e. The Mare
Serenitatis and the Mare Imbrium, in the northern hemisphere, are also
remarkable for the number of these peculiar features. They are very
plentifully distributed round the margin and in other parts of the
former, which includes besides one of the longest and loftiest on the
moon's visible surface--the great serpentine ridge, first drawn and
described nearly a hundred years ago by the famous selenographer,
Schroter of Lilienthal. Originating at a little crater under the north-
east wall of great ring-plain Posidonius, it follows a winding course
across the Mare toward the south, throwing out many minor branches, and
ultimately dies out under a great rocky promontory--the Promontory
Acherusia, at the western termination of the Haemus range. A
comparatively low power serves to show the curious structural character
of this immense ridge, which appears to consist of a number of
corrugations and folds massed together, rising in places, according to
Neison, to a height of 700 feet and more. The Mare Imbrium also affords
an example of a ridge, which, though shorter, is nearly as prominent, in
that which runs from the bright little ring-plain Piazzi Smyth towards
the west side of Plato. The region round Timocharis and other quarters of
the Mare are likewise traversed by very noteworthy features of a similar
class. The Oceanus Procellarum also presents good instances of ridges in
the marvellous ramifications round Encke, Kepler, and Marius, and in the
region north of Aristarchus and Herodotus. Perhaps the most perfect
examples of surface swellings are those in the Mare Tranquilitatis, a
little east of the ring-plain Arago, where there are two nearly equal
circular mounds, at least ten miles in diameter, resembling tumuli seen
from above. Similar, but more irregular, objects of a like kind are very
plentiful in many other quarters.

It is a suggestive peculiarity of many of the lunar ridges, both on the
Maria and elsewhere, that they are very generally found in association
with craters of every size. Illustrations of this fact occur almost
everywhere. Frequently small craters are found on the summits of these
elevations, but more often on their flanks and near their base. Where a
ridge suddenly changes its direction, a crater of some prominence
generally marks the point, often forming a node, or crossing-place of
other ridges, which thus appear to radiate from it as a centre. Sometimes
they intrude within the smaller