russell.] PRE-TERTTARY FORMATIONS. 45 
northwest of Blackfoot, namely Bio- and East buttes, are composed of 
rhvolite, and are the weathered remnants of ancient volcanic cones. 
The rock of which each of these buttes is composed, as shown by field 
examinations simply, is mainly a very light-colored rhyolite, which is 
frequently highly vesicular, and at times becomes a true pumice. On 
Big Butte fragments of pumice and also of black obsidian occur, and 
the rock is in places spherulitic. 
QUARTZITE. 
As is well known, the rocks designated by this name have resulted 
from the consolidation of sandstone, owing mainly to the deposition 
of silica from solution in water, between the grains. When this proc- 
ess has been associated with a high temperature, or b}^ movements 
which produce a shearing of the material under great pressure, a 
foliated rock, usually containing mica, is produced, which is termed 
quartz-schist. 
Quartzite which in part had the characteristic of quartz-mica-schist 
has already been referred to as occurring in the hills at the head of 
Long Tom Creek, overlooking the South Fork of Boise River. The 
only portion of this formation examined is situated in the northeastern 
portion of the Mountain Home quadrangle. It extends westward 
from its eastern border for a distance of 6 miles, forming a range of 
conspicuous peaks with rounded summits. These peaks with curved 
saddles between are thickly sheathed with fragments of their own rock. 
Their northeastern slopes descend precipitously to the South Fork of 
Boise River, but near the river granite outcrops. It is probable, 
judging from the topography, that the quartzite occurs to the north 
of the river and has a wide development in the rugged mountains in 
>Jthat region. 
The bold mountains bordering the Snake River Plains on the north- 
west, between Big Wood and Little Lost rivers, are composed largely 
of quartzite. Indeed, so far as can be judged from the observations 
made by me in that region these mountains, with the exception of the 
small area of granite near the Cinder Buttes and certain limestone 
outcrops on Lava Creek near Martin, seem to be composed entirely of 
light-colored quartzite, which extends far to the northwest toward 
the Sawtooth Range. 
The bold mountains between Big Lost and Little Lost rivers, and 
also the similar range to the northeast of Little Lost River, are in 
either case sharply defined monoclinal ridges, rising some 5,000 or 6,000 
feet above the adjacent valleys, and composed of stratified rocks which 
dip northeastward. Quartzite and limestones occur in three ranges, 
but even the prevalent character of the rocks composing them is 
unknown. 
