1890.] 
433 
[W right. 
Harney lake and Christmas lake basins are at a much higher level 
than these being 500 to 1000 feet above the lava plains of the 
Snake river. 
The fact that basaltic lava flows cover these beds is not decisive 
for either age. In the valleys of eastern Idaho, whence come some 
of the tributaries of the Snake, basaltic lavas both of Tertiary and 
of Quaternary age occur. Some of the latter fill present valley 
bottoms, extending up a short distance into the mouths of side 
canons in the adjoining ridges. These flows have had some influ- 
ence upon the direction of the present drainage, and it seems prob- 
able that before their eruption the Bear river flowed through the 
Portneuf valley into the Snake river, instead of into Salt Lake as 
it does now. 
Gilbert has shown that Lake Bonneville, the quaternary tenant 
of the Salt Lake valleys, once overflowed into the Snake river, but 
whether this overflow was contemporaneous with the existence of 
the lake in which the Nampa beds were deposited, or accompanied 
the freshets which brought down the Snake river gold-bearing grav- 
els, only a systematic study of the whole Snake river region can 
finally determine. Such a s,tudy ought to establish the time rela- 
tions of the Bonneville and Nampa beds, and a still more definite 
determination of the age of the latter might be afforded by a care- 
ful study of the canon of the Snake river below Shoshone Falls' 
and by a tracing of the upper limits of the lake in which the Nampa 
beds were deposited, and the relation of the Nampa beds to the 
last basaltic overflow of the Snake river plains. In a visit to the 
Shoshone Falls, in 1868, Mr. Clarence King found that the river 
above the falls runs in a canon about four hundred feet deep cut in 
the upper sheet of basalt which covers the present surface of these 
plains, but that the canon below the falls discloses an underlying 
mass of trachyte or andesite, which is probably of much earlier 
date. If, as seems not improbable, the basalt flows which cover 
the Nampa beds in the Boise region were contemporaneous with 
those which cover the Snake plains in the vicinity of Shoshone 
Falls, at some point in the canon of the Snake river these beds will 
be found resting on the underlying trachyte or andesite body, and 
separating it from the overlying basalt flow. It will then be proved 
that the cutting of the canon of the Snake river below the surface 
of the basalt flows has been accomplished since the drainage of the 
Nampa lake, and it will only remain to estimate the time required 
PROCEEDINGS B. S. N. H. VOL. XXIV 28 APRIL, 1890. 
