212 REPORT UNITED STATES GEOLOGICAL SURVEY. 
doubtless identical with the north fold into which the strata over the 
area of this mountain range were uplifted. Along the broken crest of 
this ridge, the reddish drab-gray Carboniferous and light buff-gray Ni- 
agara (?) limestones hold a prominent place in the rugged mountain 
masses and isolated peaks. But in the southeasterly continuation of 
the ridge the sedimentaries are, in places, removed, revealing a narrow 
belt of the Archeean nucleal rocks, so indicated by the peculiar mode of 
weathering exhibited by the rocks in the south face of the ridge and 
the presence of quantities of gneissic débris in the outlying slopes and 
stream-beds that emerge opposite this spot. This supposed Archean 
ridge, in places bears along the crest dirty yellowish deposits, recalling 
the lithologic appearance of exposures of the Primordial horizon to the 
west. But the outlying flanks on the south are plated by Carboniferous 
and Silurian strata, which rise up steeply on the lower half or more of 
the height of the mountain ridge. 
The above mentioned ridge terminates in a prominent mountain be- 
tween the forks of the stream whose eastern and lesser branch drains a 
part of the northeast flank of the north fold, and which lies to the west- 
northwest of Gros Ventre Peak between 8 and 9 miles distant. On the 
south the slope falls in an even and slightly bulging curve, correspond- 
ing to the planes of the flexed strata into the border of the basin, while 
on the west, north, and east the mountain breaks down in precipices, 
showing hundreds of feet thickness of the component strata. The ex- 
posures are mainly, if not entirely, of Carboniferous rocks, the upper part 
showing characteristic outcrops of harder and softer limestone beds 
alternating in mural exposures and steep, bare slopes that reach up to 
the summit. The basis rocks may belong to Silurian horizons.’ The 
denuded Archean ridge lies less than a mile to the north, and although 
considerably lower than the summit of the terminal mountain, its crest 
rises above the actual altitude of the lowermost sedimentary exposures 
in the precipitous north wall of the mountain. The presence of this 
mountain mass of sedimentary rocks brings out in the most vivid man- 
ner all these relationships and the enormous extent of erosive action ~ 
necessary to uncover the metamorphic core of the north fold. The ver- 
tical displacement in the fold at this point is probably not less than 
3,000 feet. Its abrupt southern flank may be traced nearly its whole 
extent from the heights in the western part of the range, sometimes ex- 
hibiting the flexed strata, in situ, on the steep mountain sides, and then, 
again, eroded so as to appear in monoclinal ridges capped here and there 
by red beds possibly of Triassic age, whose gentle northerly declination 
forms the broad plateau reaching over to the culminating mountain 
crest overlooking the Gros Ventre Basin. This part of the range is 
deeply eroded by a considerable Gros Ventre tributary, in whose bed 
the Archzean may be revealed similar to the occurrence in the mount- 
ain basin of the sources of the cascade tributary flowing into the Hoback. 
A couple of miles southeast of the last-mentioned caton the sources 
of a small, independent Hoback tributary have excavated a deep amphi- 
theater extending back into the mountains % or 3 miles from the south 
border. Where it emerges, its course is interrupted by a beautiful cas- 
cade, the stream tumbling or sliding many feet down the steep incline 
over Carboniferous ledges, which throw a heavy belt across the mouth 
of the recess and rise high up in the mountain elevations on either side. 
On the east of this amphitheater the Archean is denuded, forming 
another spur which bears atop the whole of the Primordial quartzite 
capped by remnants of Quebec limestone, which latter appears in undu- 
lating low mural masses, weathered dark brown'sh gray. The south 
