KussELL.] STREAM-CUT LAVA SHEETS. 125 
other general principle which has influenced the erosion of the Snake 
River Plains may perhaps be cited. 
Snake River, as stated, is spreading out coarse deposits in the por- 
tion of its course above American Falls, and it may reasonably be 
asked why is it that the same stream lower down its course has been 
enabled to excavate a deep canyon. The reason is evidently that owing 
to an increase in gradient in the portion of its channel below where it 
is laying aside its load, its rate of now is increased and its capacity for 
work consequently augmented. 
The scenery of Snake River Canyon presents many variations, 
dependent largely on the nature of the rocks cut through and espe- 
cially on the presence or absence of lava sheets. Where alternating, 
nearly horizontal layers of soft lake beds and hard lava occur, the 
deepening of the canyon has been retarded by the hard layer and 
accomplished mainly by the recession of cascades and waterfalls. It 
happens that the river in its course across southern Idaho passes from 
a region where lava sheets are thick with but little soft material 
between them — as, for example, from American Falls to the foot of the 
deep, narrow canyon below Shoshone Falls — to a region where the lava 
sheets in general become thinner and are separated one from another 
by soft lake beds. When the stream in its downcutting flowed over 
a nearly level sheet of basalt to its edge it quickly deepened its chan- 
nel in the soft beds encountered and a fall was produced. Such a fall 
would recede at a rate dependent on several conditions, but mainly on 
the thickness of the lava sheet and the depth of the channel below the 
cascade. In the main, the canyon of Snake River has been produced 
by the recession of a series of waterfalls of the nature just cited, and 
the process is still continuing. Each of the falls and rapids in the 
river is due to the presence of a resistant sheet of lava, in the border 
of which a notch of greater or less length has been cut. Where the 
lava sheet is thick, as at Twin Falls and Salmon Falls, a precipitous 
descent results; where the lava sheet is thin or composed of several 
thin layers of varying resistance, a succession of small leaps or a rapid 
is produced, as is the case to-day at several localities. 
An exception to the rule just stated occurs in the case of Shoshone 
Falls, the highest and grandest of all the leaps Snake River makes. 
The river is h'ere deepening its channel, mainly through horizontally 
bedded basait, with a mass of rhyolite beneath. The surface of the 
lower and harder rock was not a plane, as are the lava sheets, but is 
irregular, and a series of rapids was probably formed which devel- 
oped into a single great fall. The fall has receded about 5 miles in the 
rhyolite and at present, judging from the height to which the rhyo- 
lite above the falls rises into the lava, is as high as it will ever become. 
As the fall continues to recede it will probably decrease in height, and 
when the obstruction that causes it is cut across, will resume the 
characteristics due to thick lava sheets. 
