described by Mr Darwin. 35 



diameter of the atoll, or circle formed by the reef, varies from 

 less than one mile to 30 or 40. There is one 50 miles in length 

 by 20 in breadth ; so that, if the ledge of coral rock forming 

 the ring were extended in one line, it would be 120 miles in 

 length. Assuming it to be a quarter of a mile in breadth, and 

 150 feet deep, here is a mound compared with which the walls 

 of Babylon, the great wall of China, or the Pyramids of Egypt, 

 are but children's toys — and built too, amidst the waves of the 

 ocean, and in defiance of its storms, which sweep away the 

 most solid works of man. 



The wall of coral is generally breached in one or more places ; 

 and when the breaches are deep enough to admit a ship, the 

 atoll afibrds a convenient and safe harbour. 



Some of the atolls are perfect circles. The external side of 

 the reef often plunges to a depth of 200 or 300 fathoms, at an 

 angle of 45 degrees or more. At Cardoo Atoll no bottom was 

 found with a line of 200 fathoms (1200 feet), at the distance 

 of 60 yards from the reef. The internal side, on the other 

 hand, shelves gradually towards the centre of the lagoon, form- 

 ing a saucer-shaped cavity, the depth of which varies from one 

 fathom to fifty. In no instance has it been found entirely 

 filled up. Beyond the line where the coral ceases to grow, the 

 bottom of the lagoon consists of rolled fragments of it, or a 

 whitish mud consisting chiefly of the same substance in a com- 

 minuted state. Much of this mud is supposed to be produced 

 by certain species of fish and molluscous animals which browse 

 upon the coral ; grinding it down to fine meal, part of which 

 will pass from them and be deposited by the water. From 

 this description it will be seen that an atoll closely resembles 

 in form the cone of a submarine volcano, the coral reef repre- 

 senting the rim, the lagoon occupying precisely the place of 

 the crater. 



The islets formed on these reefs are very singular objects. 

 In storms, the sea throws up fragments of coral, sometimes 

 mixed with sand. The outer and lowest stratum of this mat- 

 ter, which is bathed by the sea at high tide, is sometimes con- 

 verted into a brecciated coral rock by calcareous infiltrations 

 from the water. Above this, and generally at the distance of 

 200 or 300 yards from the outer margin of the reef, the loose 

 fragments cast up in strong gales, mixed occasionally with sand 



