io8 CORAL-REEFS. 



massive Porites, like that at Keeling atoll, of a Meandrina, 

 Pocillopora verrucosa, and of numerous fragments of Nulli- 

 pora. From fifteen to twenty fathoms the bottom was, 

 with few exceptions, either formed of sand, or 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 frag- 

 ment of Madrepora apparently M. pocUlifera, and I believe 

 it is the same species (for I neglected to bring speci- 

 mens from both stations) which mainly forms the upper 

 margin of the reef; if so, it grows in depths varying from 



to 20 fathoms. Between twenty and thirty-three fathoms 



1 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 cir- 

 cumstance of the arming having invariably come up quite 

 clean, when sounding within a certain number of fathoms 

 off the reefs of Mauritius and Keeling atoll (eight fathoms 

 in the former case, and twelve 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 these islands, the 

 limit above assigned would be found never to vary, but I 

 conceive the facts are sufficient to show, that the exceptions 

 would be few. The circumstance of a gradual change, in 

 the two cases, from a field of clean coral to a smooth sandy 



