1100.m.

northern coast of New Holland is more than one thousand miles long: a link of three hundred and fifty miles of it is 30 m wide continuous, with no passage or opening through it. Dis3300 sq.m. appointment Islands and Duff's Group are connected by $645208 coral reefs so continuous that the natives travel over them from one island to another. Reefs occur in the Pacific ocean from one thousand one hundred to one thousand two hundred miles long, and from three hundred to four hundred miles broad, and of a thickness from thirty to sixty feet, constituting an enormous mass of calcareous matter. These Zoophytes live only in warm seas and near the surface; no indications of them are obtained from deep sea-soundings. Their growth is slow, but incessant, their numbers incalculable: they are usually attached to the shores of rocky islands, or to the crests of submarine ridges, rarely at a depth exceeding sixty feet.

77. Coral reefs are classified as Fringing, Barrier, and Circular reefs: the latter are called, by the natives of the Pacific islands, atolls.

Fringing reefs are belts of coral attached to the coasts of islands or continents. When the coast is precipitous, the belt is narrow; but when it is gently sloping, it is covered with coral until it reaches the depth of about sixty feet, where the animals cease to exist.

78. Barrier reefs are parallel to the coast, and separated from it by a deep channel. Figure 40 presents the barrier reef of Bolabola, in the Pacific ocean, encircling the island, but separated from it. stance covered with trees. forty miles in diameter. On the ocean side they terminate abruptly in deep water; but within, the slope is gradual. On the outside, the hardier species of Zoophytes

The reef is in this inThese reefs vary from three to

maintain a sturdy growth, resisting a heavy ocean surf, while the frailer varieties flourish in the placid waters within.

Fig. 40.

79. The circular reefs, or atolls, are the most common forms of coral islands. The diameter of the circles vary from one mile to forty miles, and their breadth from a few yards to more than a mile. They are not always circular;

Fig. 41.

hua

The Coral Isle.-Whitsunday.

one is thirty miles long by six broad. They enclose at space of quiet waters, called a lagoon, which communicates

гань. 220

with the ocean by one or more openings through the reef. The origin of these circular isles has been the subject of much discussion. They have been supposed to proceed sossibly formed from the growth of the coral upon the circular rims of volcanic craters beneath the surface of the ocean; and some of their phenomena favor such a view. But the subsidence of the islands, about which the corals accumulate as fringing reefs, furnishes a more satisfactory explanation of their origin. Figure 42 presents a section of the island and reef of Bolabola. When the level of the sea was at the lower line, a fringing reef attached itself to the island at A B and B A. As the island sunk in the ocean, the reef grew upward, and formed the barrier reef A' A', the upper line now constituting the ocean level. It is now a vertical section of the reef, island,

Fig. 42.

LEVEL OF SEA

Section of a Coral Island.

and intervening water of Figure 40. When the island has disappeared beneath the ocean level, we have the circular reef or atoll, enclosing a lagoon as in Whitsunday, Fig. 41.

80. But the movement of the islands is not exclusively that of subsidence; many of them have emerged from the ocean, and are still rising. The evidence of this is found in ancient reefs occurring inland and at great elevations above the sea. Upon the summit of the highest mountain in Tahiti, an island composed almost entirely of volcanic rocks, there is a reef of ancient coral attached to the rocks.

This could have grown only in the ocean, and has since been raised to its present position. In the Isle of France also occurs a bed of coral, at a distance from the ocean. A portion of. it is enclosed by two streams of lava, in which the characteristic effects of heat are exhibited in its partial ram wahn & an itmal mall pu crystalization-conversion into compact limestone or marble. Msh cartoni an 81. Coral reefs, having grown up to the surface of the sea, extend laterally, increasing their breadth. The constant action of the waves accumulates calcareous sand, shells, sea weeds and drift, upon sheltered portions of the reef in the soil thus formed, seeds, conveyed by the ture ocean or by birds from islands or the continent, spring up, and the reef becomes a habitable island.

The annexed figure presents a view of the peculiar features of a Pacific island, with its fringing and barrier reefs.

Fig. 43.

A Pacific Ocean Island, with coral reefs.

One of the most singular peculiarities of coral islands is the shore platform around them. It is a flat surface, often several hundred feet in width, but little above low tide level. Upon this lie huge masses of reef rock, worn into fantastic shapes. The platform is the result of the abrading action of the sea, and is not strictly confined to coral rocks, but occurs in sandstone shores similarly exposed, as exemplified in Figure 44.*

* Dana's Geology of the United States Exploring Expedition.

Fig. 44.

82. The minutest forms of animal life, the microscopic animalcules, commonly called Infusoria, from their occurring in great quantities in water infused with vegetable matter, are also important geological agents. Some of them exhibit the simplest conceivable conditions in which animal life can exist; but others show a complex organization, with muscular, nervous and vascular systems. Many of them are covered with shields of silica, or the oxide of iron, whose remains, after the death of the animals, constitute extensive deposits, although they are so minute that 40,000,000,000 of them occupy only one cubic inch. Ehrenberg has demonstrated the existence of monads, which do not exceed the twenty-four thousandth part of an inch in length, and so thickly crowded in the fluid as to leave intervals not greater than their own diameter. Hence he computes that each cubic line of the fluid conelemallatains 500,000,000 of these monads. A drop of water,

ian.

depan therefore, may include a number of these infusoria nearly onepoved only or quite equal to the present number of human beings on microscopic the globe. They are found in the ocean as well as in fresh Gram in a water. The beds of marl beneath peat swamps, and at Your din Consthe bottom of ponds, are composed chiefly of their shells,

and are often several feet in depth. The red scum seen