But although the fossil remains of quadrupeds and other land-animals
are found in large quantities, their number is small compared with the
enormous number of fossil sea-shells and sea-animals.

[Illustration: FOSSIL FOOTPRINTS.]

Land-animals can, as a rule, have been so preserved, only when they
have been drowned in ponds or rivers, or mired in bogs and swamps, or
overtaken by frost, or swept out to sea.

Sea-animals, on the contrary, have been so preserved on land whenever
that land has been under the sea; and this appears to have been the
case, at one or another past age, with the greater part of our
present continents. These fossil remains of sea-animals are
discovered in all quarters of the world, not only on the seashore but
also far inland, not only deep down underground but also high up on
the tops of lofty mountains--a plain proof that over the summits of
those mountains the ocean must once have rolled, and this not for a
brief space only, but through long periods of time. And not on the
mountain-summit only are these fossils known to abound, but sometimes
in layer below layer of the mountain, from top to bottom, through
thousands of feet of rock.

[Illustration: FOSSIL SHELLS.]

This may well seem puzzling at first sight. Fossils of sea-creatures
on a mountain-top are startling enough; yet hardly so startling as the
thought of fossils _inside_ that mountain. How could they have found
their way thither?