12 
ATOLLS. 
Ch. I. 
by an uneven line) round and beneath the external 
margin. Between 12 and 20 fathoms the arming came 
up an equal number of times smoothed with sand, and 
indented with coral : an anchor and lead were lost at 
the respective depths of 13 and 16 fathoms. Out of 
twenty-five soundings taken at a greater depth than 
20 fathoms, every one showed that the bottom was 
covered with sand ; whereas at a less depth than 
12 fathoms, every sounding showed 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 
zoophytes, but not, as far as I could distinguish, of a 
particle of any lamelliform genus : fragments of 
shells were rare. 
At a distance of 2,200 yards from the breakers, 
Captain FitzRoy found no bottom with a line 7,200 
feet in length ; hence the submarine slope of this coral 
formation 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 concluded 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, 
