L56 CEYLON PEARL OYSTER REPORT. 



bored with Lithodomus, and worm tubes are seen in section on the bottom surface. 

 Some of these have a few grains of sand attached to the interior of the tul >e. 



A few Nullipore patches of a white colour, scarcely distinguishable from the coral, 

 are attached to the surface, but no Polyzoa. 



A thin slice examined under the microscope shows that one or two pieces of shell 

 and quartz have been enclosed in the walls of the coral, and towards the margin the 

 thecas have been filled with secondary calcite and numerous Foraminifera. 



North Central part of East Cheval and the Middle of West 

 C h e v a 1 have calcretes exactly like those described from Periya Paar. They are 

 thickly encrusted with Polyzoa, Nullipore, and worm tubes, and in section evidence of 

 Polyzoa structure can be seen even in the middle parts of the blocks. The masses 

 are cavernous, especially near the surface, and large worm tubes adhere to one side 

 only of the cavities. The cavities have not been formed by boring animals, but are 

 probably due to influences proceeding from the movements of the worm itself in 

 setting up ciirrents and thus preventing the growth of cementing organisms in the 

 immediate neighbourhood. One block which has no worm tubes attached is not 

 cavernous. 



In the S o u t h Central part of E a s t C h e v a 1 , the Central p a r t o f 

 the Southern portion, North end of West Arm, and in the South - 

 w est part of the Paar a brown compact limestone occurs. It is usually encrusted 

 with Polyzoa, Nullipores, and worm tubes, affords attachment to the byssus of the 

 pearl oyster, and shells, especially Chama, are frequently adherent. In all cases the 

 limestone is bored by Molluscs and Clione, but only for a short distance from the 

 margin. It is slightly phosphatic, two specimens examined contained 0'21 and 0"3G 

 per cent, respectively of phosphate of lime. In section we find a fine granular, light 

 brown matrix with spots and streaks of reddish material at intervals. Traversing 

 the matrix are zig-zag and branching lines of clear calcite, evidently filled-in 

 cracks. Patches of Coral are seen showing clear calcite outlines with dark granular 

 infilling. Quartz grains, mostly angular, occur, sometimes in patches, at other times 

 in lines. Zircon, apatite, and garnet accompany the quartz, and magnetite occurs 

 sporadically, usually showing a reddish border, which is, no doubt, due to the 

 alteration of this mineral. The ground mass is mostly composed of shell fragments, 

 Foraminifera, and small broken pieces of Nullipore. Incipient oolitic structure occurs 

 round some of the grains. I should regard this limestone as the ultimate stage in the 

 alteration of a coral reef. Coral structure is still to be seen in j>laces, and we can 

 trace the changes, step by step, by which a coral rock like that occurring at the 

 north end of East Cheval is converted into compact limestone. 



Periya Paar Kerrai. Samples were obtained from three localities. Two of them 

 were calcretes, showing Polyzoa and Nullipore structure among the grains, the third 

 consisted of casts of drifted shells (Plate I., fig. 2). 



Casts of pearl oysters were most frequent. All the valves were turned in one 



