48 SNAKE EIVER PLAINS OF IDAHO. [bull. 199. 
occurs. All of these several lines of evidence when studied in the 
field lead to the same conclusions, and, I think, prove that the striking 
change in topography at junction of the Snake River Basin with the 
mountain to the northeast, in the vicinity of Shoshone, Mountain 
Home, Boise, etc. , is due primarily to faulting. This fault, however, 
is not a single, definite fracture, but a series of irregular and branch- 
ing fractures, the rocks on the northern side of which have been 
upraised or those on the southern side depressed. In the mountains 
composed of rhyolite, north of Mountain Home, the several faults are 
readily seen from many commanding summits. The trend of these 
breaks, which define the borders of prominent monoclinal ridges, 
is in general from N. 20° to N. 40° W. The strata composing the 
blocks between the faults are in most instances inclined downward 
to the northeast at angles of from 15° to 20°. Certain of these fault 
scarps, when traced southward, merge into and form a part of the 
general line of bluffs bordering the Snake River Plains. The con- 
spicuous mass of stratified rh} T olite forming Mount Bennett, is, so far 
as can be judged from its leading topographic features, a representa- 
tive example of a tilted fault block of the nature just referred to, but 
the secondary elements in the structure of the mountains are unknown. 
This block presents its bolder escarpment to the southwest, and the 
beds of which it is composed dip toward the northeast. The extremely 
bold southwest face of the mountain is a part of the general line of 
escarpments which descends to and defines the northern border of the 
Snake River Plains in Elmore and Ada counties. 
Practically nothing is known of the structure in the mountainous 
region bordering the Snake River Plains on the south, except that in 
the Rock Creek Hills a monoclinal structure similar to that of the 
Mount Bennett region is present. 
The occurrence of folds in the rocks of southern Idaho is not denied, 
but as structural features influencing the present topography, they are 
believed to be subordinate to the faults. 
Although the details in the structure of the rocks, even along the 
best known portion of the border of the Snake River Plains, with the 
exception of the area included in the Boise folio, remain to be studied, 
and even the major features in the relief are, to a great extent, 
unmapped, it seems safe to conclude, at least provisionally, that the 
great depression through which Snake River flo.ws is due in part and 
mainly to displacements along lines of fracture and an upward bend- 
ing in the region occupied b}^ the Owyhee Mountains. 
EARLY GEOLOGICAL HISTORY OF THE SNAKE RIVER BASIN. 
While the great depression in southern Idaho occupied by the Snake 
River Plains is primarily due to the upheaval of the bordering moun- 
tains, it is probable that a large amount of erosion has taken place ancf 
that the depression was thus broadened and deepened previous to its 
