Page:Collier's New Encyclopedia v. 08.djvu/366

LEFT SEA 310 SEA mals which live in, and are carried along by the currents of the ocean. In the great body of oceanic waters life is most abundant in the surface and sub-surface waters down to about 100 fathoms. Pelagic algae, such as diatoms and oscil- latoria, are abundant in this region, and are the principal and original source of food for many pelagic and nearly all deep-sea animals. In the intermediate depths of the ocean life though present is less abundant. Deposits. — All marine deposits may be divided into two classes — viz., those made up principally of the debris from the solid land of the globe, laid down in greater or less proximity to the shores of continents and islands, called "terri- genous" deposits, and those in which this continental debris is nearly or quite ab- sent, laid down in the abysmal regions of the ocean, called "pelagic" deposits. Commencing with the former, there are first the littoral and shallow-water de- posits, forming around the land masses from the shore down to a depth of about 100 fathoms, consisting of sands, gravels, and muds derived almost entirely from the disintegration of the neighboring lands. The littoral deposits, laid down between tide marks, cover about 63,000 square miles, and the shallow-water de- posits, between low-water mark and 100 fathoms, about 10,000,000 square miles. Proceeding seaward from an average depth of about 100 fathoms, the deposits gradually change in character, the pro- portion of land detritus decreasing, while the remains of oceanic organisms increase in abundance till at a considerable dis- tance from land and in comparatively deep water the terrigenous deposits pass insensibly into truly pelagic deposits. The terrigenous deep-sea deposits — i. e., those formed at depths greater than 100 fathoms — may be briefly summarized as follows : Blue mud, the most extensive, is gray- ish or bluish in color, with usually a thin reddish upper layer, and is char- acterized by the presence of fragments of rocks and mineral particles coming from the disintegration of the land. Blue mud is found along the coasts of con- tinents and continental islands, and in all inclosed and partially inclosed seas. Blue mud is estimated to cover about 14,500,000 square miles of the earth's sur- face— 4,000,000 in the Arctic, 3,000,000 in the Pacific, 2,500,000 in the Antarctic, 2,000,000 in the Atlantic, 1,500,000 in the Indian, and 1,500,000 in the Southern ocean. Red mud covers about 100,000 square miles off the coast of Brazil. Green mud and sand are similar to the blue muds, but are characterized by the presence of the mineral glauconite in isolated grains or in small concretions; the dead shells of calcareous organisms are usually filled with the glauconite, which gives the green color to the de- posits. Green mud and sand cover about 850,000 square miles — 300,000 in the At- lantic, 250,000 in the Pacific, 150,000 in the Indian, 90,000 in the Southern, and 60,000 in the Antarctic. Volcanic mud and sand are deposited around the oceanic islands of volcanic origin and the name is derived from the presence of fragments and particles of volcanic rocks and minerals, which are larger and more numerous nearer the islands, when the deposit is called a sand. Volcanic mud and sand cover about 600,- 000 square miles— 300,000 in the Pacific, 200,000 in the Atlantic, and 100,000 in the Indian ocean. Coral mud and sand occur similarly around the oceanic coral islands and off those coasts and islands fringed by coral reefs. Coral mud and sand cover about 2,557,000 square miles— 1,417,000 in the Pacific, 760,000 in the Atlantic, and 380,- 000 in the Indian ocean. Of pelagic deposits there are five types, four of organic origin, receiving their designations from the distinctive presence of the remains of calcareous or siliceous organisms, the fifth and most extensive being of inorganic origin. Globigerina ooze is so called from the presence of the dead shells of pelagic Foraminifera, those belonging to the genus Globigerina predominating, which live in the surface and sub-surface waters of the ocean, being especially abundant in tropical regions, and the shells of which after death fall to the bottom and there accumulate in moderate depths. The depth at which Globigerina ooze is found varies from less than 500 to over 2,500 fathoms, the average depth being about 2,000 fathoms. Globigerina ooze covers about 49,520,000 square miles— 17,940,000 in the Atlantic, 11,300,000 in the Pacific, 10,560,000 in the Southern, and 9,720,000 in the Indian ocean. Pteropod ooze resembles Globigerina ooze in all respects, except that there is a greater abundance of the dead shells of pelagic mollusca, such as pteropoda and heteropods; it is usually found in lesser depths than the Globigerina ooze. Pteropod ooze covers about 400,000 square miles in the Atlantic. Diatom ooze is distinguished by the presence of numerous remains of siliceous organisms, principally Diatoms, though fragments of siliceous sponge spicules and Radiolaria and Foraminifera are rarely absent. It is found in the Antarctic and Southern oceans and also in the north- west Pacific. Diatom ooze covers about 10.880,000 square miles— 10,000,000 in the