russell.] TOPOGRAPHY. 15 
The best way, perhaps, to convey to the reader an idea of the lead- 
ing topographic features of the country discussed in this report, and 
at the same time prepare the way for a more detailed study, is to sketch 
in outline the leading events in its geological history. 
Southern Idaho is a region composed of geologically old rocks 
which formed an ancient land surface having a rugged relief. In the 
depressions of this surface during later geological time extensive lake 
and stream deposits and vast lava flows were spread out. The older 
rocks, sharply separated from the younger by a long time-interval, 
during which extensive movements in the earth's crust and deep 
erosion took place, are mainly granite, rhyolite, quartzite, and lime- 
stone. The youngest of these is probably the limestone, which is 
thought to be of Carboniferous age. These rocks were variously 
folded, faulted, and upheaved into prominent mountains, and deeply 
dissected by a large river with many tributaries which was long lived. 
The valley of the main stream, the ancient representative of Sna.ke 
River, became broad and had many important tributary valleys open- 
ing from it and extending far into the bordering mountains. The 
sharp-crested mountain spurs between the lateral valleys are in some 
instances prolonged far into the main depression. 
After the topography had passed maturity — that is, after the streams 
had excavated deep valleys, leaving sharp-crested and frequently ser- 
rate divides between them — the main stream was obstructed, possibly 
by lava flows, but as seems more probable, by an upward movement 
of the rocks athwart its course in the region now included in western 
Idaho and eastern Oregon, and a lake was formed which occupied a 
large portion of the country now included in the Snake River Plains. 
This water body, named by Lindgren Lake Pa}^ette, received the sed- 
iment brought in by tributary streams and the dust blown out by 
volcanoes, and became deeply filled. These sediments, with a known 
depth of over 1,000 feet, are now well exposed, particularly in south- 
western Idaho. In places they contain impressions of leaves of trees 
which grew on the borders of the old lake, the shells of fresh-water 
mollusks, the bones of land mammals, and other remains. The fossils 
record a Tertiary (Miocene) age. 
Before Lake Payette came to an end the vast lava flows which now 
form such a conspicuous feature of the Snake River Basin began to be 
outpoured. In fact, the lava and the sediments of Lake Payette and 
of a later lake in the same basin, were contemporaneous, the lava and 
lake sediments being interbedded. Some of the lava flows entered the 
lake, and the occurrence of thick beds of volcanic fragments (lapilli), 
and of scoriaceous, glassy lava, with a torn and slag-like structure at 
the base of thick sheets of usually compact basalt, record the energy 
of the steam explosions that followed. Highly liquid lava continued 
to be poured out at various intervals from a large number of volcanic 
