472 



ISLANDS FOKMED AND DESTROYED 



[Ch. 



On the sea-coast there are eight great openings, each of 

 which has evidently, at some ancient period, served in its 

 turn as the principal channel of discharge. Although the 

 flux and reflux of the tide extend even to the heads of the 

 delta when the rivers are low, yet, when they are periodically 

 swollen by tropical rains, their volume and velocity counter- 

 act the tidal current, so that, except very near the sea, the 

 ebb and flow become insensible. During the flood season, 

 therefore, the Ganges and Brahmapootra almost assume in 

 their delta the character of rivers entering an inland sea ; the 

 movements of the ocean being then subordinate to the force 

 of the rivers, and only slightly disturbing their operations. 

 The great gain of the delta in height and area takes place 

 during the inundations ; and, during other seasons of the 

 year, the ocean makes reprisals, scouring out the channels, 

 and sometimes devouring rich alluvial plains. 



Islands formed and destroyed. — Major R. H. Colebrooke, in 

 his account of the course of the Ganges, relates examples of 

 the rapid filling up of some of its branches, and the excava- 

 tion of new channels where the number of square miles of 

 soil removed in a short time (the column of earth being 114 

 feet high) was truly astonishing. Forty square miles, or 

 25,600 acres, are mentioned as having been carried away, in 

 one place, in the course of a few years."* The immense trans- 

 portation of earthy matter by the Ganges and Brahmapootra 

 is proved by the great magnitude of the islands formed in 

 their channels during a period far short of that of a man's 

 life. Some of these, many miles in extent, have originated 

 in large sand-banks thrown up round the points at the angu- 

 lar turning of the rivers, and afterwards insulated by breaches 

 of the streams. Others, formed in the main channel, are 

 caused by some obstruction at the bottom, 

 a sunken boat, is sometimes sufficient to check the current, 

 and cause a deposit of sand, which accumulates till it usurps 



a considerable portion of the channel. The river then under- 



its 



A large tree, or 



mines its banks on each side, to supply the deficiency in 

 bed, and the island is afterwards raised by fresh deposits 



during every flood. 



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* Trans, of the Asiatic Society, vol. vii. p. 1-t. 





