in the south-
western quadrant, is scarcely half so big as the Mare Imbrium; while the
Maria Serenitatis and Tranquilitatis, about equal in area (the former
situated wholly north of the equator, and the latter only partially
extending south of it), are still smaller. The arctic Mare Frigoris, some
100,000 square miles in extent, is the only remaining large sea,--the
rest, such as the Mare Vaporum, the Sinus Medii, the Mare Crisium, the
Mare Humorum, and the Mare Humboldtianum, are of comparatively small
dimensions, the Mare Crisium not greatly exceeding 70,000 square miles,
the Mare Humorum (about the size of England) 50,000 square miles, while
the Mare Humboldtianum, according to Schmidt, includes only about 42,000
square miles, an area which is approached by some formations not classed
with the Maria. This distinction, speaking generally, prevails among the
Maria,--those of larger size, such as the Oceanus Procellarum, the Mare
Nubium, and the Mare Foecunditatis, are less definitely enclosed, and,
like terrestrial oceans, communicate with one another; while their
borders, or, if the term may be allowed, their coast-line, is often
comparatively low and ill-defined, exhibiting many inlets and
irregularities in outline. Others, again, of considerable area, as, for
example, the Mare Serenitatis and the Mare Imbrium, are bounded more or
less completely by curved borders, consisting of towering mountain
ranges, descending with a very steep escarpment to their surface: thus in
form and other characteristics they resemble immense wall-surrounded
plains. Among the best examples of enclosed Maria is the Mare Crisium,
which is considered by Neison to be the deepest of all, and the Mare
Humboldtianum.

Though these great plains are described as level, this term must only be
taken in a comparative sense. No one who observes them when their surface
is thrown into relief by the oblique rays of the rising or setting sun
can fail to remark many low bubble-shaped swellings with gently rounded
outlines, shallow trough-like hollows, and, in the majority of them, long
sinuous ridges, either running concentrically with their borders or
traversing them from side to side. Though none of these features are of
any great altitude or depth, some of the ridges are as much as 700 feet
in height, and probably in many instances the other elevations often rise
to 150 feet or more above the low-lying parts of the plains on which they
stand. Hence we