n parted company, their respective
temperatures were equal, the moon being so much the smaller of the two
would have cooled more rapidly, and the surface may have been covered by
a rigid crust when as yet that of the earth may have been molten from
heat. Hence the rigidity of the moon's surface may date back to an
immensely distant period, but she may still retain a high temperature
within this crust. Having arrived at this stage of our narrative, we are
in a position to consider by what means, and under what conditions, the
cones and craters which diversify the lunar surface have been developed.

In doing so it may be desirable, in the first place, to determine what
form of crater on our earth's surface those of the moon do not
represent; and we are guided in our inquiry by the consideration of the
absence of water on the lunar surface. Now there are large numbers of
crateriform mountains on our globe in the formation of which water has
played an important, indeed essential, part. As we have already seen,
water, though not the ultimate cause of volcanic eruptions, has been the
chief agent, when in the form of steam at high pressure, in producing
the explosions which accompany these eruptions, and in tearing up and
hurling into the air the masses of rock, scoriae, and ashes, which are
piled around the vents of eruption in the form of craters during periods
of activity. To this class of craters those of Etna, Vesuvius, and
Auvergne belong. These mountains and conical hills (the domes excepted)
are all built up of accumulations of fragmental material, with
occasional sheets and dykes of lava intervening; and where eruptions
have taken place in recent times, observation has shown that they are
accompanied by outbursts of vast quantities of aqueous vapour, which has
been the chief agent (along with various gases) in piling up the
circular walls of the crater.

It has also been shown that in many instances these crater-walls have
been breached on one side, and that streams of molten lava which once
occupied the cup to a greater or less height, have poured down the
mountain side. Hence the form or outline of many of these fragmental
craters is crescent-shaped. Such breached craters are to be found in all
parts of the world, and are not confined to any one district, or even
continent, so that they may be considered as characteristic of the class
of volcanic crater-cones to which I am now referring. In the case of the
moon, ho