24 ON CORAL REEFS AND ISLANDS. 



The ten islands here enumerated have an aggregate area of 

 1952 square miles, while the amount of actual dry habitable land 

 is but seventy-six miles, or less than one twenty-fourth. In the 

 Caroline Archipelago the proportion of land is still smaller. Men- 

 chikoff atoll covers an area of 500 square miles, and includes 

 hardly six square miles of wooded land. In the Marshall Islands 

 the dry land is not over one-hundredth of the whole surface ; 

 while in the Pescadores the proportion of land to the whole area 

 is about as 1 to 200. 



The distribution of the land upon the reef is obvious from the 

 sketches already given. It was long since remarked that the 

 windward side was in general the highest. It is also apparent 

 that there are not only great irregularities of form, but the reef 

 may at times be wholly wanting or deeply submerged on one 

 side. 



In many islands there is a ship entrance, sometimes six or eight 

 fathoms deep, through the reef to the lagoons, where good an- 

 chorage may be had ; but the larger part have only shallow pas- 

 sages, or none at all. In the Paumotus, out of the twenty-eight 

 visited by the Expedition, not one half were found to have naviga- 

 ble entrances. In the Carolines, where the islands are large and 

 not so much wooded, entrances are of more common occurrence. 

 About half of the Kingsmill Islands afford a good entrance and 

 safe anchorage. Through these openings in the reefs, there is 

 usually a rapid outward 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 Paumo- 

 tus, and as Capt. Wilkes remarks, it was difficult to pull a boat 

 against it, into the lagoon. 



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 Expe- 

 dition afford some interesting results. 



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

 1145 fathoms (6870 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 de- 

 scended 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 zoophytes. 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. 



