described by Nasmyth
and Carpenter in their book on The Moon:

"While the terrestrial crater is generally a hollow on a mountain top,
with its flat bottom high above the level of the surrounding country,
those upon the moon have their lowest points depressed more or less
deeply below the general surface of the moon, the external height being
frequently only a half or one third of the internal depth."

It has been suggested that these gigantic rings are only "basal wrecks"
of volcanic mountains, whose conical summits have been blown away,
leaving vast crateriform hollows where the mighty peaks once stood; but
the better opinion seems to be that which assumes that the rings were
formed by volcanic action very much as we now see them. If such a crater
as Copernicus or the still larger one named Theophilus, which is
situated in the western hemisphere of the moon, on the shore of the "Sea
of Nectar," ever had a conical mountain rising from its rim, the height
attained by the peak, if the average slope were about 30 deg., would have
been truly stupendous--fifteen or eighteen miles!

There is a kind of ring mountains, found in many places on the moon,
whose forms and surroundings do not, as the craters heretofore described
do, suggest at first sight a volcanic origin. These are rather level
plains of an oval or circular outline, enclosed by a wall of mountains.
The finest example is, perhaps, the dark-gray Plato, situated in 50 deg. of
north latitude, near an immense mountain uplift named the Lunar Alps,
and on the northern shore of the _Mare Imbrium_, or "Sea of Showers."
Plato appears as an oval plain, very smooth and level, about 60 miles in
length, and completely surrounded by mountains, quite precipitous on the
inner side, and rising in their highest peaks to an elevation of 6,000
to 7,000 feet. Enclosed plains, bearing more or less resemblance to
Plato--sometimes smooth within, and sometimes broken with small peaks
and craters or hilly ridges--are to be found scattered over almost all
parts of the moon. If our satellite was ever an inhabited world like the
earth, while its surface was in its present condition, these valleys
must have presented an extraordinary spectacle. It has been thought that
they may once have been filled with water, forming lakes that recall the
curious Crater Lake of Oregon.

[Illustration: THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN
HEMISPHERES). Photographed with the Lick Telescop