110 
DEPTH AT WHICH 
Ch. IV. 
a massive Porites like that at Keeling atoll, of a 
Meandrina, Pocillopora verrucosa, and of numerous 
fragments of Nullipora. From 15 to 20 fathoms the 
bottom was, with few exceptions, either formed of 
sand, oi thickly covered with Seriatopora : this delicate 
coral seems to form at these depths extensive beds 
unmingled with any other kind. At 20 fathoms, one 
sounding brought up a fragment of Madrepora, ap- 
parently M. pocillifera, and I believe it to be the 
same species as that which mainly forms the upper 
margin of the reef ; if so, it grows in depths varying 
from 0 to 20 fathoms. Between 20 and 23 fathoms I 
obtained several soundings, and they all showed a sandy 
bottom, with one exception at 30 fathoms, when the 
arming came up scooped out as if by the margin of a 
large Caryophyllia. Beyond 33 fathoms I sounded 
only once ; and from 86 fathoms, at the distance of one 
mile and a third from the edge of the reef, the arming 
brought up calcareous sand with a pebble of volcanic 
rock. The circumstance of the arming having in- 
variably come up quite clean when sounding within a 
certain number of fathoms off the reefs of Mauritius 
and Keeling atoll (8 fathoms in the former case, 
and 12 in the latter), and of its having always 
come up (with one exception) smoothed and covered 
with sand when the depth exceeded 20 fathoms, 
probably indicates a criterion, by which the limits 
of the vigorous growth of coral might in all cases be 
readily ascertained. I do not, however, suppose that 
if a vast number of soundings were obtained round 
