_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