s, approximating more or less to the
circular form, though frequently deviating considerably from it, are
among the largest enclosures on the moon. They vary from upwards of 150
to 60 miles or under in diameter, and are often encircled by a complex
rampart of considerable breadth, rising in some instances to a height of
12,000 feet or more above the enclosed plain. This rampart is rarely
continuous, but is generally interrupted by gaps, crossed by transverse
valleys and passes, and broken by more recent craters and depressions. As
a rule, the area within the circumvallation (usually termed "the floor")
is only slightly, if at all, lower than the region outside: it is very
generally of a dusky hue, similar to that of the grey plains or Maria,
and, like them, is usually variegated by the presence of hills, ridges,
and craters, and is sometimes traversed by delicate furrows, termed
clefts or rills.

_Ptolemaeus_, in the third quadrant, and not far removed from the centre
of the disc, may be taken as a typical example of the class. Here we have
a vast plain, 115 miles from side to side, encircled by a massive but
much broken wall, which at one peak towers more than 9000 feet above a
level floor, which includes details of a very remarkable character. The
adjoining _Alphonsus_ is another, but somewhat smaller, object of the
same type, as are also _Albategnius_, and _Arzachel_; and _Plato_, in a
high northern latitude, with its noble many-peaked rampart and its
variable steel-grey interior. _Grimaldi_, near the eastern limb (perhaps
the darkest area on the moon), _Schickard_, nearly as big, on the south-
eastern limb, and _Bailly_, larger than either (still farther south in
the same quadrant), although they approach some of the smaller "seas" in
size, are placed in the same category. The conspicuous central mountain,
so frequently associated with other types of ringed enclosures, is by no
means invariably found within the walled-plains; though, as in the case
of _Petavius_, _Langrenus_, _Gassendi_, and several other noteworthy
examples, it is very prominently displayed. The progress of sunrise on
all these objects affords a magnificent spectacle. Very often when the
rays impinge on their apparently level floor at an angle of from 1 deg.
to 2 deg., it is seen to be coarse, rough grained, and covered with
minute elevations, although an hour or so afterwards it appears as smooth
as glass.

Although it is a distinguishing charact