came to the conclusion that our
earth-moon system must ultimately attain a condition in which the day
is longer than the month. But this conclusion stood unsupported by any
analogous facts in the more anciently-known truths of astronomy. The
movement of the satellite of Mars, however, affords the precise
illustration we want; and this fact, I think, adds an additional
significance to the interest and the beauty of Professor Hall's
discovery.

It is of particular interest to investigate the possible connection
which the phenomena of tidal evolution may have had in connection with
the geological phenomena of the earth. We have already pointed out the
greater closeness of the moon to us in times past. The tides raised by
the moon on the earth must therefore have been greater in past ages
than they are now, for of course the nearer the moon the bigger the
tide. As soon as the earth and the moon had separated to a
considerable distance we may say that the height of the tide will vary
inversely as the cube of the moon's distance; it will therefore
happen, that when the moon was at half its present distance from us,
his tide-producing capacity was not alone twice as much or four times
as much, but even eight times as much as it is at present; and a much
greater rate of tidal rise and fall indicates, of course, a
preponderance in every other manifestation of tidal activity. The
tidal currents, for instance, must have been much greater in volume
and in speed; even now there are places in which the tidal currents
flow at four or more miles per hour. We can imagine, therefore, the
vehemence of the tidal currents which must have flowed in those days
when the moon was a much smaller distance from us. It is interesting
to view these considerations in their possible bearings on geological
phenomena. It is true that we have here many elements of uncertainty,
but there is, however, a certain general outline of facts which may
be laid down, and which appears to be instructive, with reference to
the past history of our earth.

I have all through these lectures indicated a mighty system of
chronology for the earth-moon system. It is true that we cannot give
our chronology any accurate expression in years. The various stages of
this history are to be represented by the successive distances between
the earth and the moon. Each successive epoch, for instance, may be
marked by the number of thousands of miles which separate the moon
from t