294 CORALS AND COMAL ISLANDS. 



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

 maining 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 Howland'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 under- 

 gone 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 com- 

 pact and crystalline, sometimes soft and amorphous), fre- 

 quently 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 penetrated 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 evap- 

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



