106 CORAL-REEFS. 



of reefs, both outwards or horizontally and upwards or 

 vertically, under the peculiar conditions favourable to such 

 increase, is not slow, when referred either to the standard 

 of the average oscillations of level in the earth's crust, or 

 to the more precise but less important one of a cycle of 

 years. 



Section Third. 



On the Depths at ivhich Reef-building Polypifers live. 



I have already described in detail, which might have 

 appeared trivial, the nature of the bottom of the sea 

 immediately surrounding Keeling atoll; and I will now 

 describe with almost equal care the soundings off the 

 fringing-reefs of Mauritius. I have preferred this arrange- 

 ment, for the sake of grouping together facts of a similar 

 nature. I sounded with the wide bell-shaped lead which 

 Capt. Fitzroy used at Keeling Island, but my examination 

 of the bottom was confined to a few miles of coast (between 

 Port Louis and Tomb Bay) on the leeward side of the island. 

 The edge of the reef is formed of great shapeless masses of 

 branching Madrepores, which chiefly consist of two species, 

 — apparently M. corymbosa and pocillifera^ — mingled with a 

 few other kinds of coral. These masses are separated from 

 each other by the most irregular gullies and cavities, into 

 which the lead sinks many feet Outside this irregular 

 border of Madrepores, the water deepens gradually to 

 twenty fathoms, which depth generally is found at the 

 distance of from half to three-quarters of a mile from the 

 reef. A little further out the depth is thirty fathoms, and 

 thence the bank slopes rapidly into the depths of the ocean. 

 This inclination is very gentle compared with that outside 



