russell.] LAVA STREAMS. 69 
ward with well-defined banks. On reaching the lower portion of the 
valley of Lava Creek, this lava stream expanded to a width of about 1 
mile, and continuing on to the Snake River Plains spread out still more 
widely, forming a thin sheet with an exceedingly rough surface. The 
length of this stream is not accurately known, but is probably about 
10 miles. 
Where the lava came to the surface there are no elevations, but 
instead two circular depressions now overgrown with aspens which 
make them conspicuous. These depressions are small, probably not 
over 300 feet in diameter, but discharged a large amount of molten 
rock. The absence of elevations about the summits of the conduits 
is due to the fact that explosive eruptions did not occur, and owing 
to the steepness of the slopes on which the vents were situated the 
the lava flowed away immediately on emerging and did not cool and 
thicken about them. The lava, in descending the steep slopes below, 
where it came to the surface, formed veritable cascades, and on cooling 
was left with an excessively rough surface. The crust first formed 
was broken and the fragments were cemented together by still plastic 
lava, so as to make a coarse breccia. Many masses, composed of united 
fragments of all dimensions up to 20 feet or more in diameter, were 
floated far down the stream and oat on to the border of the Snake River 
Plains. The contrast between the rough surface of this high grade, 
and consequently swift lava stream, and the smooth surfaces formed on 
sluggish lava streams on hardening, will be referred to later. 
Although the lava stream just described is comparatively small and 
made but a moderate contribution to the filling of the Snake River 
Basin, it illustrates one of the methods b}^ which the broad plains of 
that great depression were formed. 
The Martin lava stream is black and has but little soil or vegetation 
on its surface, and is plainly much younger than the lava sheets it 
crossed and partially buried after emerging on the plain. It has lost 
much of its freshness, however, and is certainly much older than the 
lava which came from Black Butte, and is much older, also, than the 
several lava streams described below, which came from the neighbor- 
ing Cinder Buttes. 
LITTLE CANYON LAVA STREAM. 
Little Canyon Creek rises in the rugged mountains about 20 miles 
northeast of Mountain Home, flows southward for about 20 miles, 
through a deep, narrow canyon cut in lava and associated sedimentary 
beds forming the Snake feiver Plains, and joins Snake River at Glenns 
Ferry. 
After a stream older than the present Little Canyon Creek had 
excavated a valley in the mountains, it was displaced by a flow of basalt 
which came to the surface about 5 miles or more to the north of the 
