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