'2:\-2 CEYLON .MURINE BIOLOGICAL REPORTS. 



Tlieso investigations Mere oairiod out on tlio west side of tlie Island (between Galle and Adam's 

 Bridge), where, of course, the pearl banks ho, and also on the Indian side, and in the Gulf of Mannar 

 generally. It is necessary at the outset to understand tliat the pearl bank area of the Ceylon coast 

 consists of a sliaUoM- water plateau, slielving out from the beach to about the 12-fathom hne, and then 

 rapidly deepening to several hundred fathoms. The overfall ranges froni 1 to 18 miles from the beach. 

 The bottom consists of alternations of sand and hmey rock, which latter is derived from decayed coral 

 re-organized into an exceedingly hard substance by the cementing growths of NuUipore and Polyzoa. 

 At mo?!t places the rock is only covered by less than four inches of sand, and is often quite bare. 



These details have been obtained as a result of continued dredging and trawling operations, 

 combined with the extensive work of skin diving and also dress diving. 



Practically parallel to the shore, whicli runs approximately north and south, there is a more or 

 less continuous, fringing, coral reef, exposed at places during low water. The rise and fall of tide is limited 

 to a few inches only. Towards the north of the plateau the submerged continuation of Karativu Island 

 rims in a northerly direction for about 10 miles as a narrow sandy shoal, and terminates very abruptly. 

 Reference to the apj)ended chart will indicate the topographical relations of the mainland. To the 

 north the pearl banks under lease he in a cul-de-sac. The only exit to the gulf is Palk's Strait, whilst 

 Southern India forms the western boundarj'. The influence of these varied factors on the surface current 

 will, however, be dealt with later. 



It is obvious that during the north-east monsoon the surface current over the entire plateau 

 has a different trend to that existing during the south-west monsoon, so that in order to fully elucidate 

 and miderstand the true condition of affairs we wiU deal with each monsoon sej)arately. 



(a) North-east Monsoon. 



This monsoon commences about the beginning of November and extends until about the end of 

 May. It is the fine weather season, as the wind is from the land. During November, December, and 

 Januaiy strong northerly winds prevail during the day, usually becoming almost easterly at or durmg 

 the night. In February these strong winds gradually die down. 



During March and April dead [Trichodesmium) calms prevail about midday. An hour or more 

 later a gentle breeze springs up from the west, and as it strengthens it veers to the north, and by nightfall 

 is due east from the land, where it continues during the night, shifting again to the south in the morning 

 hours and gradually dying away at daybreak. 



These conditions have been noted during a great number of years, and although the sequence 

 of these daily changes of the wind is occasionally broken, they still remain a very noticeable feature 

 about this time. During the fisheiy season the westerly afternoon wuid enables the fleet to sail ashore, 

 whilst the early morning easterly winds waft them back again to the fishing grounds after they have 

 disposed of their oysters. 



In May these rhythmic winds are almost entirely broken up, and are succeeded by cakn mornings 

 and exceedingly squally afternoons, characterized by heavy rains ^dth hghtniag and thmider. Thi 

 condition marks the approach of the south-west monsoon. 



In thus considering the currents prevalent durmg the north-east monsoon, it will be noticed from 

 the cliart that the pearl bank area under lease hes in a cul-de-sac, where it is entirely protected by the 

 luaiixland and the line of fringing reefs. 



The banks south of Tallaivillu Point are swept by an oceanic current, wliich, running north, is 

 deflected at this point to the west and on to the Lidian coast. 



North of TaUaivOlu Point, however, it may be said that there are no true surface currents, except 

 far out at sea. Wliat surface currents exist on the plateau are transient and temporary, and vary 

 even during the day with the direction of the wind, bj- which thej' are controlled and to which they 

 entirely owe their existence. This area hes securely under the lee of the land, and is further protected 

 by the line of fringing reefs previously mentioned. The situation of the banks renders them immune 



