Russell] PRE-TERTIARY FORMATIONS. 47 
above, is northwest and southeast. On the west side of the Snake 
River Plains, in the Lost River country, the ranges, judging from dis- 
tant views, are prevailingly raonoclinal; that is, they are formed by 
the upturned edges of large blocks of the earth's crust, in which the 
bedding planes dip in one direction. Under the generally accepted 
explanation of the origin of ridges of the nature of those just cited, 
they are considered as having been produced by breaks along which 
differential movements on the two sides have occurred. The rocks on 
one side of a break have been elevated or those on the opposite side 
depressed, producing what is termed a fault. The steeper sides of 
the ranges are fault scarps more or less eroded and perhaps largely 
concealed beneath rock waste in the form of talus slopes and alluvial 
fans. The monoclinal ridges forming the Lost River Mountains are 
seemingly of the Great Basin type, but, unlike the representatives of 
that t} 7 pe in Utah, Nevada, southeastern California, etc., trend north- 
west and southeast instead of at right angles to that course, as is com- 
mon throughout much of the Great Basin." 
The geological structure of the various ranges and ridges composed 
principally of quartzite situated southwest of the Lost River Moun- 
tains is little known, except that the presence of faults is indicated by 
the diverse inclination of the strata in neighboring ridges and that 
much movement along breaks trending in general northwest and 
southeast has taken place. Farther west, from near Shoshone to 
beyond Boise, as previously stated, extensive faulting has occurred, 
as may be readiy seen in the portion of that region which is com- 
posed of stratified rhyolite. Near Boise, where the nearly level 
Snake River Plains meet the mountains on their northern border, the 
bold mountain front, composed principally of granite, has been ascribed 
by Lindgren^ to faulting. My studies in the same region and in 
the area east of it to King Hill Creek and near Shoshone, serve to con- 
firm this view. The leading facts tending to support this conclusion 
are: First, the alignment of the steep precipices which descend from 
the mountains to meet the plain, and the fact that both the granite and 
the rhyolite of which the mountains arc 1 principally composed termi- 
nate abruptly at the escarpment referred to; second, from this line of 
escarpments branching faults extend into the mountains formed of 
rhyolite; these branching faults form in fact the main escarpment for 
a distance sometimes of several miles before departing from it suffi- 
ciently to become individualized; third, the presence of recent faults 
at and near the base of the mountain in the vicinity of Mountain 
Home, and in the basalt co vering the adjacent depression; fourth, the 
presence of hot springs along the base of the straight mountain face 
where it meets the plain and where evidence of recent movement 
« A comprehensive account of the leading structural features of the Great Basin may be found in 
G. K. Gilbert's Lake Bonneville: Mon. U. S. Geol, Survey, Vol. I, 1890, pp. 340-392. 
Kieol, Atlas U. S., Boise folio, 1898. 
