It is in consequence of circumstances of this kind, and of others that I
might mention to you, that our conclusions on and interpretations of
the record are really and strictly only valid so long as we confine
ourselves to one vertical section. I do not mean to tell you that there
are no qualifying circumstances, so that, even in very considerable
areas, we may safely speak of conformably superimposed beds being older
or younger than others at many different points. But we can never be
quite sure in coming to that conclusion, and especially we cannot
be sure if there is any break in their continuity, or any very great
distance between the points to be compared.

Well now, so much for the record itself,--so much for its
imperfections,--so much for the conditions to be observed in
interpreting it, and its chronological indications, the moment we pass
beyond the limits of a vertical linear section.

Now let us pass from the record to that which it contains,--from the
book itself to the writing and the figures on its pages. This writing
and these figures consist of remains of animals and plants which, in the
great majority of cases, have lived and died in the very spot in which
we now find them, or at least in the immediate vicinity. You must all of
you be aware--and I referred to the fact in my last lecture--that there
are vast numbers of creatures living at the bottom of the sea. These
creatures, like all others, sooner or later die, and their shells and
hard parts lie at the bottom; and then the fine mud which is being
constantly brought down by rivers and the action of the wear and tear of
the sea, covers them over and protects them from any further change
or alteration; and, of course, as in process of time the mud becomes
hardened and solidified, the shells of these animals are preserved
and firmly imbedded in the limestone or sandstone which is being thus
formed. You may see in the galleries of the Museum up stairs specimens
of limestones in which such fossil remains of existing animals are
imbedded. There are some specimens in which turtles' eggs have been
imbedded in calcareous sand, and before the sun had hatched the young
turtles, they became covered over with calcareous mud, and thus have
been preserved and fossilized.

Not only does this process of imbedding and fossilization occur with
marine and other aquatic animals and plants, but it affects those land
animals and plants which are drifted away to sea, or