100 



water, but none of them working beyond one hundred feet oi 

 the surface. When reef structure is found beyond this depth, 

 it has been caused by the subsidence of the ocean bottom on 

 which such formations were originally laid. Should the reef 

 region be slowly sinking, at a rate not faster than that at which 

 the Polyps can make the reef rise, then almost any thickness may 

 be obtained through long periods of operation. The observations 

 made by Professor Dana about the Coral regions of the Pacific, 

 hare led to the conclusion "that some of the reefs have a thickness 

 of two or three thousand feet, or more, and which has been acquired 

 during such a slow subsidence." ^ Daewin^ says that thick beds 

 of Coral are formed only at small depths beneath the surface 

 of the sea, and that Captain Moresby, — whose opporttmities 

 for observation during his survey of the Maldiva and Chagos 

 Archipelagoes have been unrivalled, — informed him that the 

 upper part or zone of the steep-sided reefs, on the inner and 

 outer coasts of the Atolls in both groups, invariably consists of 

 Coral, and the lower parts of sand. At seven or eight fathoms 

 depth, the bottom is formed, as could be seen through the clear 

 water, of great living masses of Coral, which at about ten 

 fathoms generally stand some way apart from each other, with 

 patches of white sand between them, and at a little greater 

 depth these patches become united into a smooth steep slope 

 without any Coral; and when we know that the reefs round 

 these islands do not differ from other Coral formations in their 

 form and structure, we may conclude that in ordinary cases 

 reef-building Polypifers do not flourish at greater depths than 

 between twenty and thirty fathoms. Eheenbeeg^ says of the 

 Coral reefs of the Red Sea, which he carefully studied : — " The 

 living Corals do not descend there into great depths. On the 

 edges of islets and near reefs, where the depth was small, very 

 many Hved ; but we found no more, even at six fathoms. The 

 pearl-fishers at Yemen and Massaua asserted that there was no 

 Coral near the pearl-banks at nine fathoms depth, but only 

 sand." 



1 Dana, Text Book of Geology, p. 268. 



2 Dakwin, Structure and Distribution of Coral Reefs, pp. 82-83. 



" ^ Ehrenberg, iJber die Natur und BUdung der CoraUeninseln im Rothen-Meere' 

 p. 56. 



