means invariably found within the walled-plains; though, as in the case
of _Petavius_, _Langrenus_, _Gassendi_, and several other noteworthy
examples, it is very prominently displayed. The progress of sunrise on
all these objects affords a magnificent spectacle. Very often when the
rays impinge on their apparently level floor at an angle of from 1 deg.
to 2 deg., it is seen to be coarse, rough grained, and covered with
minute elevations, although an hour or so afterwards it appears as smooth
as glass.

Although it is a distinguishing characteristic 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