1885.] Alfred Carpenter — Tlic Simfcli of no Ground. 125 



XI. — " The Swatch of no Ground ". — By Commander Alfred Carpenter, 

 R. N., in charge of Marine Survey of India. Communicated hy the 

 Natural History Secretary. 



[Recived June 6th ;— Eead July 1st, 1885.] 



The Bay of Bengal, the entrance to which between Ceylon and the 

 Nicobar Islands has a depth of some 2000 fathoms, gradually shoals 

 northward to the 19th parallel of latitude, where it has a depth of about 

 1400 fathoms. In the next 60 miles northward, the head of the Bay 

 shoals rapidly, as might be expected when it is entered by such rivers 

 as the Ganges and the Brahmaputra. 



The particles of mud discharged by a river debouching through a 

 flat delta are very minute and are held in suspension for days. It is 

 these which slowly settling form here a bank of olive-coloured mud and 

 grey sand. If we now look at a chart of the Bay of Bengal and examine 

 the mouths of this great delta and the shoals formed off them, we shall 

 notice with surprise that the direction of every channel through these 

 shoals is such as to tend to throw the ebbing waters towards the 

 region called the Swatch. This is especially noticeable in Chart 829 

 Coconada to Bassien river. 



Presuming then that this tendency is actually followed by the 

 ebbing water, the result is a number of whirls and eddies just in that 

 locality, the position of the Swatch being central with regard to the 

 deltaic mouths. 



The condition necessary to admit of mud in suspension settling to 

 the bottom is perfect quiescence of the supporting medium. This 

 never occurs here during the ebb tides. During the flood, which only lasts 

 5 hours against 7 hours' ebb, the water is only muddy for the first hour, 

 while the outflow partly returns ; the green ocean water then comes in 

 carrying little or nothing in suspension. In this region, then, we have 

 only one hour's settle of mud against 6 hours or 7 hours over the adjoin- 

 ing banks. During the course of ages the banks on either side have 

 grown seaward and their southern face falls abruptly into deep water. 

 But in the Swatch the banks have never been able to meet and the 

 depth still remains considerable. 



This submarine ravine is 1800 feet deep at 15 miles from the flat 

 mangrove islands of the delta, whilst at that distance off shore the 

 bank on each side of the Swatch is only 100 feet below water. 



The ' Investigator' in the Spring of this year, 1885, re-sounded the 

 whole bank of soundings from False Point to the Mutla river entrance. 

 Compared with the surveys of some forty years ago there has been 



