382 AUSTRALIA : 



fact of the small circular coral islet rising up to the surface 

 of the Pacific from unfathomed depths around it ; or other 

 singularities of these constructions which we have no room to 

 detail, but which are fully described by naturalists. 



These considerations bring us nearer to the actual theory 

 of coral rocks, but with the intervention still of another 

 question (of great moment, as we shall see, to any general 

 conclusions), viz. the depth of ocean at which the reef- 

 building corals can live and work ? This, it will be obvious, 

 is a question distinct from that of the gross thickness or 

 depth of coralline masses, and somewhat easier of solution, 

 yet not without its appropriate difficulties. From a large 

 collation of facts Mr. Darwin is led to conclude that, in ordi- 

 nary cases, the corals which build reefs do not flourish at 

 greater depths than from 20 to 30 fathoms, and that the 

 greatest activity of their existence is on the surface and 

 outer edges of reefs. Other observers have limited their 

 range of operation still more closely to the surface ; but take 

 what estimate we may, it seems certain that no increment 

 can take place to coral growth below a comparatively small 

 depth ; none whatsoever above the surface washed by the 

 spray of the sea. Whence, then, the vast masses and lofty 

 coral pinnacles which the sounding-line follows downwards 

 to the depth of several hundred fathoms, with evidences of 

 the same structure and origin, and leaving it uncertain 

 whether they may not descend deeper still? If these 

 zoophytes work only thus near to the surface, how are we to 

 explain the origin and actual position of all that lies beneath ? 

 This is the problem most interesting in the theory of coral 

 formations ; and the solution of which, whatever it be, asso- 

 ciates them most closely with the great geological phenomena 

 of the globe. In the article before alluded to we have given 

 an outline of the question and of Mr. Darwin's views upon 

 it. In the short space now remaining to us we shall put 



