1 2 ATOLLS. Ch. I. 



twenty-five soundings taken at a greater depth than 

 20 fathoms, every one shewed that the bottom was 

 covered with sand; whereas at a less depth than 

 12 fathoms, every sounding shewed that it was 

 exceedingly rugged, and free from all extraneous 

 particles. Two soundings were obtained at the 

 depth of 360 fathoms, and several between 200 and 

 300 fathoms. The sand brought up from these depths 

 consisted of finely triturated fragments of stony zoo- 

 phytes, but not, as far as I could distinguish, of a par- 

 ticle of any lamelliform genus : fragments of shells 

 were rare. 



At a distance of 2,200 yards from the breakers, Cap- 

 tain FitzRoy found no bottom with a line 7,200 feet in 

 length ; hence the submarine slope of this coral forma- 

 tion is steeper than that of any volcanic cone. Off the 

 mouth of the lagoon, and likewise off the northern 

 point of the atoll, where the currents act violently, the 

 inclination, owing to the accumulation of sediment, is 

 less. As the arming of the lead from all the greater 

 depths showed a smooth sandy bottom, I at first con- 

 cluded that the whole consisted of a vast conical pile of 

 calcareous sand, but the sudden increase of depth at 

 some points, and the fact of the line having been cut, 

 when between 500 and 600 fathoms were out, indicates 

 the probable existence of submarine cliffs of rock. 



On the margin of the reef, close within the line 

 where the upper surface of the Porites and of the 

 Millepora is dead, three species of Nullipora flourish. 

 One grows in thin sheets, like a lichen on old trees ; the 



