Around Copernicus an extensive area of
the moon's surface is whitened with something resembling the rays of
Tycho, but more irregular in appearance. Copernicus lies within the edge
of the great plain named the _Oceanus Procellarum_, or "Ocean of
Storms," and farther east, in the midst of the "ocean," is a smaller
crater mountain, named Kepler, which is also enveloped by a whitish
area, covering the lunar surface as if it were the result of extensive
outflows of light-colored lava.
In one important particular the crater mountains of the moon differ from
terrestrial volcanoes. This difference is clearly 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.
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