THE COMPLETED ATOLL. 293 



guano is shallow, is also quite apparent here, and the conse- 

 quent characteristic 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 par- 

 tial, and, on breaking some of these, in the centre was usually 

 found a nucleus or core 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. 0° 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 Bakers 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 detritus, while the 

 whole island has undergone some elevation. It therefore pre- 

 sents 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 surface of the island is gently 

 depressed. 



Within this depression there are other ridges, parallel to 



