ts, but all had originated in mid-ocean. These considerations
alone render it almost certain that the areas now occupied by the great
oceans have never, during known geological time, been occupied by
continents, since it is in the highest degree improbable that every
fragment of those continents should have completely disappeared, and
have been replaced by volcanic islands rising out of profound oceanic
abysses; but recent research into the depth of the oceans and the nature
of the deposits now forming on their floors, adds greatly to the
evidence in this direction, and renders it almost a certainty that they
represent very ancient if not primaeval features of the earth's surface.
A very brief outline of the nature of this evidence will be now given.

The researches of the _Challenger_ expedition into the nature of the
sea-bottom show, that the whole of the land debris brought down by
rivers to the ocean (with the exception of pumice and other floating
matter), is deposited comparatively near to the shores, and that the
fineness of the material is an indication of the distance to which it
has been carried. Everything in the nature of gravel and sand is laid
down within a very few miles of land, only the finer muddy sediments
being carried out for 20 or 50 miles, and the very finest of all, under
the most favourable conditions, rarely extending beyond 150, or at the
utmost, 300 miles from land into the deep ocean.[164] Beyond these
distances, and covering the entire ocean floor, are various oozes formed
wholly from the debris of marine organisms; while intermingled with
these are found various volcanic products which have been either carried
through the air or floated on the surface, and a small but perfectly
recognisable quantity of meteoric matter. Ice-borne rocks are also found
abundantly scattered over the ocean bottom within a definite distance of
the arctic and antarctic circles, clearly marking out the limit of
floating icebergs in recent geological times.

Now the whole series of marine stratified rocks, from the earliest
Palaeozoic to the most recent Tertiary beds, consist of materials
closely corresponding to the land debris now being deposited within a
narrow belt round the shores of all continents; while no rocks have been
found which can be identified with the various oozes now forming in the
deep abysses of the ocean. It follows, therefore, that all the
geological formations have been formed in comparativel