42 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
granite is light-colored, coarse-grained, and resembles the Boise 
granite, but there is no evidence of a connection between the two. It 
it possible, however, that they are of the same age, and perhaps por- 
tions of a single intrusion. 
As stated by Lindgren in the Boise folio, granite is present in the 
Owyhee Mountains, in southwestern Idaho to the south of Snake 
River, but little is known of that region. No other granitic areas are 
reported to occur in the part of Idaho here considered, but it is to be 
remembered that large portions of that region have never been visited 
by geologists, and even the general nature of many of the mountain 
ranges is unknown. Certain bold peaks to the south of Snake River, 
to the east of Albion, rise well above timber line, and when seen from 
a distance present the appearance of weathered granite, but were not 
visited by me. 
RHYOLITE. 
The extensive area of rhyolite whose western border, as stated 
above, is on the divide between Ditto and Syrup creeks, extends south- 
eastward to the vicinity of King Hill Creek, a distance of about 30 
miles. The width of the area is 6 or 8 miles, but its full extent to the 
northward is unknown. It forms Mount Bennett, which rises 5,000 
feet above the adjacent portion of the Snake River Plains, as well as the 
rugged country drained by Syrup and Long Tom creeks, lying north 
of Mountain Home. On the south it is bordered by the Snake River 
lava, and on the north, at the head of Long Tom Creek, by a quartzite 
ridge which slopes steeply on its northern side to the South Fork of 
Boise River, where granitic outcrops occur. 
The rocks in this extensive area are stratified and present two well- 
defined varieties, one a compact, light-purplish rock with a granular 
structure and conspicuous crystals of feldspar, and the other a black, 
vitreous rock which also contains large feldspar crystals. In each 
variety there is frequently present a pronounced flow structure, which is 
especially well shown on weathered surfaces. This structure was pro- 
duced by the flow of the still plastic magma after crystals had formed 
in it. In many instances parallel laminae were produced in this manner, 
which are a small fraction of an inch thick, but may be traced laterally 
for several feet. An alternation of beds of granular rocks with a dull 
fracture and of black, vitreous layers gives the formation a distinctly 
bedded structure. Some of the light purplish beds, particularly near 
the base of the formation, on the divide between Dry and Syrup 
creeks, are of the nature of a tuff — that is, they consist of consolidated 
dust-like fragments, but contain distinct crystals of feldspar. This for- 
mation, which may be termed provisionally the Mount Bennett rhyolite, 
is tilted in various ways, but usually dips toward the northeast. From 
