9-1 
CONDITIONS OF GROWTH. 
Ch. IV. 
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 
late 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 Peros Banhos atoll is in the same 
condition. These facts, as will hereafter be shown, 
render it probable that the whole group at some 
former period subsided seven or eight fathoms ; and 
that the coral perished on the outer margins of those 
atolls which are now submerged, but that it continued 
alive and grew up to the surface on the others now 
perfect. If all these atolls did formerly subside, and if 
from the suddenness of the movement or from any other 
cause, those species of corals which are best adapted 
to live at a certain depth, once got possession of the 
knolls, supplanting their former occupants, they would 
have little or no power to grow upwards. To illustrate 
this, I may observe that if the corals of the upper 
zone on the outer edge of Keeling atoll were to perish, 
it is improbable that those of the lower zone would grow 
to the surface, and thus become exposed to conditions 
for which they do not appear to be adapted. The con- 
