94 CORAL-REEFS, 



that no sensible change has taken place during eighty 

 years in the coral-knolls, and considering that every 

 single reef has reached the surface in other atolls, which 

 do not present the smallest appearance of being older 

 than Diego Garcia and Peros Banhos, and which are 

 placed under the same external conditions with them, one 

 is led to conclude that these submerged reefs, although 

 covered with luxuriant coral, have no tendency to grow 

 upwards, and that they would remain at their present levels 

 for an almost indefinite period. 



From the number of these knolls, from their position, 

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

 hundred yards across, with a rounded outline, and pre- 

 cipitous 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 small 

 proportional amount more 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, whilst 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 



