eussell j CHANGES MADE IN LAVA SHEETS AFTER COOLING. 127 
SPRING-FORMED ALCOVES ON THE SIDES OF CANYONS. 
At Blue Lakes, about 5 miles below Shoshone Falls, where the walls 
of Snake River Canyon are about 700 feet high, and nearly vertical, 
except for the talus lodged against their bases, there is a tributary 
canyon-like alcove in the north wall which is about 2 miles long, and 
about 2,000 feet across at the top and which heads in a semicircular 
amphitheater with vertical walls about 300 feet high. - There is no 
stream entering the alcove from the surface of the neighboring plain, 
but springs discharging probably several hundred cubic feet of water 
per second boil up in the amphitheater at its head. One of the strik- 
ing features in this alcove is the small quantity of talus at the bases of 
the bordering cliffs. This is most noticeable in the semicircular 
expansion of the alcove at its head. In the view of the Blue Lakes 
alcove presented on PI. XXIV some of the features referred to may 
be recognized. 
On the north side of Snake River Canyon, about 2 miles above the 
mouth of Salmon River, which comes into the Snake from the south, 
the canyon wall is again nearty vertical and about 300 feet high, and 
there are two side branches, known as Little and Box canyons. These 
are similar in all their leading features to the Blue Lakes alcove, and, 
like it, are lateral openings from the main canyon. The}^ are also 
without surface streams entering them from the plain above. Box 
Canyon is approximately 3 miles long, and Little Canyon is perhaps a 
mile less in length. Each of these secondary canyons or alcoves has 
vertical walls about 200 feet high. There are comparatively small 
talus slopes along their sides and practically none at the base of the 
wall encircling their heads. At the head of each alcove there is again 
an immense spring. The stream issuing from Box Canyon has a vol- 
ume of about 1,000 cubic feet per second, and the stream from Little 
Canyon is perhaps one-third as great. A view in Little Canyon look- 
ing out into Snake River Canyon is reproduced on PI. XXIII, B. 
Malade River, which is formed by the union of Big Wood and Little 
Wood rivers, in recent years has become dry in summer, owing to the 
demands made on its tributaries for irrigation. The river, as I was 
informed, was dry a few miles below where its two branches unite, 
late in August, 1901, but on crossing its canyon, where it emerges into 
Snake River Canyon, I found a roaring stream of clear water 8 feet 
wide and fully 1 feet deep discharging by estimate about 3,000 cubic 
feet of water per second. All of this water is contributed by springs 
in the lower 4 miles of the canyon. The main supply, as I have 
been informed by Mr. N. Uhrlaub, comes from a great spring which 
rises at the foot of a precipice in the canyon, and which causes a cas- 
cade in the Malade when water comes from the upper part of its 
course in winter and spring. The volume of the spring referred to is 
1 
