CEYLON PEARL BANKS. 216 



ridge«; of sand may possibly be sufficient to cause silting, and it is 

 still more likely that the movement of water, which is sufficient to 

 disintegrate seaweed growdng on the bottom to such an extent as to 

 make it difficult for the divers to see, is quite enough to disturb the 

 sand and produce silting. 



It is obvious then that SouthwelFs and Kerkham's opinion that 

 there are no bottom currents, even if true, does not dispose of 

 Herdman's suggestion that silting of sand takes place to such an 

 extent as to be an important cause of mortality. So long as there is 

 movement in the bottom layers of water, whether caused by a 

 definite current or by the transmission of the surface Avaves , it is 

 probable that silting takes place. 



In support of this view, I am able to publish, through the kindness 

 of Captain J. A. Legge, Master Attendant, Colombo, an extract 

 from his report to Government in December, 1903, in his capacity 

 as Inspector of Pearl Banks : — 



" From November 19 until my return I used a diving dress and air 



pump and dived personally daily What impressed me most 



was that the spots I div^ed on last March, which Avere then level rock, 

 with a coating of 3 or 4 inches of sand, had now as much as a 

 foot of sand in places. All over the sand was in fairly deep ridges, 

 not so deep as the ridges off the paar pro^aer, but quite distinct from 

 the appearance of the sea bottom last March. Now this was not 

 the case on the South-east Cheval, and the impression I have formed 

 from this is that the shoal running up from the Karativu Islands 

 arrests the sand and shelters this part of the Cheval and also the North 

 and South Modragam Paars. One of my reasons for this impression 

 is that after a protracted search for the old tanks sunk by Captain 

 Donnan in four fathoms on the extreme north end of this shoal 

 I was quite unsuccessful, though these tanks are seen from quite a 

 little way off by their shadow on the surface of the water Avhen 

 the water is clear, and I have never experienced clearer water than 

 when I made this search last month. I am informed that Captain 

 Donnan on only one occasion found these tanks in November, and 

 I was unsuccessful last year. The soundings on or about the 

 position of these tanks showed a shallowmg of the water. I havg . 

 therefore, come to the conclusion that the south-west monsoon 

 washes up the sand and covers these tanks, and the north-east 

 scours them clear again of sand. If this opinion is confirmed, it 

 will explain the absence of sand ridging on the South-east Cheval , 

 and the marked ridging and increase in quantity of sand on the 

 parts of the Cheval not sheltered by this shoal." 



The information collected by Southwell and Kerkham regarding 

 currents is interesting, since much of it was obtained from personal 

 observation in the diving dress , but these conclusions can hardly be 

 regarded as convincing. More work is needed on this important 

 point. 



