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30 On Coral Reefs and Islands. 



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Off the sontheasE^ side of Ahii (another of the Paumotns), 

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

 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. 



A mile and a half south of the larger Disappointment Island, 

 there was no bottom at 550 fathoms. 



Near the eastern end of Metia, 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, among whom Beechey stands pre-em- 

 inent, correspond with this statement. At considerable depths, 

 as would appear from the above facts, the sides of the coral struc- 

 ture may be vertical or even may overhang the bottom below. 



There are examples also of less abrupt slopes. Northwest of 

 the Hawaiian Group, Lisiansky, at the island bearing his name, 

 found shallow water for a distance of six or seven miles : the water 

 deepened to ten or eleven fathoms the first mile, fifteen the sec- 

 pnd, and at the last throw of the lead there were still but tvyenty- 

 five fathoms.f Christmas Island affords on its western side anoth- 

 er example of gradually deepening waters. Yet these shallow 

 waters terminate finally in a rapid declivity of forty or fifty de- 

 grees. Off the prominent angles of an atoll, soundings generally 

 continue much beyond the distance elsewhere, as was first ob- 

 served by Beechey. At Washijjgton Island, mostly abrupt in its 

 shores, there is a bank, according to the surveys of the Expedi- 

 tion, extending from the east point to a distance of half a mile, 

 and another on the west extending to a distance of nearly two 



* Beechey, whose observations on soundings are the fullest hitherto published, 

 states many facts of ^^reat interest At Carysfort Island, he found the depth 60 

 yards from the surf hne, 6 fathoms:— 80 yards, 13 fathoms;— 120 yards, 18 fath- 

 oms;— 200 yards, 24 fathoms;— and immediately beyond, no bottom with 35 fath- 

 oms. At Hendersons 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 1500 feet. 



Darwin states many-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 



fathom?. At Egmont Island, 50 fathoms from the reef, soundings tv ere struck in 

 150 Mhoms. At Cardoo Atoll, only 60 yards from the reef, no bottom was ob- 

 tained witli a Une of %)0 fathoi|^. Off Keeling Island, 2200 yards from the breakers, 

 ^ Captain Fitzroy found no bottom at 1200 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 ijock ; and deduces from the fact "the probable 

 existenc^uf submarine cliffs.** iH. 



+ "^^oj^ge round the world, in the years 1803-6, in the ship Neva,U X. Lisiansky, 

 Captain iathe Russian ^avy, 4to, London ;^j)p. 254-257. 



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