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