CONDITIONS OF SEDIMENTARY DEPOSITION. 499 



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 deposi- 

 tion. A lee -bank is a submarine terrace of construction. Where 

 such a terrace extends into an abyss it argues prolonged devel- 

 opment, and, therefore, antiquity of relation between continental 

 platform and oceanic basin. 



(/z) Subside?we 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 con- 

 tinents 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 dif- 

 ferent formations. From this map we may gather that terrigen- 

 ous 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." 



