nt on the surface of some of them. For example, the
Mare Imbrium and the Mare Frigoris appear under certain conditions to be
of a dirty yellow-green hue, the central parts of the Mare Humorum dusky
green, and part of the Mare Serenitatis and the Mare Crisium light green,
while the Palus Somnii has been noted a golden-brown yellow. To these may
be added the district round Taruntius in the Mare Foecunditatis, and
portions of other regions referred to in the catalogue, where I have
remarked a very decided sepia colour under a low sun. It has been
attempted to account for these phenomena by supposing the existence of
some kind of vegetation; but as this involves the presence of an
atmosphere, the idea hardly finds favour at the present time, though
perhaps the possibility of plant growth in the low-lying districts, where
a gaseous medium may prevail, is not altogether so chimerical a notion as
to be unworthy of consideration. Nasmyth and others suggest that these
tints may be due to broad expanses of coloured volcanic material, an
hypothesis which, if we believe the Maria to be overspread with such
matter, and knowing how it varies in colour in terrestrial volcanic
regions, is more probable than the first. Anyway, whether we consider
these appearances to be objective, or, after all, only due to purely
physiological causes, they undoubtedly merit closer study and
investigation than they have hitherto received.

There are twenty-three of these dusky areas which have received
distinctive names; seventeen of them are wholly, or in great part,
confined to the northern, and to the south-eastern quarter of the
southern hemisphere--the south-western quadrant being to a great extent
devoid of them. By far the largest is the vast Oceanus Procellarum,
extending from a high northern latitude to beyond latitude 10 deg. in the
south-eastern quadrant, and, according to Schmidt, with its bays and
inflections, occupying an area of nearly two million square miles, or
more than that of all the remaining Maria put together. Next in order of
size come the Mare Nubium, of about one-fifth the superficies, covering a
large portion of the south-eastern quadrant, and extending considerably
north of the equator, and the Mare Imbrium, wholly confined to the
northeastern quadrant, and including an area of about 340,000 square
miles. These are by far the largest lunar "seas." The Mare Foecunditatis,
in the western hemisphere, the greater part of it lying