44 PHILOSOPHICAL TRANSACTIONS. [ANNO 1781. 



Some of these islands, 4 or 5 miles in extent, are formed at the angular turnings 

 of the river, and were originally large sand banks thrown up round the points, 

 but afterwards insulated by breaches of the river. Others are formed in the 

 straight parts of the river, and in the middle of the stream ; and owe their origin 

 to some obstruction lurking at the bottom. Whether this be the fragments of 

 the river bank ; or a large tree swept down from it ; or a sunken boat ; it is 

 sufficient for a foundation ; and a heap of sand is quickly collected below it. 

 This accumulates amazingly fast : in the course of a few years it peeps above 

 water, and having now usurped a considerable portion of the channel, the river 

 borrows on each side to supply the deficiency in its bed ; and in such parts of the 

 river we always find steep banks on both sides.* Each periodical flood brings an 

 addition of matter to this growing island ; increasing it in height as well as ex- 

 tension, until its top is perfectly on a level with the banks that include it : and 

 at that period of its growth it has mould enough on it for the purposes of culti- 

 vation, which is owing to the mud left on it when the waters subside, and is in- 

 deed a part of the economy which nature observes in fertilizing the lands in 

 general. 



While the river is forming new islands in one part, it is sweeping away old 

 ones in other parts. In the progress of this destructive operation, we have op- 

 portunities of observing, by means of the sections of the falling bank, the 

 regular distribution of the several strata of sand and earths, l)ing above each 

 other in the order in which they decrease in gravity. As they can only owe this 

 disposition to the agency of the stream that deposited them, it would appear that 

 these substances are suspended at different heights in the stream, according to 

 their respective gravities. We never find a stratum of earth under one of sand ; 

 for the muddy particles float nearest the surface.-j- I have counted 7 distinct 

 strata in a section of one of these islands. Indeed, not only the islands, but 

 most of the river banks wear the same appearance : for as the river is always 

 changing its present bed, and verging towards the site of some former one now 

 obliterated, this must necessarily be the case. Asa strong presumptive proof of 

 the wanderings of the Ganges from the one side of the Delta to the other, there 

 is no appearance of virgin earth between the Tiperah hills on the east, and the 

 province of Bardwan on the west; nor on the north till we arrive at Dacca and 

 Bauleah. In all the sections of the numerous creeks and rivers in the Delta, 

 nothing appears but sand and black mould in regular strata, till we arrive at the 



* This evidently points out the means for preventing encroachments on a river bank in the straight 

 parts of its course, viz. to remove the shallows in the middle of its channel — Orig. 



■f A glass of water taken out of the Ganges, when at its height, yields about 1 part in 4 of mud- 

 No wonder then that the subsiding waters should quickly form a stratum of earth ; or that the Delta 

 should encroach on the sea ! — Orig. 



