oons revolve round him more quickly than the planet rotates:
hence with them the process is inverted, and they must be approaching
him and may some day crash along his surface. The inner moon is now
about 4,000 miles away, and revolves in 7-1/2 hours. It appears to be
about 20 miles in diameter, and weighs therefore, if composed of rock,
40 billion tons. Mars rotates in 24-1/2 hours.

A similar fate may _possibly_ await our moon ages hence--by reason of
the action of terrestrial tides produced by the sun.

LECTURE XVIII

THE TIDES, AND PLANETARY EVOLUTION

In the last lecture we considered the local peculiarities of the tides,
the way in which they were formed in open ocean under the action of the
moon and the sun, and also the means by which their heights and times
could be calculated and predicted years beforehand. Towards the end I
stated that the subject was very far from being exhausted, and
enumerated some of the large and interesting questions which had been
left untouched. It is with some of these questions that I propose now to
deal.

I must begin by reminding you of certain well-known facts, a knowledge
of which I may safely assume.

And first we must remind ourselves of the fact that almost all the rocks
which form the accessible crust of the earth were deposited by the
agency of water. Nearly all are arranged in regular strata, and are
composed of pulverized materials--materials ground down from
pre-existing rocks by some denuding and grinding action. They nearly all
contain vestiges of ancient life embedded in them, and these vestiges
are mainly of marine origin. The strata which were once horizontal are
now so no longer--they have been tilted and upheaved, bent and
distorted, in many places. Some of them again have been metamorphosed by
fire, so that their organic remains have been destroyed, and the traces
of their aqueous origin almost obliterated. But still, to the eye of the
geologist, all are of aqueous or sedimentary origin: roughly speaking,
one may say they were all deposited at the bottom of some ancient sea.

The date of their formation no man yet can tell, but that it was vastly
distant is certain. For the geological era is not over. Aqueous action
still goes on: still does frost chip the rocks into fragments; still do
mountain torrents sweep stone and mud and _debris_ down the gulleys and
watercourses; still do rivers erode their channels, and carry mud and
silt far out to sea.