STRUCTURE OF CORAL ISLANDS. 



139 



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 surt 

 line, 5 fathoms; 80 yards, 13 fathoms; 120 yards, 18 

 fathoms ; 200 yards, 24 fathoms ; and immediately beyond, 

 no bottom with 35 fathoms. At Henderson's Island, sound- 

 ings 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 moudi 

 of the lagoon of Diego Garcia, Captain Moresby found no 

 bottom with 150 fathoms. At Egmont Island, 50 fathoms 

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

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

 obtained 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 five and six hundred fathoms, the line was 

 partly cut as if it had rubbed against a projecting ledge of 

 rock ; and deduces from the fact " the probable existence of 

 submarine cliffs." 



Prof. Agassiz states that the Bahamas and the reefs north- 

 east of Cuba have very great depth close alongside. 



There are examples also of less abrupt slopes. North-west 

 of the Hawaiian group. Captain Lisiansky, who commanded 

 the Russian ship Neva in a voyage round the world in the 

 years 1860-61, at the island bearing his name, found 



