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°, 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° 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.
THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN HEMISPHERES).
Photographed with the Lick Telescope.
It is not my intention to give a complete description of the various lunar features, and I mention but one other—the "clefts" or "rills," which are to be seen running across the surface like cracks. One of the most remarkable of these is found in the Oceanus Procellarum, near the crater-mountain Aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. The cleft, or crack, in question is very erratic in its course, and many miles in length, and it terminates in a ringed plain named Herodotus not far east of Aristarchus, breaking through the wall of the plain and entering the interior. Many other similar chasms or cañons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on the subject of the lunar clefts:
"If, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now a 'changeless world,' that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. Should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for."
Mr. Elger then proceeds to describe his discovery in 1883, in the ring-plain Mersenius, of a cleft never noticed before, and which seems to have been of recent formation.[15]