56 MADRAS FISHERIES BULLETIN [VOL. XVI, 



Madura a converse process is in operation, the land slowly gaining 

 upon the sea along the coast line facing the central division of the 

 pearl banks. Two factors are at work — the extension seawards of 

 fringing coral reefs along the coast and the distribution upon the 

 sea bottom of considerable quantities of sand and mud brought 

 down by the rivers Tambraparni, Vaippar, and Vembar. There is 

 also a constant movement of sand northwards during the prevalence 

 of the South-West Monsoon, whereby the depth of the water on 

 this pearl bank plateau may possibly be rendered shallower in 

 the central and northern sections. 



The most recent oscillations of level that appear to have affected 

 the shore line of the Gulf of Mannar and Palk Bay seem to me to 

 be as follows : — 



I. An early period of depression cutting off Ceylon entirely 

 from India and giving wide and free water communication between 

 what are now the Gulf of Mannar and the Coromandel Coast. The 

 land at this period must have been at least 12 to 1 5 feet lower 

 than now, and probably still more. The whole of the Jaffna penin- 

 sula and all of Adam's Bridge, Rameswaram and Mannar Islands, 

 and much of the lowlying lands of the present coastal districts of 

 Tinnevelly, Ramnad and Tanjore were submerged. At this period 

 the local varietal divergencies now seen among certain species of 

 marine animals without a free-swimming larval stage, probably 

 did not exist. Fossil evidence indicates this depression to have 

 occurred during the Miocene period. 



II. Next succeeded a period of elevation, when Ceylon was 

 connected with India by a fairly wide belt of land in the Palk Bay 

 area. The level of the land was considerably higher than now, at 

 least sufficient to convert the present inland sea of Palk Bay and 

 Strait (now with depths of 7 fathoms over a considerable area) into 

 a shallow marine lagoon, open to the sea probably only at its 

 northern end. It was during this period probably that many 

 members of the Carnatic fauna passed over into Ceylon. 



III. Finally, at a comparatively recent date, a small subsidence 

 occurred, restoring partially the old sea connexion between Palk 

 Bay and the Gulf of Mannar. This isolated Ceylon once more and 

 allowed of a new commingling of the marine fauna of the opposite 

 sides of Adam's Bridge. This last oscillation, though comparatively 

 recent and almost within the historic period, was antecedent, I 

 believe, to the formation of the shell deposits in the Tambraparni 

 delta, in Tuticorin harbour and in Pulicat Lake. 



