know formed, and still form, 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 lique-
