Sect. III. DEPTH. 107 



knolls from their harbours, that their growth can 

 hardly be very slow. I may add, that M. Duchassaing 

 broke off all the Madrepores growing on a marked 

 place in a bay at Ghiadaloupe ; and in the course of 

 two months he found there a greater number of corals 

 than before. 1 



From the facts given in this section, it may be 

 concluded, first, that considerable thicknesses of rock 

 have certainly been formed within the present geo- 

 logical era by the growth of corals and the accumu- 

 lation of their detritus ; and, secondly, that the 

 increase of individual corals and of reefs, both out- 

 wards or horizontally, and upwards or vertically, under 

 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 which Beef -building Corals live. 



I have already described in detail the nature of the 

 bottom of the sea immediately surrounding Keeling 

 atoll ; and I will here describe with almost equal 

 care the soundings off the fringing-reefs of Mauritius. 

 I sounded with the wide bell-shaped lead which Captain 

 FitzRoy used at Keeling Island. My examination 

 of the bottom was confined to a few miles of coast 



1 L'Institut, 1846, p. 111. 



