252 
GILBERT. 
of steam. By that explosion a body of rock is broken into 
fragments and thrown outward. Such of the fragments as 
descend outside the cavity are heaped about its margin, con¬ 
stituting a rim, which is smooth if the fragments are small 
and rugged, and irregular if they are large. Less than fifty 
craters of this type are known, and they are all small, the 
largest being less than two miles wide. They resemble the 
craters of the moon, in that 
their bottoms are depressed 
below the general level, and 
in that the volumes of their 
rims are approximately equal 
to the capacities of their cavi¬ 
ties. They lack the wreath, 
the inner terraces, the inner 
plain, and the central hill. Thus characterized, they differ 
widely from the lunar craters of medium and maximum 
size, but they resemble those of smaller size. It is possible 
that the resemblance depends in part on the invisibility of 
minor features of the small craters of the moon ; but it is 
perhaps equally possible that better seeing would disclose 
yet other elements of similarity. 
If we accept this resemblance as satisfactory, half of the 
moon’s hollows are explained. Can we adjust the explana¬ 
tion to the remaining half, or must we draw an arbitrary 
line through what appears to be a continuous series of phe¬ 
nomena and study the two parts separately? To adapt the 
explosive hypothesis to the larger craters it is necessary not 
merely to think of a greater explosion, but to imagine some 
phase or accompaniment of explosive action which will fur¬ 
nish the rim with a system of concentric ridges and the 
cavity with a level bottom and a central eminence. If the 
attempt at adaptation fails, as I think it must, then the ex¬ 
planation can be accepted for the small craters only by 
divorcing them from the large—and, whether it be accepted 
or not, inquiry must be continued. 
Before passing to the examination of other theories, it is 
