russell] PRE-TERTIARY FORMATIONS. 41 
of rock fragments has become so deep that rain water is not retained 
at the surface, and no .stream channels are in sight. The concave sur- 
faces of these partially filled depressions meet the convex curve of the 
bordering- uplands, and where the mantle of rock waste is not broken 
by residual crags the landscape has broadly undulating outlines. 
Traversing the Boise granite in the region to the north of Boise, 
and described by Lindgren, there are numerous dikes of both acid and 
basic rock which form long narrow outcrops, differing from the inclos- 
ing granite. As is well known, they are due to the filling of fissures 
with molten material injected from below and to the hardening and 
crystallizing of the intruded magma on cooling. To the east of the 
portion of the granitic area included in the Boise folio dikes are 
less common, although several conspicuous examples occur in the 
mountains drained by Slater, Ditto, and Indian credits. These dikes 
arc composed of purplish rhyolite, trend nearly due north and south, 
and have a width of from 30 to 80 feet or more. The dikes are ver- 
tical, and owing" to the greater resistance thev offer to erosion, stand 
in bold relief above the surface of the inclosing granite. The most 
characteristic examples thus far examined occur in the mountains 
adjacent to the border of the Snake River Plains, between Black and 
Slater creeks; others, but less numerous and less bold and conspicuous, 
were seen in the mountains between Indian and Ditto creeks. The 
rock forming these dikes is of the same general character as the 
rhyolite occurring in well-defined sheets in the mountains to the 
northeast of Mountain Home and in Mount Bennett, etc., and suggests 
that much of the granite now exposed was formerly covered with 
rhyolitic flows, which have been eroded away. The rhyolite is younger 
than the granite, and an epoch of erosion intervened between the 
formation of the two terranes. 
An outcrop of granite about 5 or 6 square miles in area occurs on the 
west side of the Snake River Plains, about 75 miles west of Blackfoot, 
and near the Cinder Buttes. This small isolated area is surrounded 
by quartzite on all sides except the eastern, where it is bordered by 
a recent flow of lava which came from the Cinder Buttes. The con- 
tact of the granite with the surrounding quartzite is sharply defined, 
and at one locality on its southern margin a mining shaft has been 
sunken to a depth of 80 feet, following the plane of contact, which is 
inclined southward at an angle of about 80°. The bedding planes in 
the quartzite also dip southward, but at an angle of only 15° to 18°. 
There is no evidence of an alteration of the quartzite at its junction 
with the granite, and the contact is evidently a result of faulting. 
The diverse dips observable in neighboring quartzite ridges show 
that much movement has occurred along fault planes in the hills about 
the granite, and it seems evident that the granite itself has been ele- 
vated by this process high enough to be exposed by erosion. The 
