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On the N. is a large peculiar depressed plain with a gently sloping wall, within which are three short rill-like valleys and a crater. MERSENIUS. With its extensive rill-system and interesting surroundings, one of the most notable ring-plains in the third quadrant.
They abound in minute detail bright little mountains and ridges and include some clefts pertaining to the Mersenius rill-system; but their most noteworthy feature is the long bright mountain-arm, branching out from the eastern wall of Gassendi, and extending for more than 50 miles towards the south-east.
A fine mountain range, the Percy Mountains, is connected with the E. flank of Gassendi, extending in a S.E. direction towards Mersenius, and defining the N.E. side of the Mare Humorum. BULLIALDUS. A noble object, 38 miles in diameter, forming with its surroundings by far the most notable formation on the surface of the Mare Nubium, and one of the most characteristic ring-plains on the moon.
The border is broken on the E. by a crater, and on the W. by the well-known crater Byrgius A, from which a number of bright streaks radiate, mostly towards the E. One on the W. extends to Cavendish, and another to Mersenius, traversing the ring-plain Cavendish C. North-east of Byrgius there is a mountain arm which includes a peak 13,000 feet in height.
Mersenius is a formation about 40 miles in diameter, with a prominently convex interior, containing much detail which has received more than ordinary attention from observers. It has, moreover, been specially mapped by Schmidt and others, yet no trace of this rill was noted, though objects much more minute and difficult have not been overlooked.
CAVENDISH. A notable ring-plain, 32 miles in diameter, S.E. of Mersenius, with a prominently terraced border, rising at one point on the S. to a height of 6000 feet above the interior, on which are a few low ridges. There is also a bright crater on the N.W. border. The W. glacis is very broad, and includes two large shallow depressions.
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.
Cavendish d includes a coarse cleft on its floor, running from N. to S., which I have frequently glimpsed with a 4 inch achromatic. There are two other delicate clefts running from the Gassendi region to the S.W. side of Mersenius, which are in part crater-rills.
ZUPUS. A formation about 12 miles in diameter with a dark floor, situated in the hilly region N.E. of Mersenius. FONTANA. A noteworthy ring-plain, about 20 miles in diameter, E.N.E. of Zupus, with a bright border, exhibiting a narrow gap on the S. and two large contiguous craters on the N.W. The faint central mountain stands on a dusky interior.
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.
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