THE COMPLETED ATOLL, 



253 



a]ipearance somewhat resembling porcelain, but is usually 

 coloured more or less by organic matter. Generally it is very 

 hard, and strongly cohesive, though sometimes friable, and it 

 lies unevenly on the surface in rough fragments that are warped 

 and curved by the heat of the sun. It consists chiefly of phos- 

 phoric acid and lime, but, owing to the variable amount of 

 sulphate of lime, wnth which it is mechanically mixed, there is 

 a lack of uniformity in different samples. Hence the per- 

 centage of phosphoric acid varies from over 50 per cent, to 

 less than 30 per cent. 



The gypsum or sulphate of lime is usually soft and amor- 

 phous, sometimes crystalline, and, at a depth of eighteen inches 

 or two feet, occurs in hard, compact, crystalline beds. It is of 

 a light snuff colour, and, where it underHes guano, is mixed with 

 considerable phosphate of lime, which has been washed down 

 from the surface. Similar deposits of sulphate of lime occur 

 on many other elevated lagoon islands of the Pacific. 



Starbuck's, Starve or Hero Island, is an elevated atoll, and is 

 worthy of mention, because like Jarvis's, McKean's, and other 

 islands of similar structure, it contains a large deposit of gyp- 

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

 hydrated sulphate of lime, containing about twelve percent. 

 of phosphate of lime, and coloured by a little organic matter. 

 So far as my observation extends, all elevated lagoons 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 lati;:udes 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 unfavourable 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. 

 Julien, gives an account of. the various phosphatic minerals 



