Skct. I. THE GROWTH OF CORAL-REEFS. 93 



tendency to grow upwards, and that they would remain 

 at their present levels for an indefinite period. 



From the number of these knolls, from their posi- 

 tion, size, and form, — many of them being only one or 

 two hundred yards across, with a rounded outline and 

 precipitous sides, — it is indisputable that they have been 

 formed by the growth of coral; and this makes the case 

 much more remarkable. In Peros Banhos and in the 

 Great Chagos bank, some of these almost columnar 

 masses are 200 feet high, and their summits lie only from 

 two to eight fathoms beneath the surface ; therefore, 

 a little greater proportional amount of growth would 

 cause them to attain the surface, like those numerous 

 knolls which rise from an equally great depth within 

 the Maldiva atolls. We can hardly suppose that time 

 has been wanting for the upward growth of the coral ; 

 as in Diego Garcia, the broad annular strip of land, 

 formed by the continued accumulation of detritus, shows 

 how long this atoll has remained at its present level. 

 We must look to some other cause than the rate of 

 growth ; and I suspect it will be found in the reefs being 

 formed of different species of corals, adapted to live at 

 different depths. 



The Great Chagos bank is situated in the centre of 

 the Chagos group, and the Pitt and Speaker banks at 

 its two extreme points. These banks resemble atolls, 

 except in their external rim being about eight fathoms 

 submerged, and in being formed of dead rock, with very 

 little living coral on it : a portion nine miles long of 

 the annular reef of Peroi Banhos atoll is in the same 



