y shallow water,
and always adjacent to the continental land of the period. The great
thickness of some of the formations is no indication of a deep sea, but
only of slow subsidence during the time that the deposition was in
progress. This view is now adopted by many of the most experienced
geologists, especially by Dr. Archibald Geikie, Director of the
Geological Survey of Great Britain, who, in his lecture on "Geographical
Evolution," says: "From all this evidence we may legitimately conclude
that the present land of the globe, though consisting in great measure
of marine formations, has never lain under the deep sea; but that its
site must always have been near land. Even its thick marine limestones
are the deposits of comparatively shallow water."[165]

But besides these geological and physical considerations, there is a
mechanical difficulty in the way of repeated change of position of
oceans and continents which has not yet received the attention it
deserves. According to the recent careful estimate by Mr. John Murray,
the land area of the globe is to the water area as .28 to .72. The mean
height of the land above sea-level is 2250 feet, while the mean depth of
the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic
miles, and that of the waters of the ocean 323,800,000 cubic miles; and
it follows that if the whole of the solid matter of the earth's surface
were reduced to one level, it would be everywhere covered by an ocean
about two miles deep. The accompanying diagram will serve to render
these figures more intelligible. The length of the sections of land and
ocean are in the proportion of their respective areas, while the mean
height of the land and the mean depth of the ocean are exhibited on a
greatly increased vertical scale. If we considered the continents and
their adjacent oceans separately they would differ a little, but not
very materially, from this diagram; in some cases the proportion of land
to ocean would be a little greater, in others a little less.

[Illustration: FIG. 32.]

Now, if we try to imagine a process of elevation and depression by which
the sea and land shall completely change places, we shall be met by
insuperable difficulties. We must, in the first place, assume a general
equality between elevation and subsidence during any given period,
because if the elevation over any extensive continental area were not
balanced by some subsidence of approximately equal amount,