Russell.] PRE-TERTIARY FORMATIONS. 43 
commanding- summits it is easily seen that the tilting- is due to move- 
ments of large blocks, bounded b} r faults. The larger features in 
the relief are produced by fault scarps, but great erosion has also 
occurred. The faults have a general southeast-northwest trend, but 
while nearly parallel in several instances they frequently branch and 
diverge at low angles from the main belt of displacement which 
determines the bold southwest border of the mountains, overlooking 
the Snake River Plains. The precipitous southwest escarpment of 
Mount Bennett is a fault scarp, from which a step-like recession of 
cliffs has occurred and on which there are prominent talus slopes. 
The leading features of the structure, as well as of the relief of 
the Mount Bennett rhyolite area, as is also the case on a yet larger 
scale in the Lost River country, are due to normal faults. The moun- 
tains are of the Great Basin type, but instead of trending northeast 
and southwest, as is the case in much of Utah and Nevada, they have 
a direction about at right angles to that course. 
Rocks similar to those forming Mount Bennett and the dark brown 
and purple mountains north of Mountain Home occur also in Snake 
River Canyon at Shoshone Falls. Beginning on the west, in the 
bottom of Snake River Canyon near the mouth of the lateral alcove 
leading to the Blue Lakes, this rock is exposed almost continuously 
on each side of the river up to Shoshone Falls and for a short way 
above, a total distance of about 7 miles. The surface of the forma- 
tion is irregular. In places horizontally bedded basalt rests directly 
on the terrane referred to, but usually there intervenes a sheet of 
coarse unconsolidated breccia, made up of angular fragments, fre- 
quently 6 to 8 feet in diameter, of the lower formation, loosely united 
by an earthy connecting material. The thickness of this irregular 
sheet of angular fragments varies, but in some instances is fully 70 
feet. The breccia is not stratified and does not present other evidence 
of the action of water, but is overlain in places by well-stratified lay- 
ers of sandy material from a few inches to 20 or 30 feet thick, which 
usually has the color and texture of a red brick, especially at its upper 
surface. The color is due to the heat of the overlying basalt. This 
sheet of moderately metamorphosed sedimentary material, possibly 
in part composed of volcanic dust and lapilli, is well exposed in the 
sides of the canyon below the falls, and can be easily traced for a dis- 
tance of at least a mile. Other similar layers, from a few inches to 5 
or feet thick in the central part, but thinning toward each end of the 
exposed portion, and having a length of a few hundred feet, are to be 
seen at several localities, interbedded with the basalt forming the walls 
of the main canyon and of the Blue Lakes alcove. 
The rock exposed in Snake River Canyon and forming the precipice 
over which the river plunges at Shoshone Falls, as well as the frag- 
mental material resting on it, exhibits both the massive, granular. 
