1890 .] 
431 
[Wright. 
great lava plains of the Snake river I could form no idea, since 
unfortunately both eastward and westward-going trains pass over 
that country in the night. 
On the banks of the Boise river, just below where it emerges 
from the canon I observed that the gravels, which form the first 
bench to the south of the river, rest on an eroded, white sandstone, 
which might correspond to the “ original sandstone” struck at the 
bottom of the drill hole and form part of the bottom of the lake in 
which these beds were deposited ; this assumes a descent of about 
700 feet in twenty miles giving a rather unusual but not impossible 
slope to the bed of the lake, and makes it probable that the canon 
of the Boise is near the northern shore line of this lake. 
I had been unable to find any fossil evidence of the age of these 
beds, but on other grounds had assumed that they were late Ter- 
tiary or early Quaternary. They had a younger appearance than 
the pliocene deposits of Nevada, and on the other hand looked 
older in some respects than the Quaternary deposits of Lakes 
Bonneville and Lahontan. 
The character of the Snake river valley in this region was of 
special significance to me. Instead of meandering to and fro in a 
broad alluvial bottom with large cottonwood trees along its banks, 
as is usually the case with such large streams, it runs in a compar- 
atively straight course through a slightly rolling sage bush country, 
filling its bed from bank to bank with a swift, deep, turbid stream, 
with only a scanty growth of young willows along its shores, which 
are being constantly undermined and carried away. This, to me, 
was an evidence that, at some point below this, its bed had been 
lowered in recent geological time by the breaking of some barrier 
that had formerly held it back, and that it was now rapidly cutting 
back and deepening its bed in an endeavor to reach a baselevel of 
erosion. Its slope from the base of the mountains in eastern Idaho 
is very rapid for so large a river. It has not been surveyed so that 
it cannot be accurately determined, but if it ran in a straight line 
its descent would be ten feet to the mile, and, as its course 
through the lava beds is generally direct, it can hardly be less than 
seven or eight feet, making allowance for its meanderings and the 
falls it passes over. Of these there are several, the greatest being 
the Shoshone Falls which are 212 feet high. 
It is also significant that after its junction with the Boise river 
