250 CORALS AND CORAL ISLANDS. 



island, and stretches, with some interruptions of intervening 

 sand, nearly from the north to the south end. Its surface is 

 even, and in many places covered by a thick growth of purslane, 

 whose thread-like roots abound in the guano where it grows. 

 The deposit rests on a hard coral bottom, and varies in depth 

 from six inches to four feet. The fact already observed at 

 Baker's, that vegetation flourishes most where the guano is 

 shallow, is also quite apparent here, and the consequent charac- 

 teristic difference between the guano of the deep and shallow 

 parts is distinctly marked. 



Some interesting pseudomorphs occur buried in the guano 

 of this island. Coral fragments of various species were found 

 that had long been covered up under the deposit, and in some 

 of which the carbonic acid had been almost entirely replaced 

 by phosphoric acid. In such I have found seventy per cent, 

 of phosphate of lime. In many others the change was only 

 partial, and, on breaking some of these, in the centre was 

 usually found a nucleus or co7X of coral, still retaining its 

 original hardness and composition, while the external parts had 

 been changed from carbonate to phosphate, which, though soft 

 and friable, still preserved the structure and appearance of the 

 coral. . 



Jarvis's Island is situated in lat. g° 22' south, and long. 159° 

 58' west, from Greenwich. It is nearly two miles long by one 

 mile wide, trending east and west, and containing about 1,000 

 acres. Like Baker's and Howland's, it has the general features 

 of a coral island, but it differs from them essentially in the fact 

 that it once contained a lagoon which has gradually been filled 

 up with sand and cietritus, while the whole island has undergone 

 some elevation. It therefore presents a basin-like form, the 

 surface being depressed from the outer edge 'toward the centre. 

 It is encircled by a fringing reef, or shore platform, about 300 

 feet wide ; from this a gradually sloping beach recedes, the 

 crown of which is from eighteen to twenty-eight feet high, 

 forming a ridge or border, of varying width, which surrounds 

 the island like a wall, from the in-shore edge of which the sur- 

 face of the island is gently depressed. 



