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 number of craters, all so nearly of the same altitude. In shallow tracts of sea, coral reefs no doubt tend to assume the well-known circular form, but the difficulty was to account for the numerous atolls which rise to the surface from the abysses of the ocean, while the coral-forming zoophytes can only live near the surface.