nature, you must have an eternity of witnesses or an infinity of
circumstances, and neither of these is attainable. It is utterly
impossible that such evidence should be carried beyond a certain point
of time; and all that could be said, at most, would be, that so far as
the evidence could be traced, there was nothing to contradict the
hypothesis. But when you look, not to the testimonial evidence--which,
considering the relative insignificance of the antiquity of human
records, might not be good for much in this case--but to the
circumstantial evidence, then you find that this hypothesis is
absolutely incompatible with such evidence as we have; which is of so
plain and simple a character that it is impossible in any way to escape
from the conclusions which it forces upon us.

[Illustration: FIG. 1.--IDEAL SECTION OF THE CRUST OF THE EARTH.]

You are, doubtless, all aware that the outer substance of the earth,
which alone is accessible to direct observation, is not of a homogeneous
character, but that it is made up of a number of layers or strata, the
titles of the principal groups of which are placed upon the accompanying
diagram. Each of these groups represents a number of beds of sand, of
stone, of clay, of slate, and of various other materials.

On careful examination, it is found that the materials of which each of
these layers of more or less hard rock are composed are, for the most
part, of the same nature as those which are at present being formed
under known conditions on the surface of the earth. For example, the
chalk, which constitutes a great part of the Cretaceous formation in
some parts of the world, is practically identical in its physical and
chemical characters with a substance which is now being formed at the
bottom of the Atlantic Ocean, and covers an enormous area; other beds of