concentric terraces and intermediate valleys. The inner declivity of the
north-eastern wall of Plato exhibits what to all appearance is an
undoubted landslip, as does also that of Hercules on the northern side,
and numerous other cases might be adduced; but in all of them the
appearance is very different from that of the true terrace.

The _glacis_, or outer slope of a ring-plain, is invariably of a much
gentler inclination than that which characterises the inner declivity:
while the latter very frequently descends at an 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, w