russell.] PRE-TERTIARY FORMATIONS. 39 
area of which it treats, and should be carefully studied not only by 
professional geologists, but more particularly by the people of Idaho. 
A considerable portion of the Snake River Plains lying south of the 
area embraced in the Boise folio has also been explored by Lindgren 
in connection with detailed studies of neighboring mining districts, 
and the results have been presented by him in a paper published by 
the United States Geological Survey." In this paper there is a sum- 
mary of the observations made by other geologists who have visited 
southern Idaho. 
From the reports just mentioned, as well as my own explorations, 
it is evident that the geological discussion of the Snake River Plains 
may be consistently divided into two portions, the first dealing with 
the older rocks which were upraised into mountains and deeply eroded 
before the partial filling of the Snake River Basin, and the second 
with the lacustral and stream deposits, lava sheets, etc., now forming 
the plains themselves. The former, in geological language, embraces 
mostly if not entirely, Paleozoic rocks; and the second, Tertiary and 
Recent rocks. Between the formation of these two great rock divi- 
sions there was a long interval, during which deep erosion and the 
development of a rugged topography took place. 
PRE-TERTIARY FORMATIONS. 
The rocks of which the mountains surrounding the Snake River 
Plains are most largely composed are granite, rhyolite, quartzite, and 
limestone. 
GRANITE. 
The terrane named the "Boise granite" by Lindgren, from which 
the rugged mountains which lie to the north and east of Boise have 
been sculptured, extends eastward from this area and occupies an exten- 
sive and exceedingly rugged region lying north of Mountain Home. 
The eastern border of this area forms a moderately irregular line with 
an approximately north-south direction, situated between the canyons 
of Ditto and Syrup creeks. On this dividing ridge the granite passes 
beneath extensive surface flows of rhyolitic lava which form the moun- 
tains drained principally by Syrup and Long Tom creeks, as well as 
Mount Bennett, and other high land still farther east. The junction 
of the granite and rhyolite is sharply defined and is easily traced, 
owing, principally, to the light color of the granite and the dark 
browns, purples, and reds of the later-formed terranes. The granite 
is massive, without bedding planes, while the rhyolite is in distinct 
layers, which are sharply tilted. Near the "Overland road," which 
skirts the base of the mountains lying northwest of Mountain Home, 
a Eighteenth Ann. Rept. U. S. Geol. Survey, Pt. Ill, pp. 617-719. This report is accompanied by a 
geological map of a portion of the western part of the Snake River Plains. 
