moon once split open along this line?

The _Mare Serenitatis_ is encircled by mountain ranges to a greater
extent than any of the other lunar "seas." On its eastern side the
Caucasus and the Apennines shut it in, except for a strait a hundred
miles broad, by means of which it is connected with the _Mare Imbrium_.
On the south the range of the Haemus Mountains borders it, on the north
and northwest the Caucasus and the Taurus Mountains confine it, while on
the west, where again it connects itself by a narrow strait with another
"sea," the _Mare Tranquilitatis_, it encounters the massive uplift of
Mount Argaeus. Not far from the eastern strait is found the remarkable
little crater named Linne, not conspicuous on the gray floor of the
_Mare_, yet easily enough found, and very interesting because a
considerable change of form seems to have come over this crater some
time near the middle of the nineteenth century. In referring to it as a
crater it must not be forgotten that it does not form an opening in the
top of a mountain. In fact, the so-called craters on the moon, generally
speaking, are simply cavities in the lunar surface, whose bottoms lie
deep below the general level, instead of being elevated on the summit of
mountains, and inclosed in a conical peak. In regard to the alleged
change in Linne, it has been suggested, not that a volcanic eruption
brought it about, but that a downfall of steep walls, or of an
unsupported rocky floor, was the cause. The possibility of such an
occurrence, it must be admitted, adds to the interest of the observer
who regularly studies the moon with a telescope.

Just on the southern border of the _Mare_, the beautiful ring Menelaus
lies in the center of the chain of the Haemus Mountains. The ring is
about twenty miles across, and its central peak is composed of some
highly reflecting material, so that it shines very bright. The streak or
ray from Tycho which crosses the _Mare Serenitatis_ passes through the
walls of Menelaus, and perhaps the central peak is composed of the same
substance that forms the ray. Something more than a hundred miles
east-southeast from Menelaus, in the midst of the dark _Mare Vaporum_,
is another brilliant ring mountain which catches the eye, Manilius. It
exceeds Menelaus in brightness as well as in size, its diameter being
about twenty-five miles. There is something singular underlying the dark
lunar surface here, for not only is Manilius extraordinarily bril