eristic that there is no great
difference in level between the outside and the inside of a walled-plain,
there are some very interesting exceptions to this rule, which are termed
by Schmidt "Transitional forms." Among these he places some of the most
colossal formations, such as _Clavius_, _Maurolycus_, _Stofler_,
_Janssen_, and _Longomontanus_. The first, which may be taken as
representative of the class (well known to observers as one of the
grandest of lunar objects), has a deeply sunken floor, fringed with
mountains rising some 12,000 feet above it, though they scarcely stand a
fourth of 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