an important part of the earth's
substance.

As the surface layers of the earth must have been the lightest, they
would necessarily, when broken up by this gigantic convulsion, have come
together to form the exterior of the new satellite, and be soon adjusted
by the forces of gravity and tidal disturbance into a more or less
irregular spheroidal form, all whose interstices and cavities would be
filled up and connected together by the liquid or semi-liquid mass
forced up between them. Thence-forward, as the moon increased its
distance and reduced its time of rotation, in the way explained by Sir
Robert Ball, there would necessarily commence a process of escape of the
imprisoned gases at every fissure and at all points and lines of
weakness, giving rise to numerous volcanic outlets, which, being
subjected only to the small force of lunar gravity (only one-sixth that
of the earth), would, in the course of ages, pile up those gigantic
cones and ridges which form its great characteristic.

But this small gravitative power of the moon would prevent its retaining
on its surface any of the gases forming our atmosphere, which would all
escape from it and probably be recaptured by the earth. By no process of
external aggregation of solid matter to such a relatively small amount
as that forming the moon, even if the aggregation was so violent as to
produce heat enough to cause liquefaction, could any such
long-continued volcanic action arise by gradual cooling, in the absence
of internal gases. There might be fissures, and even some outflows of
molten rock; but without imprisoned gases, and especially without water
and water-vapour producing explosive outbursts, could any such amount of
scoriae and ashes be produced as were necessary for the building up of
the vast volcanic cones, craters, and craterlets we see upon the moon's
surface.

I am not aware that either Sir Robert Ball or Sir George Darwin have
adduced this highly volcanic condition of the moon's surface as a
phenomenon which can _only_ be explained by our satellite having been
thrown off a very much larger body, whose gravitative force was
sufficient to acquire and retain the enormous quantity of gases and of
water which we possess, and which are _absolutely essential_ for that
_special form of cone-building volcanic action_ which the moon exhibits
in so pre-eminent a degree. Yet it seems to me clear, that some such
hypothetical origin for our satellite would have ha