44 SNAKE KIVER PLAINS OF IDAHO. [bull. 199. 
purplish variety and the glossy, black variety, each porphyrinic, 
observed in the Mount Bennett rhyolite. Both varieties also show 
well-defined flow structure. My observation shows that the Mount 
Bennett rhyolite and the rock forming- the lower portions of the cliffs 
at Shoshone Falls are essentially identical, but no critical examination 
of the rocks of either terrane has been made. In the reports of the 
Fortieth Parallel Survey a the rock beneath the basalt at Shoshone 
Falls is termed trachyte, and no mention is made of the conspicuous 
layer of coarse breccia between the older massive igneous rock at the 
base and the basalt which forms the upper portion of the cliffs. 
In the hills drained in part by Rock Creek, and situated between 
12 and 20 miles south of Shoshone Falls, rocks of the same general 
character a^ those forming the mountains to the northeast of Moun- 
tain Home, and similar also to the lower terrane at Shoshone Falls, 
again appear at the surface and form an extensive range of hills or 
low mountains. These hills, judging from their form, color, etc., 
when seen from a distance, are formed of the same kind of rock, prob- 
ably throughout an area of between 150 or 200 square miles, but 
were not examined by me except for a distance of about 8 miles along 
Rock Creek to the south of Rock Creek post-office. In the Rock Creek 
Hills, as on the mountains to the northeast of Mountain Home, the 
rhyolite is in well-defined layers, and exhibits diverse dips ranging 
from 10° to 20° in adjacent ridges. 
On Rock Creek two flowing wells have been obtained, and will be 
described more fully in the second part of this report, and springs are 
numerous. These wells and springs indicate that the rocks are com- 
posed of alternating layers of pervious and impervious material, and 
that when penetrated by the drill or broken by faults, water may rise 
to the surface. There are reasons, however, for the suggestion that 
the sheet of rhyolite forming the surface portion of the Rock Creek 
Hills is comparatively thin and rests on Tertiary sedimentary beds. 
Should this surmise be sustained b} 7 future studies, the hope of obtain- 
ing flowing water over a wide extent of country to the south of Sho- 
shone Falls will be greatly enhanced. Springs are also numerous in 
the mountains composed of rhyolite to the northeast of Mountain 
Home, and the success attending the drilling of wells in the similar 
formation on Rock Creek suggests that favorable artesian conditions 
exist there also. Success in drilling artesian wells in a region of vol- 
canic rocks like that drained by Rock Creek is unusual and inspires 
a hope that much of the country underlain by stratified rhyolite will 
be found to be water bearing. 
Two of the three prominent buttes described on a previous page, 
situated in the central part of the Snake River Plains about 25 miles 
« Vol. I, pp. 592-593. 
