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