Page:EB1911 - Volume 19.djvu/1013

Rh reared the greater part of the structure of our present knowledge on the subject. The classification adopted was a double one, taking account both of the origin and of the distribution in depth of the various deposits, thus:—

Krümmel prefers to simplify this by grouping the deposits in a single category arranged according to their position into:

As so defined the hemipelagic deposits are those which occur in general on the slope from the continental shelves to the ocean depths and also in the deep basins of enclosed and fringing seas. The eupelagic deposits are subdivided by Krümmel into two main groups; (a) epilophic, including the pteropod, globigerina and diatom oozes occurring on the rises and ridges and in the less deep troughs. (b) Abyssal, including the radiolarian ooze and red clay of the deepest abysses.

The littoral deposits include those of the actual shore on the wash of the waves and of the surface of the continental shelf.

Shore Deposits are the product of the waste of the land arranged and bedded by the action of currents or tidal streams. On the rocky coast of high latitudes blocks of stone detached by frost fall on the beach and becoming embedded in ice during winter are often drifted out to sea and so carry the shore deposits to some distance from the land. Similar effects are produced along the boulder-clay cliffs of the Baltic. Where the force of the waves on the beach produces its full effect the coarser material gets worn down to gravel, sand and silt, the finest particles remaining long suspended in the water to be finally deposited as mud in quiet bays. A particularly fine-grained mud is formed on the low coasts of the eastern border of the North Sea by a mixture of the finest sediment carried down by the slow-running rivers with the calcareous or siliceous remains of plankton. Pure calcareous sand and calcareous mud are formed by wave action on the shores of coral islands where the only material available is coral and the accompanying calcareous algae, crustacea, molluscs and other organisms secreting carbonate of lime. Recent limestones are being produced in this way and also in some places by the precipitation of calcium carbonate by sodium or ammonium carbonate which has been carried into the sea or formed by organisms. The precipitated carbonate may agglomerate on mineral or organic grains which serve as nuclei, or it may form a sheet of hard deposit on the bottom as occurs in the Red Sea, off Florida, and round many coral islands in the Pacific. Only the sand and the finest-grained sediments of the shore zone are carried outwards over the continental shelf by the tides or by the reaction-currents along the bottom set up by on-shore winds. The very finest sediment is kept in a state of movement until it drops into the gulleys or furrows of the shelf, where it can come to rest together with the finer fragments of the remains of littoral or bank vegetation. Thus are formed the “mud-holes” of the Hudson Furrow so welcome as guides telling their position to ship captains making New York harbour in a fog. Sand may be taken as the predominating deposit on the continental shelves, often with a large admixture of remains of calcareous organisms, for instance the deposits of maërl made up of nullipores off the coasts of Brittany and near Belle Isle. Amongst the most widely distributed of the

deposits actually formed on the continental shelf are phosphatic nodules; these are especially abundant on the east coast of the United States and on the Agulhas Bank, where the amount of calcium phosphate in the nodules is as much as 50%. Sir John Murray finds the source of the phosphoric acid to be the decomposition of large quantities of animal matter, and he illustrates this by the well-known circumstance of the death of vast shoals of fish when warm Gulf-Stream water displaces the cold current which usually extends to the American coast. Glacial detritus naturally plays a great part in the deposits on the polar continental shelves.

Hemipelagic deposits are a mixture of deposits of terrigenous and pelagic origin. The most abundant of the terrigenous materials are the finest particles of clay and calcium carbonate as well as fragments derived from land vegetation, of which twigs, leaves, &c., may form a perceptible proportion as far as 200 m. from land. Blue mud, according to Murray and Renard, is usually of a blue or slaty or grey-green colour when fresh, the upper surface having, however, a reddish tint. The blue colouring substance is ferrous sulphide, the upper reddish layer contains more ferric oxide, which the predominance of decomposing organic matter in the substance of the mud reduces to ferrous oxide and subsequently by further action to sulphide. The proportion of calcium carbonate varies greatly according to the amount of foraminifera and other calcareous organisms which it contains. Blue mud prevails in large areas of the Pacific Ocean from the Galapagos Islands to Acapulco. In the Indian Ocean it covers the Bay of Bengal, the Arabian Gulf, the Mozambique Channel and the region to the south-west of Madagascar. In the Atlantic it is the characteristic deposit of the slopes of continental shelves of western Europe and of New England, being largely mixed with ice-borne material to the south of Newfoundland. It is particularly in evidence round the whole of the Antarctic Shelf, where it occurs down to depths of 2500 fathoms. It is the chief deposit, according to Nansen, of the North Polar Basin and, according to Schmelck and Böggild, of the Norwegian Sea also, where it is largely mixed with the shells of the bottom-living foraminifer Biloculina. Max Weber states that blue mud occurs in the deep basins of the eastern part of the Malay Sea. In the form of volcanic mud it is common round the high volcanic islands of the South-Western Pacific.

Red mud may be classed as a variety of blue mud, from which it differs on account of the larger proportion of ochreous substance and the absence of sufficient organic matter to reduce the whole of the ferric oxide. This variety surrounds the tropical parts of the continental shelves of South America, South Africa and eastern China.

Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of glauconite, which is formed inside the cases of foraminifera, the spines of echini and the spicules of sponges in a manner not yet understood. It occurs in such abundance in certain geological formations as to give rise to the name of green-sand. Green mud abounds off the east coast of North America seawards of Cape Hatteras, also to the north of Cuba, and on the west off the coast of California. The “Challenger” expedition found it on the Agulhas Bank, on the eastern coasts of Australia, Japan, South America and on the west coast of Portugal. When the proportion of calcium carbonate in the blue mud is considerable there results a calcareous ooze, which when found on the continental slope and in enclosed seas is largely composed of remains of deep-sea corals and bottom-living foraminifera, pelagic organisms including pteropods being less frequently represented. The floors of the Caribbean, Cayman and Mexican Basins in the Central American Sea are covered with a white calcareous ooze, which is clearly distinguished from the eupelagic pteropod and globigerina oozes by the presence of abundant large mineral particles and the remains of land plants. In this deposit the occurrence of calcareous concretions is very characteristic, as L. F. de Pourtalès pointed out in 1870; they consist of remains of deep-sea corals, serpulae, echinoderms and mollusca united