less than 600 teeth and 100 ear-bones), often
semi-fossil, and which from their great density had remained intact for
ages, long after all the softer parts had perished and disappeared.

The greatest depth of the Ocean appears to coincide roughly with the
greatest height of the mountains. There are indeed cases recorded in
which it is said that "no bottom" was found even at 39,000 feet. It is,
however, by no means easy to sound at such great depths, and it is now
generally considered that these earlier observations are untrustworthy.
The greatest depth known in the Atlantic is 3875 fathoms--a little to
the north of the Virgin Islands, but the soundings as yet made in the
deeper parts of the Ocean are few in number, and it is not to be
supposed that the greatest depth has yet been ascertained.

CORAL ISLANDS

In many parts of the world the geography itself has been modified by the
enormous development of animal life. Most islands fall into one of three
principal categories:

Firstly, Those which are in reality a part of the continent near which
they lie, being connected by comparatively shallow water, and standing
to the continent somewhat in the relation of planets to the sun; as,
for instance, the Cape de Verde Islands to Africa, Ceylon to India, or
Tasmania to Australia.

Secondly, Volcanic islands; and

Thirdly, Those which owe their origin to the growth of Coral reefs.

[Illustration: Fig. 49.--Whitsunday Island.]

Coral islands are especially numerous in the Indian and Pacific Oceans,
where there are innumerable islets, in the form of rings, or which
together form rings, the rings themselves being sometimes made up of
ringlets. These "atolls" contain a circular basin of yellowish green,
clear, shallow water, while outside is the dark blue deep water of the
Ocean. The islands themselves are quite low, with a beach of white sand
rising but a few feet above the level of the water, and bear generally
groups of tufted Cocoa Palms.

It used to be supposed that these were the summits of submarine
volcanoes on which the coral had grown. But as the reef-making coral
does not live at greater depths than about twenty-five fathoms, the
immense number of these reefs formed an almost insuperable objection to
this theory. The Laccadives and Maldives for instance--meaning literally
the "lac of or 100,000 islands," and the "thousand islands"--are a
series of such atolls, and it was impossible to imagine so great a
num