is clearly divisible into strongly contrasted areas, differing both in
colour and structural character. Somewhat less than half of what we see
of it consists of comparatively level dark tracts, some of them very many
thousands of square miles in extent, the monotony of whose dusky
superficies is often unrelieved for great distances by any prominent
object; while the remainder, everywhere manifestly brighter, is not only
more rugged and uneven, but is covered to a much greater extent with
numbers of quasi-circular formations, differing widely in size, classed
as walled-plains, ring-plains, craters, craterlets, crater-cones, &c.
(the latter bearing a great outward resemblance to some terrestrial
volcanoes), and mountain ranges of vast proportions, isolated hills, and
other features.

Though nothing resembling sheets of water, either of small or large
extent, have ever been detected on the surface, the superficial
resemblance, in small telescopes, of the large grey tracts to the
appearance which we may suppose our terrestrial lakes and oceans would
present to an observer on the moon, naturally induced the early
selenographers to term them Maria, or "seas"--a convenient name, which
is still maintained, without, however, implying that these areas, as we
now see them, are, or ever were, covered with water. Some, however,
regard them as old sea-beds, from which every trace of fluid, owing to
some unknown cause, has vanished, and that the folds and wrinkles, the
ridges, swellings, and other peculiarities of structure observed upon
them, represent some of the results of alluvial action. It is, of course,
possible, and even probable, that at a remote epoch in the evolution of
our satellite these lower regions were occupied by water, but that their
surface, as it now appears, is actually this old sea-bottom, seems to be
less likely than that it represents the consolidated crust of some semi-
fluid or viscous material (possibly of a basaltic type) which has welled