in the form of
obscure rings (mere scars on the surface), dusky craters, circular
arrangements of isolated hills, reminding one of the monoliths of a
Druidical temple; all of which we are justified in concluding were at one
time formations of a normal type. It has been held by some selenologists
--and Schmidt appears to be of the number,--that, seeing the comparative
scarcity of large ring-plains and other massive formations on the Maria,
these grey plains represent, as it were, a picture of the primitive
surface of the moon before it was disturbed by the operations of interior
forces; but this view affords no explanation of the undoubted existence
of the relics of an earlier lunar world beneath their smooth superficies.

MARIA.--Leaving, however, these considerations for a more particular
description of the Maria, it is clearly impossible, in referring to their
level relatively to the higher and brighter land surface of the moon, to
appeal to any hypsometrical standard. All that is known in this respect
is, that they are invariably lower than the latter, and that some sink to
a greater depth than others, or, in other words, that they do not all
form a part of the same sphere. Though they are more or less of a
greyish-slaty hue--some of them approximating very closely to that of the
pigment known as "Payne's grey"--the tone, of course, depends upon the
angle at which the solar rays impinge on that particular portion of the
surface under observation. Speaking generally, they are, as would follow
from optical considerations, conspicuously darker when viewed near the
terminator, or when the sun is either rising or setting upon them, than
under a more vertical angle of illumination. But even when it is possible
to compare their colour by eye-estimation under similar solar altitudes,
it is found that not only are some of the Maria, as a whole, notably
darker than others, but nearly all of them exhibit _local_ inequalities
of hue, which, under good atmospheric and instrumental conditions, are
especially remarkable. Under such circumstances I have frequently seen
the surface, in many places covered with minute glittering points of
light, shining with a silvery lustre, intermingled with darker spots and
a network of streaks far too delicate and ethereal to represent in a
drawing. In addition to these contrasts and differences in the sombre
tone of these extended plains, many observers have remarked traces of a
yellow or green ti