The Altai Mountains form a magnificent chain, 275 miles in length, commencing on the outer eastern slope of Piccolomini, and following a tolerably direct north-east course, with a few minor bendings, to the west side of Fermat, where they turn more towards the north, ultimately terminating about midway between Tacitus and Catherina.  The region situated on the south-east is a great table-land, without any prominent features, rising gently towards the mountains, which shelve steeply down to an equally barren expanse on the north-west, to which they present a lofty face, having an average altitude of about 6000 feet.  The loftiest peak, over 13,000 feet, rises west of Fermat.

The Riphaean Mountains, a remarkably bright group, occupying an isolated position in the Mare Procellarum south of Landsberg, and extending for more than 100 miles in a meridional direction.  They are most closely aggregated at a point nearly due west of Euclides, from which they throw off long-branching arms to the north and south, those on the north bifurcating and gradually sinking to the level of the plain.  The loftiest peaks are near the extremity of this section, one of them rising to 3000 feet.  Two bright craters are associated with these mountains, one nearly central, and the other south of it.

The Percy Mountains.—­This name is given to the bright highlands extending east of Gassendi towards Mersenius, forming the north-eastern border of the Mare Humorum.  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.

The principal ranges on the limb are the Leibnitz Mountains, extending from S. lat. 70 deg. on the west to S. lat. 80 deg. on the east limb.  They include some giant peaks and plateaus, noteworthy objects in profile, some of which, according to Schroter and Madler, rise to 26,000 feet.  The Dorfel Mountains, between S. lat. 80 deg. and 57 deg. on the eastern limb, include, if Schroter’s estimate is correct, three peaks which exceed 26,000 feet.  On the eastern limb, between S. lat. 35 deg. and 18 deg., extend the Rook Mountains, which have peaks, according to Schroter, as high as 25,000 feet.  Next in order come the Cordilleras, which extend to S. lat. 8 deg., and the D’Alembert Mountains, lying east of Rocca and Grimaldi, closely associated with them, and probably part of the same system.  Some of the peaks approach 20,000 feet.  In addition to these mountain ranges there are others less prominent on the limb in the northern hemisphere, which have not been named.

ISOLATED MOUNTAINS are very numerous in different parts of the moon, the most remarkable are referred to in the appendix.  Many remain unnamed.