russell.] CRATER RINGS NEAR CLEFT. Ill 
these features may be distinguished in the photograph reproduced in 
PL XVII, B. 
The larger of the pits is less markedly circular than its neighbor 
and about 1,100 feet in diameter at the bottom. Its walls are approxi- 
mately of the same height as those of the smaller "pit, but less con- 
tinuous. There are two breaks in the rim rock, one on the northeast 
and the other on the southwest, down which a horseman can easily 
ride to the plain at the bottom. About the outer border of the larger 
pit. and in general a few rods away from its brink, there are irregu- 
lar piles of lapilli with indefinite shapes, the higher ones forming 
broad hills that rise perhaps 50 feet above the adjacent plain. 
The fact that these circular pits are depressions in a broad plain, 
composed of horizontally bedded and relatively thick sheets of basalt, 
indicates that they were formed after the basalt had cooled and 
hardened. The presence near them of considerable but not conspicu- 
ous deposits of lapilli— although there are no cinder cones or other 
evidence of explosive volcanic eruption nearer than Kuna Butte, 7 
miles distant — indicates that the fragmental material was blown out 
of the pit near which it occurs. The amount of this material in sight, 
however, is far too small to refill the adjacent depression if trans- 
ferred to it; and is absent or concealed by more recently deposited 
soil from about the smaller depressions. The tentative explanation 
that these facts indicate is that two small volcanoes formerly existed 
where the crater rings now occur, and probably built cinder cones 
similar to Kuna Butte, and that later explosions occurred, which blew 
away the piles of fragments and enlarged the conduits above which 
they have been deposited. This suggested explanation, adopted from 
the studies of others in reference to similar crater rings elsewhere, is 
not entirely satisfactory, however, as in case such explosions as sug- 
gested did occur, we should expect to find blocks of lava torn from 
the walls of the conduits in the process of enlarging them, scattered 
over the surrounding surface. More than this, the two circular pits 
arc less than 1,000 feet apart, and unless the explosions which pro- 
duced them occurred simultaneously, one would be expected to be 
more or less completely filled by the debris from its neighbor. That 
an explosive eruption occurred in the case of the larger pit, however, 
is evident from the lapilli piles referred to. The only alternative 
hypothesis suggested by these depressions, and by others elsewhere, 
some of them several miles in diameter, is that the walls of the volca- 
noes broke away and fell in on account of the withdrawal of the sup- 
porting lava formerly filling their conduits, after the manner in 
which the great crater-like depressions in the summits of certain 
Hawaiian volcanoes are known to have been enlarged. 
Although it is necessary at present to leave these crater rings with- 
out a definite explanation, it is hoped that the facts recorded may 
