CORAL-REEFS. 23 



is confined to a zone outside, and beneath the surface, 

 where it must be very abundant. Fragments of the Mille- 

 pora alcicornis and of an Astrsea were also numerous ; 

 the former is found, but not in proportionate numbers, 

 in the hollows on the reef; but the Astraea I did not see 

 living. Hence we may infer, that these are the kinds of 

 coral which form the rugged sloping surface (represented 

 in the woodcut 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 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 sound- 

 ings 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 of 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 



