72 SPOLIA ZEYLANICA. 



by the topography of the bottom over which they are traveUiug ; 

 and other reasons. It cannot therefore definitely be said how the 

 outer wall of the plateau acts as a deviator of the ocean currents in 

 this case. If the current is deep, it would of course play a very 

 important part, but if shallow, less so. In any case the increase 

 which Avould be caused in the ocean temperature by the sudden 

 alteration in depth from 1 ,000 fathoms to 35 fathoms within a few 

 miles must have a profoundly modifjdng effect. It may also be con- 

 jectured that the gully in the middle of the plateau may produce 

 some alteration in the speed of the currents, if the currents reach so 

 far down. 



Swell. — As regards swell, it is possible to speak more- definitely. 

 How and where the motion through the water known uiider that 

 name originates it would be difficult to say ; but it is certain that 

 its effect is greatly increased, and finally converted into motion of 

 the water — i.e., breaking waves — on coming into shaUows. 



This is clearly seen along the south-west coast of Ceylon, where 

 it is no doubt aggravated by its 12-mile journey over the plateau 

 before it reaches sufficiently shallow water to break, as it does, in 

 enormous roUers on the beach. 



The shoaling of the plateau to the northward, and especially the 

 presence of the banks before mentioned (which lie exactly in the 

 line of the south-west sweU on its path towards Colombo), make an 

 extremely weU-marked difference in the amount of the swell, which 

 is both higher and more constant in this locality than, say, 30 miles 

 farther south, as at Barberyn (Bentota district). 



Had this fact been known, it should have been an additional 

 reason against the attempt to build a harbour at Colombo which 

 should be clear of swell — an attempt which, as we see, has hitherto 

 been attended by failure. Almost any other notch in the coast to 

 the southward (and especially Galle, off which bay the plateau is 

 considerably deeper and narrower) would have offered a better 

 chance of success from this cause. 



Character of the Bottom. — During the course of sounding opera- 

 tions it is usual to obtain specimens of the material forming the floor 

 of the sea. On Fig. I. the general appearance of these specimens is 

 given beneath the figures representing the depths. Dr. Willey, to 

 whom the samples were sent, reports that the specimens from the 

 shore plateau consist of sheUy and coralline debris, worm tubes, 

 bryozoa, echinoderm spines, foraminifera, &c., almost entirely 

 calcareous. A sample from 185 fathoms, on the other hand, from 

 outside the plateau, though on its exterior slopes, consists of caked 

 and powdery mud, principally calcareous, with a few minute quartz 

 grains, siliceous spicules, and organic particles. In fact the bottom 

 in the deeper water over the edge of the plateau consists of fine mud, 

 essentially the same at all depths and aU stations, containing 

 numerous calcareous remams of forammifera, chiefly Pulvinulma. 



