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: