STRUCTURE OF CORAL ISLANDS. 171 



current, especially during the ebbing tide. At Depeyster 

 Island, it was found to run at the rate of two and a half miles 

 an hour. It was as rapid at Raraka, in the Paumotus, and, as 

 Capt. Wilkes remarks, it was difficult to pull a boat against 

 it into the lagoon. 



II. SOUNDINGS ABOUT CORAL ISLANDS. 



The water around coral islands deepens as rapidly and in 

 much the same way as off the reefs about high islands. The 

 atoll usually seems to stand as if stilted up in a fathomless sea. 

 The soundings of the Expedition afford some interesting re- 

 sults. 



Seven miles east of Clermont Tonnerre, the lead ran out to 

 1,145 fathoms (6,870 feet), without reaching bottom. Within 

 three quarters of a mile of the southern point of this island, the 

 lead, at another throw, after running out for a while, brought 

 up an instant at 350 fathoms, and then dropped off again and 

 descended to 600 fathoms without reaching bottom. On the 

 lead, which appeared bruised, a small piece of white coral was 

 found, and another of red ; but no evidence of living zoo- 

 phytes. On the east side of the island, three hundred feet 

 from the reef, a bottom of coral sand was found in 90 fathoms ; 

 at one hundred and eighty feet, the same kind of bottom in 85 

 fathoms ; at one hundred and thirty feet, a coral bottom in 7 

 fathoms ; and from this it decreased irregularly to the edge of 

 the shore reef. 



Off the southeast side of Ahii (another of the Paumotus), 

 about a cable's length from the shore, the lead, after descend- 

 ing 150 fathoms, struck a ledge of rock, and then fell off and 

 finally brought up at a depth of 300 fathoms. 



Two miles east of Serle's Island, no bottom was found at 

 600 fathoms. 



