[Illustration:  THE LUNAR ALPS, APENNINES, AND CAUCASUS.  Photographed with the Lick Telescope.]

Copernicus, a crater mountain situated about 10 deg. north of the equator, in the eastern hemisphere of the moon, is another wonderful object, 56 miles in diameter, a polygon appearing, when not intently studied, as a circle, 11,000 or 12,000 feet deep, and having a group of relatively low peaks in the center of its floor.  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., 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