274 
GILBERT. 
differences in form, but it has not been found possible to 
reproduce them on a small scale. 
Arched Floors .—In general, the inner plains of the craters 
are level, except as interrupted by central hills and by craters 
of subsequent origin. In some of the larger it is possible to 
note,, under favorable conditions of light and shadow, a 
gentle convexity corresponding to the normal curvature of 
the moon’s surface. In a few instances, not more than a 
dozen in all, the convexity is seen to be greater, the central 
portion distinctly rising above the level of the margins. 
This peculiarity is most strongly marked in the floor of the 
crater Mersenius, which is a dome 1,500 feet high and thirty 
miles across. A similar but lower dome in the crater of 
Petavius bears on its crest a group of peaks exhibiting the 
ordinary characters of the central hills. Two explanations 
have occurred to me, each based upon the idea of a relatively 
soft substratum. It appears possible that deep seated matter 
which had been displaced horizontally by the original 
collision might slowly return under gravitational stress, 
lifting the middle of the crater floor after congelation of the 
impact liquid had given it a level character. It also appears 
possible that the strains produced by the tendency of viscous 
material to flow upward were not effective until reinforced 
by the shock associated with the formation of some later but 
neighboring crater. In either case the result was produced 
by the flow of a viscous solid analogous to the viscous move¬ 
ment supposed to accompany the broader terrestrial uplifts. 
The fact that the arch of Mersenius is cracked at the crest and 
that the arch of Petavius is traversed by a system of cracks 
is consistent, I think, with either of these explanations, but 
is perhaps not specially significant, as cracks occur in con¬ 
siderable abundance in many parts of the moon’s surface. 
Distribution and Overlap .—Consideration will presently be- 
given to the fact that in certain districts craters are thickly 
set and elsewhere they are less numerous, but in other re¬ 
spects their distribution exhibits no system. They are not 
arranged in lines or other patterns, large and small are in- 
