40 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
and at a distance of 3 to 4 miles west of Canyon Creek, small exposures 
of granite occur on the border of the lava forming the plain. Rising 
steeply from these isolated outcrops, which are significant in reference 
to the relative age of the two classes of rock forming the mountains, 
are bold precipices of dark rhyolite. Significant, too, in respect to 
the geological structure of the region and the relation the plains bear 
to the mountains, is the fact that the precipitous escarpment bordering 
the plain on the northeast, between Mount Bennett and Boise, is com- 
posed to the eastward of rhyolite and to the westward of granite, but 
the alignment is unbroken at the junction of the two formations. 
The major features of this great escarpment, about 50 miles in length, 
are due to faulting or to the uplifting of the mountains along a line of 
fracture. 
The Boise granite, as has been stated by Lindgren, is intrusive and 
of ancient date (perhaps pre-A]gonkian) — that is, the great granitic 
area referred to, embracing several hundred square miles, is composed 
of rocks which were forced upward in a molten condition from deep 
within the earth's crust, but, so far as known, failed to roach the sur- 
face so as to produce volcanic eruptions. The rising magma uplifted 
the rocks beneath which it was intruded, probably into the form of a 
dome, which has since been removed by erosive agencies, and, cooling 
slowly under great pressure, produced a massive, coarsely crystalline 
rock. Not only has the thick cover beneath which the granitic magma 
was intruded been worn away, but streams have cut deeply into the 
intrusive rock itself, and probably many hundreds of feet of its upper 
portion have been removed. This process of denudation is still going 
on, as is shown by the soft, broken, and deeply deca} r ed conditions of 
the portion of the granite now exposed, and by the nearly universal 
mantle of fragments that covers its surface and extends far down the 
channels of the draining streams. Owing principally to the decay of 
its constituent crystals, mica and feldspar, the granite is usually 
altered to a depth of 100 feet or more below the surface, and expo- 
sures of fresh rock, compact enough to be used in finished masonry, are 
not to be seen. Decay, disintegration and erosion have given to the 
granitic area an exceedingly rugged surface, the major features of 
which are deep, generally smooth-sided valleys and bold, rounded 
hills. One characteristic is the frequent presence of great numbers of 
crags and castle -like forms of bare rock which project through the 
nearly universal covering of surface debris. 
The stream channels in the granite, and far down their courses after 
reaching the Snake River Plains, are deeply filled w T ith rock waste. 
The disintegration of the rocks is evidently progressing rapidly and 
furnishing more fragments, mostly of the nature of coarse, angular 
sand and gravel, than the streams are able to carry away. In many 
depressions, and even in valleys of considerable size, the accumulation 
