TEE COMPLETED ATOLL. 297 



gypsum. Its supposed guano I have found to consist of the 

 hydrated sulphate of lime, containing about twelve per 

 cent, of phosphate of lime, and colored by a little organic 

 matter. So far as my observation extends, all elevated la- 

 goons have similar deposits of gypsum. 



.As regards the distribution of these phosphatic guano de- 

 posits, I believe them, in this region of the Pacific, to be con- 

 fined to latitudes very near the equator, where rain is compara- 

 tively of rare occurrence. In latitudes more remote from the 

 equator than 4° or 5°, heavy rains are frequent, and this cir- 

 cumstance is not only directly unfavorable to the formation of 

 guano deposits, but it encourages vegetation, and when an 

 island is covered with trees and bushes, the birds preferring 

 to roost in them, there is no opportunity for the accumulation 

 of guano deposits. 



An article in the same Journal (vol. xl., 1865) by A. A. Ju- 

 lien, gives an account of the various phosphatic minerals 

 formed from the guano deposits on a coral island, Sombrero, 

 in the Caribbean Sea. 



Lord Byron, of the Blonde, mentions that phosphate of 

 lime (apatite) was collected by him on Mauke, an elevated 

 coral island of the Hervey Group, west of the Society Islands, 

 but its exact condition in the rock is not stated. 



Coral islands are exposed to earthquakes and storms like 

 the continents, and occasionally a devastating wave sweeps 

 across the land. During the heavier gales, the natives some- 

 times secure their houses by tying them to the cocoanut trees, 

 or to a stake planted for the purpose. A height of ten or 

 twelve feet, the elevation of their land, is easily overtopped by 

 the more violent seas ; and great damage is sometimes expe- 

 rienced. The still more extensive earthquake-waves, such as 



