On recent Changes in the Delta of the Ganges. 555 



was named Scapheus ancylochelis, approached most closely to Mega- 

 c/ieirus longimanus, from the Solenhofen limestone, species of which 

 genus occur also in the Oxford Clay of Wiltshire and the Oolite 

 and Lias of Germany. 



April 1. — Professor A. C. Ramsay, President, 

 in the Chair. 



The following communication was read : — 



" On recent Changes in the Delta of the Ganges." By James 

 Fergusson, Esq., F.R.G.S. Communicated by the President. 



Before describing the local phenomena of the Ganges, the author 

 explained, first, the laws that govern the extent of the oscillations 

 in reaches of rivers, either laterally or in the direction of their course ; 

 secondly, the causes operating to raise the banks of rivers flowing 

 through very flat plains above the level of the country at a little 

 distance from their margins ; and lastly, the immense relative thick- 

 ness of the early deposits in deltas over those of later periods, when 

 the conditions of the river had come more nearly in equilibrium. 



Mr. Fergusson then proceeded to point out that in historical 

 times the Brahmapootra and Ganges, on entering the plains of 

 Bengal— passing Goalparah and Rajmahal respectively — ran ori- 

 ginally to the sea in a nearly due north and south course, parallel to 

 one another. This symmetry was first disturbed by the upheaval of 

 the Modopore jungle, north of Dacca, by which the Brahmapootra, 

 was diverted in a south-eastern direction into the depression known 

 as the Sylhet Jheels, which were the result of the upheaval just 

 described. He then further explained how the river, having filled 

 up these Jheels, had returned to its former bed within the limits of 

 the present century. 



The paper then described the effect this change had already pro- 

 duced in reopening the rivers of the western half of the Delta, and 

 showed that, if it were maintained, it would have the effect of so 

 raising the eastern half as to restore the course of the two great 

 rivers very nearly to the position they occupied before the disturb- 

 ance above alluded to. 



The next point adverted to was the gradual retrocession of all 

 the mouths of the tributaries of the Ganges, in consequence of the 

 tilting back of the plain, by the gradual rise of the Deltaic plains. 



Mr. Fergusson then stated that he conceived we had sufficient 

 historical indications to show that within the last 5000 years the 

 plain of Bengal has been nearly in the same condition that the 

 valley at Assam now is — a jungly swamp, with only a few habit- 

 able spots here and there on the banks of the larger rivers. 



The last phenomenon alluded to was the " Swatch of low ground" 

 m the Bay of Bengal. This was ascribed to the action of the tides, 

 which, being accelerated on either shore of the bay, acquired a 

 rotatory motion at the sand-heads, and, meeting in the centre of the 

 bay, scooped or swept out this depression in the centre, and by 

 this action prevented the growth of the Delta seaward to the extent 

 that would otherwise take place. 



