class (well known to observers as one of the grandest of lunar objects), has a deeply sunken floor, fringed with mountains rising some 12,000 feet above it, though they scarcely stand a fourth of this height above the plain on the west, which ascends with a very gentle gradient to the summit of the wall.  Hence the contrast between the shadows of the peaks of the western wall on the floor at sunrise, and of the same peaks on the region west of the border at sunset is very marked.  In Gassendi, Phocylides, and Wargentin we have similar notable departures from the normal type.  The floor of the former on the north stands 2000 feet above the Mare Humorum.  In Phocylides, probably through “faulting,” one portion of the interior suddenly sinks to a considerable depth below the remainder; while the very abnormal Wargentin has such an elevated floor, that, when viewed under favourable conditions, it reminds one of a shallow oval tray or dish filled with fluid to the point of overflowing.  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.

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.  In some cases the ring consists of isolated dark sections, with here and there a bright mass of rock interposed; in others, of low curvilinear ridges, forming a more or less complete circumvallation.  They vary in size from 60 or 70 miles to 15 miles and less.  The great ring north of Flamsteed, 60 miles across, is a notable example; another lies west of it on the north of Wichmann; while a third will be found south-east of Encke;—­indeed, the Mare Procellarum abounds in objects of this type.  The curious formation on the Mare Imbrium immediately south of Plato (called “Newton” by Schroter), may be placed in this category, as may also many of the low dusky rings of much smaller dimensions found in many quarters of the Maria.  As has been stated elsewhere, these features have the appearance of having once been formations of a much more prominent and important character, which have suffered destruction, more or less complete, through being partially overwhelmed by the material of the “seas.”