.SEA-BOTTOMS AND CALCRETES. 159 



action of the rivers and ocean currents, and organisms living in the surrounding 

 .seas are responsible for the calcareous material which forms a considerable proportion 

 of the deposits laid down in places distant from the shore. 



As no rocks of undoubted Tertiary age are found on the adjacent coasts, it would 

 appear that all through that period the district lias been in a state of equilibrium. 

 SuBSS* has remarked on the tact that no distinct line can he drawn between the 

 Miocene and succeeding formations in the east, no break is seen in the deposition, 

 and newer beds have quietly overlapped those of earlier date. In the absence of any 

 signs of tectonic movements during the Tertiary period, which, if they had existed, 

 would certainly have left some traces in the rocks of Ceylon and India, we are driven 

 to the conclusion that the shallow platform surrounding Ceylon and connecting it 

 with India on the north is due to the filling up of the sea by detritus derived from 

 the land (see map, p. 161). 



On the west coast of Ceylon, near the mouths ot rivers, spits of sand stretch 

 across the submerged platform, towards the north-west, while across the Gulf of 

 Manaar from the opposing shores of .Southern India similar hanks of sand extend to 

 the north-east. Near the coasts the spits consist of coarse fragments, while further 

 out the sands become successively of finer grain. Long continued growth of these 

 spits would result in the formation of a platform arching to the north and produce 

 exactly the conditions Ave find in the Gulf of Manaar and Palk Bay. Waltheki 

 records that to the north of Palk Bay, between Calimene Point and Jaffnapatam. 

 there extends a string of shoals less than 3 fathoms under the surface, and these may 

 represent a bank in the act of forming. 



In describing the deposits it has been shown that in the Gulf of Manaar the loose 

 material is at the present day being cemented, into calcareous sandstones or calcretes 

 at the " paars," chiefly through the agency of Polyzoa and Nullipores. 



The important part played by Polyzoa in this connection has not hitherto been 

 recognised. 1 am convinced that many square miles of the rocky paars are due to 

 this cause alone. < 'oral reefs in all stages of decay, from living reefs to compact lime- 

 stones showing hut few traces of coral structure, are associated with the calcretes. 



If an area of this character were raised above the sea level and acted upon by the 

 waves of the sea, we should expect the harder paars and limestones to exist as islands, 

 between which would be areas of loose drifting sand. Such is exactly the structure 

 of Adam's Bridge a remarkable chain of islands and shoals which stretches across the 

 platform from Ceylon to India, According to Foote| and WALTHER one of these 

 islands, Rameswaram, has an ancient coral reef along its northern border, and a 



* ' Das Antlitz der Erde,' vol. 2, p. G4*. 



t "Die Adamsbrucke uiid die Korallenriffe der Palk-Strasse." Erganzungsheft No. 102 zu Petek- 

 mann's ' Mittheilungen,' 1891. 



I 'Memoirs of the Geological Survey of India.' "On the Geology of the Madura and Tinnevelly 

 Districts." 



8 Op. cit. 



