1863.] fergussojj — delta of the Ganges. 351 



current slackens, there is a great tendency for rivers to deposit their 

 mud in their own beds, and to heal the wounds that have been made 

 in their banks ; so that even during that period it is doubtful if more 

 than half is carried off. For instance, careful simultaneous experi- 

 ments were made, two years ago, as to the quantity of solid particles 

 held in suspension in the waters of the Matabangah, — first, at leaving 

 the Ganges, when it was found to be 1 in 294 parts, while nearly 

 at its junction with the Hoogly the quantity was only 1 in 884, 

 proving that two -thirds had been deposited en route in that short 

 distance *. 



Sometimes an acre or two of a bank will fall in in a single night, 

 and, consequently, the stream will be unusually turbid for the next 

 twenty -four hours; but, in such an abnormal instance, one-half 

 at least probably never leaves the local stream, but is deposited again 

 a few miles further down; and, in fact, every stream and every 

 locality has its peculiar regimen in this respect, and until they are 

 more carefully examined than they have hitherto been, it will be 

 safer to look to such indications as history affords us, and to our 

 charts. These last show that little or no change or extension 

 seawards has taken place, during the last 100 years, between the 

 Hoogly and the Horringotta, or about halfway across the seaward 

 face of the delta. But the eastern half is in a state of rapid change, 

 having remained behind, I believe, principally in consequence of the 

 absorption of the Brahmapootra's silt by the Sylhet Jheels; but 

 probably in little more than a century or two from this time the gap 

 may be repaired, and the Sunderbuns bounded by a nearly straight 

 line east and west. 



As regards the elevation of the delta, by far the safest test is the 

 progress of the junction of the tributary streams, such as the Soane 

 and Gogra. If the former be carefully surveyed from time to time, and 

 the retrocession of the tributaries carefully noted, we shall gather far 

 more satisfactory evidence of the gradual elevation of the delta than 

 can be obtained by dipping tumblers from the sides of ' Budgerows,' 

 which operation has hitherto been supposed to be sufficient to gauge 

 the growth of continents. 



2. Swatch of " No Ground.'''' — There is still one other phenomenon 

 which it is necessary to allude to, in order to understand the 

 present or prospective condition of the seaward face of the delta 

 of Bengal. This is the existence of a great depression, or hole, in 

 the middle of the Bay of Bengal, known in the charts as the 

 " Swatch of No Ground." Its exact position is shown on the map 

 accompanying this paper (Plate XII.), and its sides are so steep and 

 well defined that it affords mariners the best possible sea-mark — the 

 lead suddenly dropping, especially on its western face, from 5 and 10 

 to 200 and even 300 fathoms, with " no ground." 



It seems quite impossible to ascribe this sinking to volcanic 



action, inasmuch as we know that no such violent convulsion has 



taken place in Lower Bengal, during the last 200 years, as could 



have caused such a chasm ; and it is not conceivable that so large 



* < The Ganges and the Hoogly,' by F. Prestage. Calcutta, 1861. 



