russell.] IDAHO FORMATION. 55 
white sand and evenly laminated nearly white clays, together with a 
thin sheet of basalt, which soon disappears when traced southward. 
The section observed at this locality is as follows: 
Section of Snake River Canyon 10 miles below Glenns Ferry. 
Feet. 
1. Light-gray, laminated, line-grained clay or silt, changing to white below. 
By estimate 150-170 
(This stratum forms the surface for many miles south of Snake 
River, and rises into low hills; at the border of the canyon it is deeply 
notched and well exposed.) 
2. Scoriaceous basalt 4-10 
(Near at hand two sheets of basalt are present at this horizon; the 
upper are 6 and the lower are 10 feet thick, with 10 inches to 2 feet of 
sandy sedimentary material between; this layer is of a brick-red color 
and is traversed by vertical but tortuous steam holes. ) 
3. Associated with No. 2 is a bed of coarse, well-worn gravel 4-6 
(This bed possibly indicates the summit of the Payette formation, 
as the beds below contain an abundance of Miocene fossils; those above, 
so far as known, are barren of fossils. ) 
4. Loose, well-worn pebbles, of a variety of rocks, including basalt, cross bedded, 
ferruginous, in part loosely cemented 3-10 
5. Unconsolidated, fine, nearly white sand, with occasional shaly layers, 
cemented at times into irregular concretionary bands, charged with 
fresh-water shells throughout. a Thickness above the river 300 
Total, about 650 
The basalt exposed at Shell Mountain is the attenuated margin of a 
widely extended sheet which underlies the plain to the north of Snake 
River, but which has been removed by the erosion of Bennett and other 
creeks, as well as by the main stream to which they are tributary. 
This is the only layer in the section that reveals the inclination of the 
beds composing it; it dips very gently southward. 
The facts briefly presented in the last few pages and others that 
might be recorded, did space permit, show that Snake River in its 
course for at least 50 miles below the mouth of Salmon River, in gen- 
eral skirts the southern margins of lava sheets which underlie the plain 
to the north, but occasionally cuts across portions of the lower sheets 
which extend farthest southward. Where the river crosses a portion 
of a lava sheet and has cut a channel in it its canyon becomes narrow 
with steep walls. In such localities, if the basalt is thick, the canyon 
walls are nearly vertical; and if thin, a conspicuous rim rock appears 
above steeply sloping bluffs in part concealed by talus and landslides. 
When two or more sheets of basalt are present, rim rocks occur below 
the highest one, which forms the rim or ''break" of the canyon. 
When only soft sedimentary beds are present, and this is only on the 
south side of the river, except in the vicinity of Glenns Ferry, the 
aA list of the shells collected at this locality is given on p. 56. 
