Russell] IDAHO FORMATION. 53 
blocks of basalt 10 to 1-L inches in diameter. In places the bed of lapilli 
is compact and stands in vertical walls from 20 to 30 feet high, more or 
less sculptured by wind erosion. In this stratum, also, there are at 
times local beds, usually less than afoot thick, of thinly laminated soft 
sandstone 1 . About 2 miles east of the locality first mentioned, and at 
the end of a prominent bluff' (PI. VI, J?), still more conspicuous varia- 
tions were observed in the strata forming the canyon wall, as is shown 
by the following section: 
Section of wall of Snake Hirer Canyon 5 wiles below mouth of ('inn/on Creek. 
Feet. 
1. Coarse basaltic 1 >reccia, fragments 3 feet in diameter 40 
2. Compact, columnar basalt 6 
3. Coarse volcanic breccia 20 
4. Evenly laminated volcanic sand 9 
5. Yellowish stratified lapilli 70 
6. Fine, white, laminated calcareous clay 2 
7. Yellowish stratified lapilli 7 
8. Fine, white, horizontally laminated clays, interbedded with fine, loose, 
cross-bedded, light-colored sand, becoming pebbly and cross-stratified at 
the top 120 
9. Unexposed to the river, about 100 
Total 374 
The highest rim rock of basalt, usually present in the portion of the 
canyon here referred to, is absent from above the summit of the prom- 
ontory at which the section just given was observed, but is present 
near at hand. A capping of some 20 to 30 feet of basalt above the 
highest bed of breccia observed may at one time have existed, but has 
crumbled and fallen away. 
From the mouth of Bruneau River upstream for about 6 or 8 miles 
the Snake flows between precipitous walls capped on each side by a 
conspicuous rim rock, as is illustrated on PI. VI, A. This view was 
taken from the summit of the northern wall of the canyon, just below 
the mouth of the Bruneau, looking downstream to the southwest, and 
into the broader portion of the canyon, which is excavated in soft sed- 
iment. Beneath the thick sheets of basalt, of which four are easily 
recognized in the lower part of the canyon of Canyon Creek, there 
are thick beds of gravel, sand, and clay, but lapilli are seemingly 
absent. At a locality on the left bank of the river near where it 
enters the wider portion of its canyon the following section is revealed: 
Section of Snake River Canyon near month of Bruneau River. 
Feet. 
1. Compact columnar basalt 40 
2. Stratified unconsolidated white sand 30 
3. Waterworn pebbles 15 
4. Fine unconsolidated white sand 12 
5. Finely laminated, horizontally bedded sandy clay 250 
Total 347 
