172 CORALS AND CORAL ISLANDS. 



A. mile and a half south of the larger Disappointment 

 Island, there was no bottom at 550 fathoms. 



Near the eastern end of Metia, an island nearly north of 

 Tahiti, no bottom was found with a line of 150 fathoms ; and, 

 a mile distant, no bottom was reached at 600 fathoms. 



In general, for one to five hundred yards from the margin 

 of the shore reef, the water slowly deepens, and then there is an 

 abrupt descent at an angle of 40 or 50 degrees. The results 

 of earlier voyagers correspond with this statement. At con- 

 siderable depths, as would appear from the above facts, the 

 sides of the coral structure may be vertical or even may over- 

 hang the bottom below. 



Beechey, whose observations on soundings are the fullest 

 hitherto published, states many facts of great interest. At 

 Carysfort Island, he found the depth, 60 yards from the surf 

 line, 5 fathoms ; — 80 yards, 13 fathoms; — 120 yards, 18 fath- 

 oms ; — 200 yards, 24 fathoms ; — and immediately beyond, no 

 bottom with 35 fathoms. At Henderson's Island, soundings 

 continued out 250 yards, where the depth was 25 fathoms, 

 and then terminated abruptly. Off Whitsunday, 500 feet 

 out, there was no bottom at 1,500 feet. 



Darwin states other facts bearing upon this subject, of 

 which we may cite the following: — At Heawandoo Pholo (one 

 of the Maldives), Lieutenant Powell found 50 or 60 fathoms 

 close to the edge of the reef. One hundred fathoms from the 

 mouth of the lagoon of Diego Garcia, Captain Moresby found 

 no bottom with 150 fathoms. At Egmont Island, 50 fath- 

 oms from the reef, soundings were struck in 150 fathoms. At 

 Cardoo Atoll, only 60 yards from the reef, no bottom was ob- 

 tained with a line of 200 fathoms. Off Keeling Island, 2,200 

 yards from the breakers, Captain Fitzroy found no bottom at 

 1,200 fathoms. Mr. Darwin also states that, at a depth between 



