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Ch. XLIX.] THE THEOEY OF SUBSIDENCE CONSIDERED. ■ G05 



within it^ tlie reef, as was before stated (p. 593) ^ constantly 

 contracting its dimensions as it sinks. Thirdly^ witliin the 

 lao'oon the accnmnlation of calcareons matter is chiefly sedi- 

 mentary, a kind of chalky mud derived from the decay of the 

 softer corallines, with a mixtnre of calcareous sand swept by 

 the winds and waves from the surrounding circular reef. 

 Here and there, but only in partial clumps, are found livi] 

 corals, which grow in the middle of the lagoon, and mixed 



and sand, a great variety of 



& 



mud 



shells, and fragments of testacea and echinoderms. 



We owe to Lieutenant Nelson the discovery that in the Ber- 

 mudas the calcareous mud resulting from the decomposition 

 of the softer corallines is absolutely undistinguishable w^hen 

 dried from the ordinary white chalk of Europe,"^ and this mud 

 is carried to great distances by currents, and spread far and 

 wide over the floor of the ocean. We also have opportunities 

 of seeing in upraised atolls, such as Elizabeth Island, Tonga, 

 and Hapai, which rise above the level of the sea to heights 

 varying from 10 to 80 feet, that the rocks of which they 



consist do not differ in structure or in the state of preserva- 

 tion of their included zoophytes and shells from some of the 

 oldest limestones known to the geologist. Captain Beechey 

 remarks that the dead coral in Elizabeth's Island is ^ more or 



combed 



f 



7D 



into a compact rock which has the fracture of secondary lime- 



t 



Mas, off Sumatr 



feet high, is described by Dr. Jack as being overspread by 

 coral and large shells of the Chama {Tridacna) gigas, which 

 rest on quartzose and arenaceous rocks, at various levels from 

 the sea-coast to the summit of the highest hills. 



The cliffs of the island of Timor in the Indian Ocean are 

 composed, says Mr. Jukes, of a raised coral reef abounding 

 in Astrway Mceandrina^ and Porites^ with shells of Stromhus^ 

 Conus^ Nerita^ Arca^ Pecten^ VemtSy and Lucina. On a ledge 

 about 150 feet above the sea, a Tridacna (or large clam shell), 

 two feet across, was found bedded in the rock with closed 



^ Trans. Geol. Soc. London, 2d series, 



t Bcechey's Voyage, vol. i. p. 45. 



vol. V. 



