THE COMPLETED ATOLL. 25 1 



Within this depression there are other ridges, parallel to the 

 outer one, and old beach-lines and water-marks, the remaining 

 traces of the waters of the lagoon, marking its gradual decrease 

 and final disappearance. 



This flat depressed surface in the centre of the island is 

 about seven or eight feet above the level of the sea. It bears 

 but little vegetation, consisting of long, coarse grass, Mesem- 

 bryanthemum and Portulacca, and that is near the outer edges 

 of the island, where the surface is formed of coral sand mixed 

 with more or less guano. In the central and lower parts the 

 surface is composed of sulphate of lime (gypsum), and it is on 

 this foundation that the principal deposit of guano rests. 



In examining the foundation of the guano deposit on Baker's 

 or Rowland's Island, by sinking a shaft vertically, the hard 

 conglomerate reef-rock is found directly underlying the guano. 

 Resting on this foundation the guano has undergone only such 

 changes as the climate has produced. On Jarvis's Island, 

 however, after sinking through the guano, one first meets with 

 a stratum of sulphate of lime (sometimes compact and crystal- 

 line, sometimes soft and amorphous), frequently two feet thick, 

 beneath which are successive strata of coral sand and shells, 

 deposited one above the other in the gradual process by which 

 the lagoon was filled up. These horizontal strata were pene- 

 trated to a depth of about twenty feet. They were composed 

 chiefly of fine and coarse sand with an occasional stratum of 

 coral fragments and shells. 



Of the origin of this sulphate of lime there can hardly be 

 any doubt. .As the lagoon was nearly filled up, while, by the 

 gradual elevation of the island, the communication between 

 the outer ocean and the inner lake was constantly becoming 

 less easy, large quantities of sea-water must have been eva- 

 porated in the basin. By this means deposits would be formed 

 containing common salt, gypsum, and other salts found in the 

 waters of the ocean. From these the more soluble parts would 

 gradually be washed out again by the occasional rains, leaving 

 the less soluble sulphate of lime as we find it here. 



Some additional light is thrown on this matter by the 



