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The conspicuous central mountain, so frequently associated with other types of ringed enclosures, is by no 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.
If we study any good modern lunar map, it is evident how constantly they appear near the borders of mountain ranges, walled-plains, and ring-plains; as, for instance, at the foot of the Apennines; near Archimedes, Aristarchus, Ramsden, and in many other similar positions.
Somewhat less than half of what we see of it consists of comparatively level dark tracts, some of them very many thousands of square miles in extent, the monotony of whose dusky superficies is often unrelieved for great distances by any prominent object; while the remainder, everywhere manifestly brighter, is not only more rugged and uneven, but is covered to a much greater extent with numbers of quasi-circular formations, differing widely in size, classed as walled-plains, ring-plains, craters, craterlets, crater-cones, &c.
The ring-plains vary in diameter from sixty to less than ten miles, and are far more regular in outline than the walled-plains. Their ramparts, often very massive, are more continuous, and fall with a steep declivity to a floor almost always greatly depressed below the outside region.
There is a longitudinal range in the interior, and on the W. and N.W. the remains of two large walled-plains, the more westerly of which is a noteworthy object under suitable conditions. A short distance S. is a large, irregular, and very dark marking. On the N., lies an immense bright plain, extending nearly to the border of Endymion. WEST LONGITUDE 60 deg. TO 40 deg.
MOUNTAIN RINGS. These objects, usually encircled by a low and broken border, seldom more than a few hundred feet in height, are closely allied to the walled-plains. They are more frequently found on the Maria than elsewhere.
These distinguishing titles, as adopted by Schroter, Lohrmann, and Madler, and accepted by subsequent observers, are WALLED-PLAINS, MOUNTAIN RINGS, RING-PLAINS, CRATERS, CRATER-CONES, CRATERLETS, CRATER-PITS, DEPRESSIONS. WALLED-PLAINS. These formations, approximating more or less to the circular form, though frequently deviating considerably from it, are among the largest enclosures on the moon.
While many of these features are so deep that they remain visible for a considerable time under a low sun, others, though perhaps of greater extent, vanish in an hour or so. As in the case of the walled-plains, the ramparts of the ring-plains exhibit gaps and are broken by craters and depressions, but to a much less extent.
These examples, very far from being exhaustive, will be sufficient to show that the walled-plains exhibit noteworthy differences in other respects than size, height of rampart, or included detail. Still another peculiarity, confined, it is believed, to a very few, may be mentioned, viz., convexity of floor, prominently displayed in Petavius, Mersenius, and Hevel.
There is a deep crater with a bright rim on its S.W. wall, and E. of this a notable gap. There is also a wide opening on the N. The E. wall is of the linear type. A cleft crosses the interior. GUERIKE. The most southerly member of a remarkable group of partially destroyed walled-plains, standing in an isolated position in the Mare Nubium.
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