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CONDITIONS OF SEDIMENTARY DEPOSITION.
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capable of distributing the sediment, and of shoals and deeps in the sea. The amount of difference in depths is not indicated, but the rapid descent from the edge of the bank to the foot is essential to diffusion of the current and the consequent deposition. A lee-bank is a submarine terrace of construction. Where such a terrace extends into an abyss it argue prolonged development, and, therefore, antiquity of relation between continental platform and oceanic basin.

(h) Subsidence from oceanic circulation.—The greater part of terrigenous sediment must be deposited in deltas and estuaries, on continental platforms, and in silt banks along great deeps. But a very considerable amount of fine silt brought out by rivers and undertow, quantities of volcanic dust fallen on the ocean, and the calcareous and silicious parts of pelagic organisms are taken into oceanic circulation, and find a resting-place more or less remote from their place of origin. These deposits constitute the deep-sea formations; they are not clearly recognized among the strata of past geological periods now exposed in land surfaces, and on this fact rests the principal argument for the antiquity of the continents and oceans. They have been fully described by Murray,[1] and their mode of deposition need here be indicated only by reference to the blue muds of the Bay of Bengal and the Arabian Sea.

The blue muds are composed of minute mineral fragments derived from the disintegration of the land, of a diameter of .05 mm., or less, which may contain calcareous remains amounting to 50 per cent. of the whole, or may be almost free from lime. The description of a typical sample, taken about 275 miles south of the mouth of the Ganges, is given by Murray[2] in an article which is accompanied by a map showing the distribution of different formations. From this map we may gather that terrigenous deposits form a belt, 50 to 125 miles wide, along the eastern coast of Africa, the western coast of Australia, and the Malay

  1. ↑ Challenger Reports; Narr. of the Cruise, Vol. I, Part II.
  2. ↑ Scott. Geog. Mag., Vol. V, No. 8, Aug., 1889, p. 420.John Murray on "Marine Deposits in the Indian, Southern and Antarctic Oceans."
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