Fig. 707.—Coral (Madrepora brachiata).
The coral insect is a zoophyte (Anthozoa), which, as may be seen from the illustrations, assumes curious and elegant forms, and the coral it produces is a limy or calcareous deposit, which is fixed upon a rocky base. As years go on these accretions become greater and greater, and at length rise above the water. When a little distance below it, the reefs form dangerous and frequently unsuspected barriers, upon which ships are wrecked. The red coral is dredged up from the Mediterranean, where there are extensive coral fisheries. This coral is found deep in the water, and never rises to the surface. Formerly there were coral reefs in the European seas, but the changes of temperature stopped their production. The “atolls,” or circular coral reefs with an opening at one side, have been described by Professor Darwin. “Who,” says the great naturalist, “would not be struck with wonder and admiration on catching sight for the first time of this vast ring of coral rock, often many miles in diameter? Sometimes a low green island is seen beyond it, with a shore of dazzling whiteness; outside is the foaming surf of the ocean, and within it a broad expanse of tranquil water, of pale green colour and exquisite purity.” These “atolls” mark the situation of sunken islands, and the extension of them and the barrier reefs would seem to indicate a slow but decided sinking of the bottom of the Indian and other oceans; but the “reefs” tell us that the land to which they are attached has not become depressed, and may have become elevated. We may then conclude that a continual rising and depression of the land is taking place in various oceans, indicating a sinking of the ocean bed in one locality and the result of volcanic activity in another, for no active volcanoes are found in the regions of depression.
Fig. 708.—Spicules of Gorgonia (magnified).
We must now leave the sea and come to land again, to consider volcanoes and volcanic action there.
Volcanoes and Earthquakes.
The various phenomena of volcanoes form a subject very difficult to be explained, as it is impossible to ascertain positively the cause of volcanic action. Whether the earth is interiorly a mass of molten rock and fire, or whether the heat is created by the intense contractile force and movement of rocks, and their motion thus developed into heat aided by chemical combination, we cannot absolutely determine. The theory restricting volcanic phenomena to the upper crust of the earth, by supposing the local accumulations of hot liquid masses of rock, which are forcibly emptied by the expansion of vapours, may perhaps be found the true one.
The majority of the volcanoes are found near, or at no very great distance from, the sea. We may therefore expect to find that water has something to do with the eruptions as it has in the case of the Geysers. But this hypothesis will scarcely hold good in every case, though volcanoes of later ages are limited to regions very different from those in which volcanic action used to be. For instance, in America we have only volcanoes on the Pacific side, and the Andes furnish several. Mexico, Central America, and California possess many volcanoes, and as far north as Alaska we find Mount Elias. There are plenty of extinct volcanoes in Europe, but the Mediterranean produces the active vents; and about the Red Sea and the Caspian, and even in the central chain of Asia, there are volcanoes far from water. The Hawaii isles, on the other hand, are all volcanic, and Australasia furnishes us with remarkable specimens; so altogether the testimony tends to prove that where volcanic remains are apparent the sea had at one time been, or now is, near at hand.
Burning mountains have been familiar to us from our childhood in pictures, and by stirring narrative of destruction wrought by them. The volcano is generally a mountain rising to a cone, but Vesuvius presented quite the appearance of a hollow basin at the top, before it suddenly broke forth and buried Herculaneum in ashes. Von Buck visited it in 1799, and declares it had at one time risen, like an island, from the sea. There are about two hundred and seventy volcanoes at present in activity; four in Europe; eleven in Iceland and Jan Meyen’s land; in Asia, ninety-three; in Africa, twenty-six; forty-six in North America and the Aleutian Isles; twenty-seven in Central America and the Antilles; in South America, thirty-one; and twenty-four islands with volcanic tendencies largely developed. There may be many more “resting.”