The Moon | Page 8

this height above the plain on the west, which ascends with
a very gentle gradient to the summit of the wall. Hence the contrast
between the shadows of the peaks of the western wall on the floor at
sunrise, and of the same peaks on the region west of the border at
sunset is very marked. In Gassendi, Phocylides, and Wargentin we

have similar notable departures from the normal type. The floor of the
former on the north stands 2000 feet above the Mare Humorum. In
Phocylides, probably through "faulting," one portion of the interior
suddenly sinks to a considerable depth below the remainder; while the
very abnormal Wargentin has such an elevated floor, that, when viewed
under favourable conditions, it reminds one of a shallow oval tray or
dish filled with fluid to the point of overflowing. These examples, very
far from being exhaustive, will be sufficient to show that the
walled-plains exhibit noteworthy differences in other respects than size,
height of rampart, or included detail. Still another peculiarity, confined,
it is believed, to a very few, may be mentioned, viz., convexity of floor,
prominently displayed in Petavius, Mersenius, and Hevel.
MOUNTAIN RINGS.--These objects, usually encircled by a low and
broken border, seldom more than a few hundred feet in height, are
closely allied to the walled-plains. They are more frequently found on
the Maria than elsewhere. In some cases the ring consists of isolated
dark sections, with here and there a bright mass of rock interposed; in
others, of low curvilinear ridges, forming a more or less complete
circumvallation. They vary in size from 60 or 70 miles to 15 miles and
less. The great ring north of Flamsteed, 60 miles across, is a notable
example; another lies west of it on the north of Wichmann; while a
third will be found south- east of Encke;--indeed, the Mare Procellarum
abounds in objects of this type. The curious formation on the Mare
Imbrium immediately south of Plato (called "Newton" by Schroter),
may be placed in this category, as may also many of the low dusky
rings of much smaller dimensions found in many quarters of the Maria.
As has been stated elsewhere, these features have the appearance of
having once been formations of a much more prominent and important
character, which have suffered destruction, more or less complete,
through being partially overwhelmed by the material of the "seas."
RING-PLAINS.--These are by far the most numerous of the ramparted
enclosures of the moon, and though it is occasionally difficult to decide
in which class, walled-plain or ring-plain, some objects should be
placed, yet, as a rule, the difference between the structural character of
the two is abundantly obvious. The ring-plains vary in diameter from

sixty to less than ten miles, and are far more regular in outline than the
walled-plains. Their ramparts, often very massive, are more continuous,
and fall with a steep declivity to a floor almost always greatly
depressed below the outside region. The inner slopes generally display
subordinate heights, called terraces, arranged more or less
concentrically, and often extending in successive stages nearly down to
the interior foot of the wall. With the intervening valleys, these features
are very striking objects when viewed under good conditions with high
powers. In some cases they may possibly represent the effects of the
slipping of the upper portions of the wall, from a want of cohesiveness
in the material of which it is composed; but this hardly explains why
the highest terrace often stands nearly as high as the rampart. Nasmyth,
in his eruption hypothesis, suggests that in such a case there may have
been two eruptions from the same vent; one powerful, which formed
the exterior circle, and a second, rather less powerful, which has
formed the interior circle. Ultimately, however, coming to the
conclusion that terraces, as a rule, are not due to any such freaks of the
eruption, he ascribes them to landslips. In any case, we can hardly
imagine that material standing at such a high angle of inclination as that
forming the summit ridge of many of the ring-plains would not
frequently slide down in great masses, and thus form irregular plateaus
on the lower and flatter portions of the slope; but this fails to explain
the symmetrical arrangement of the 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