98 



now been ascertained by the investigations of Qxjot, Gaimard, 

 Beechet, Fitzkot, Nelson, Darwin, Dana, Couthotit, Jukes, 

 and Agassiz, tbat the vital operations of reef-btiilding Polyps 

 are limited within a certain Bathymetrical range; that beyond 

 that depth they cannot live; and that the forms which reefs 

 assume depend upon the elevation or subsidence of the ocean's 

 bed on which their foundations are laid. When we compare the 

 stupendous results obtained by the operations of a community 

 of Polyps with the boasted monuments of man, the latter sink 

 into insignificance. The great wall of China, or the pyramids 

 on Egypt's plains, are as nothing when contrasted with the 

 Atolls that stud the Coral Sea, and the Barrier reefs that stretch 

 along the shore of New Caledonia to the length of four hundred, 

 or those which extend along the north-east coast of AustraHa 

 for upwards of twelve hundred mUes. How marvellous the fact, 

 that these masses of calcareous rock have been secreted, through 

 successive ages, by generations of tiny architects, amidst the 

 waters of the ocean, and in defiance of the violence of its 

 ever-restless waves ! The study of such phenomena prepares 

 the mind of the geologist for the investigation of operations of 

 a similar character, that have taken place in the seas of former 

 periods of the earth's history; for many palaeozoic and secondary 

 rocks may be said to be ancient Coral reefs, which appear to 

 have been formed under conditions analogous to those now in 

 operation in the waters of the Pacific Ocean. 



Coral reefs are masses of limestone accumulated through long 

 periods of time, under certain physical conditions, by the living 

 energies of reef-building Actinozoa : they assume various forms, 

 in accordance with the outline of the coast, or the contour 

 of the submarine rock or bottom on which they commence to 

 buUd; such structures, however, are not entirely composed of 

 dead and living Coral, for as these islands are the favourite 

 abodes of many Mollusca, Crustacea, Echinodermata, and other 

 Eadiata, their skeletons after death largely contribute to 

 augment the growth of the reef. 



The principal reef-building Actinozoa belong to the groups 

 Poritidw, Astrceidce, and Meandrinidce; these form the large solid 



