The Moon | Page 9

angle varying from 60
deg. to 50 deg. at the crest of the wall, to from 10 deg. to 2 deg. at the
bottom, where it meets the floor; the former extends for a great distance
at a very flat gradient before it sinks to the general level of the
surrounding country. It differs likewise from the inner descent, in the
fact that, though often traversed by valleys, intersected by deep gullies

and irregular depressions, and covered with humpy excrescences and
craters, it is only rarely that any features comparable to the terraces,
usually present on the inner escarpment, can be traced upon it.
Elongated depressions of irregular outline, and very variable in size and
depth, are frequently found on the outer slopes of the border. Some of
them consist of great elliptical or sub-circular cavities, displaying many
expansions and contractions, called "pockets," and suggesting the idea
that they were originally distinct cup-shaped hollows, which from some
cause or other have coalesced like rows of inosculating craters. While
many of these features are so deep that they remain visible for a
considerable time under a low sun, others, though perhaps of greater
extent, vanish in an hour or so.
As in the case of the walled-plains, the ramparts of the ring-plains
exhibit gaps and are broken by craters and depressions, but to a much
less extent. Often the lofty crest, surmounted by aiguilles or by blunter
peaks, towering in some cases to nearly double its altitude above the
interior, is perfectly continuous (like Copernicus), or only interrupted
by narrow passes. It is a suggestive circumstance that gaps, other than
valleys, are almost invariably found either in the north or south walls,
or in both, and seldom in other positions. The buttress, or
long-extending spur, is a feature frequently associated with the ring-
plain rampart, as are also numbers of what, for the lack of a better name,
must be termed little hillocks, which generally radiate in long rows
from the outer foot of the slope. The spurs usually abut on the wall, and,
either spreading out like the sticks of a fan or running roughly parallel
to each other, extend for long distances, gradually diminishing in height
and width till they die out on the surrounding surface. They have been
compared to lava streams, which those round Aristillus, Aristoteles,
and on the flank of Clavius a, certainly somewhat resemble, though, in
the two former instances, they are rather comparable to immense ridges.
In addition to the above, the spurs radiating from the south-eastern
rampart of Condamine and the long undulating ridges and rows of
hillocks running from Cyrillus over the eastern glacis of Theophilus,
may be named as very interesting examples.

Neison and some other selenographers place in a distinct class certain
of the smaller ring-plains which usually have a steeper outer slope, and
are supposed to present clearer indications of a volcanic origin than the
ring-plains, terming them "Crater-plains."
CRATERS.--Under this generic name is placed a vast number of
formations exhibiting a great difference in size and outward
characteristics, though generally (under moderate magnification) of a
circular or sub-circular shape. Their diameter varies from 15 miles or
more to 3, and even less, and their flanks rise much more steeply to the
summit, which is seldom very lofty, than those of the ring-plains, and
fall more gradually to the floor. There is no portion of the moon in
which they do not abound, whether it be on the ramparts, floors, and
outer slopes of walled and ring plains, the summits and escarpments of
mountain ranges, amid the intricacies of the highlands, or on the grey
surface of the Maria. In many instances they have a brighter and newer
aspect than the larger formations, often being the most brilliant points
on their walls, when they are found in this position. Very frequently too
they are not only very bright themselves, but stand on bright areas,
whose borders are generally concentric with them, which shine with a
glistening lustre, and form a kind of halo of light around them. Euclides
and Bessarion A, and the craters east of Landsberg, are especially
interesting examples. It seems not improbable that these areas may
represent deposits formed by some kind of matter ejected from the
craters, but whether of ancient or modern date, it is, of course,
impossible to determine. Future observers will perhaps be in a better
position to decide the question without cavil, if such eruptions should
again take place. Like the larger enclosures, these smaller objects
frequently encroach upon each other-- crater-ring overlapping
crater-ring, as in the case of Thebit, where a large crater, which has
interfered with the continuity of the east wall, has, in its turn, been
disturbed by a smaller crater on its own east wall. The craters in many