountain-sides, and even occupying 

 their crests in places, dipping almost uniformly in the direction of pre- 

 existing depressions of erosion, they are presumably amongst the ear- 

 liest products of volcanic effusion which were poured out and overflowed 

 a vast extent of country about the sources of the Columbia. 



These deposits consist chiefly of trachytic materials, but which exhibit 

 in their local occurrence great variety in physical constitution, as will 

 have appeared from the notices of these rocks in the foregoing chapters 

 on regional geology. However, it may be deemed permissible briefly to 

 review the salient local features which these deposits assume in different 

 portions of the district, in order that they may be the more readily com- 

 pared with the observations of my colleagues in adjacent regions. 



The farthest west exposures of these rocks observed by myself were on 

 the western flank of the Blackfoot Bange. Here they are found to occn py 

 a sort of basin area intervening between the foot of the range and the 

 high basaltic-caj)ped barrier on the west through which Blackfoot Biver 

 has excavated a canon on its way out into the Snake plains. The rock 

 consists of flag-like, brown, drab, and pale brick-red trachyte ? portions of 

 which have a mottled appearance from the presence of small light specks, 

 and sometimes small amygdaloid cavities, and crowns three or four 

 conical, rounded hills which descend to the westward. The rock usually 

 weathers a pinkish color, and at one point the ledges seemed to have a 

 very gentle inclination to the northeastward, or in the direction of the 

 Blackfoot Bange, from which they are separated by a narrow belt of 

 rolling, grassy hills. In a high rounded summit, 7,200 feet above sea- 

 level, about four miles little south of east of the above locality, and occu- 

 pying a recess in the range opening to the southwest, occurs a heavy cap- 

 ping of brownish-red and drab trachyte, in heavy and thin slabs or layers, 

 dipping gently northward. The heavy-bedded layers are rounded by 

 atmospheric action, even weathering by exfoliation. Traces of similar 

 deposits were observed to the southeast, but of scarcely more impor- 

 tance than to indicate the excessive denudation which these early vol- 

 canic flows have undergone in this region. In the northern portion of 

 the range, a highish ridge connecting the east and west spur-branches 

 of the range shows heavy ledges of a softish dark-brown and drab mot- 

 tled, vesicular trachytic lava at an elevation of about 6,200 feet, which, 

 though apparently in situ, is too obscurely exposed to show more than 

 its general appearance. It occurs in large weather-abraded masses in a 

 bench facing Blackfoot Peak, which latter lies about one and a half 

 miles to the eastward. Similar and probably identical lava-flows were 

 elsewhere encountered, sometimes associated in the vicinity of the ordi- 

 nary trachytes, though at no place were the relations of these rocks 

 clearly shown. 



At the northern extremity of the Willow Creek Basin hills, a couple 

 of miles north of Station XVI, a heavy bed of gray or bluish-drab, com- 

 pact trachytic rock unconformably overlies Laramie Group deposits, in 



