he earth.

But we have another system of chronology derived from a wholly
different system of ideas; it too relates to periods of vast duration,
and, like our great tidal periods, extends to times anterior to human
history, or even to the duration of human life on this globe. The
facts of geology open up to us a majestic chronology, the epochs of
which are familiar to us by the succession of strata forming the crust
of the earth, and by the succession of living beings whose remains
these strata have preserved. From the present or recent age our
retrospect over geological chronology leads us to look through a
vista embracing periods of time overwhelming in their duration, until
at last our view becomes lost, and our imagination is baffled in the
effort to comprehend the formation of those vast stratified rocks, a
dozen miles or more in thickness, which seem to lie at the very base
of the stratified system on the earth, and in which it would appear
that the dawnings of life on this globe may be almost discerned. We
have thus the two systems of chronology to compare--one, the
astronomical chronology measured by the successive stages in the
gradual retreat of the moon; the other, the geological chronology
measured by the successive strata constituting the earth's crust.
Never was a more noble problem proposed in the physical history of our
earth than that which is implied in the attempt to correlate these two
systems of chronology. What we would especially desire to know is the
moon's distance which corresponds to each of the successive strata on
the earth. How far off, for instance, was that moon which looked down
on the coal forests in the time of their greatest luxuriance? or what
was the apparent size of the full moon at which the ichthyosaurus
could have peeped when he turned that wonderful eye of his to the sky
on a fine evening? But interesting as this great problem is, it lies,
alas! outside the possibility of exact solution. Indeed we shall not
make any attempt which must necessarily be futile to correlate these
chronologies; all we can do is to state the one fact which is
absolutely undeniable in the matter.

Let us fix our attention on that specially interesting epoch at the
dawn of geological time, when those mighty Laurentian rocks were
deposited of which the thickness is so astounding, and let us consider
what the distance of the moon must have been at this initial epoch of
the earth's history. All we know f