analogy with the lava-craters present on our globe, we must suppose
them to be due to the extrusion, and piling up, of lava through central
pipes, followed in some cases by the subsidence of the floor of the
crater. It seems not improbable that it was in this way the greater
number of the circular craters lying around Tycho, and dotting so large
a space round the margin of the moon, were constructed. (See Fig. 38.)
In general they appear to consist of an elevated rim, enclosing a
depressed plain, out of which a central cone arises. The rim may be
supposed to have been piled up by successive discharges of lava from a
central orifice; and after the subsidence of the paroxysm the lava still
in a molten condition may have sunk down, forming a seething lake within
the vast circular rampart, as in the case of the Hawaiian volcanoes. The
terraces observable within the craters in some instances have probably
been left by subsequent eruptions which have not attained to the level
of preceding ones; and where a central crater-cone is seen to rise
within the caldron, we may suppose this to have been built up by a later
series of eruptions of lava through the original pipe after the
consolidation of the interior sea of lava. The mamelons of the Isle of
Bourbon,[15] and some of the lava-cones of Hawaii, appear to offer
examples on our earth's surface of these peculiar forms.

Such are the views of the origin of the physical features of our
satellite which their form and inferred constitution appear to suggest.
They are not offered with any intention of dogmatising on a subject
which is admittedly obscure, and regarding which we have by no means all
the necessary data for coming to a clear conclusion. All that can be
affirmed is, that there is a great deal to be said in support of them,
and that they are to some extent in harmony with phenomena within range
of observation on the surface of our earth.

The far greater effects of lunar vulcanicity, as compared with those of
our globe, may be accounted for to some extent by the consideration that
the force of gravity on the surface of the moon is only one-sixth of
that on the surface of the earth. Hence the eruptive forces of the
interior of our satellite have had less resistance to overcome than in
the case of our planet; and the erupted materials have been shot forth
to greater distances, and piled up in greater magnitude, than with us.
We have also to recollect that the abrading actio