296 CORALS AND COMAL ISLANDS. 



similar origin to that just described above ; part rests on a 

 coral formation; while still another part, covering a large 

 tract, has been by the action of water mixed, with coral mud. 



The first named deposit, lying on the sulphate of lime bed, 

 has a peculiar character. It is covered by, or consists of, a 

 hard crust, that is from one-fourth of an inch to an inch and 

 a half in thickness, beneath which lies a stratum of guano 

 varying in depth from one inch to a foot. In many places 

 where the guano was originally shallow, the whole is taken 

 up and formed into the hard crust which then lies immedi- 

 ately on the sulphate. This crust, when pure, is snow-white, 

 with an appearance somewhat resembling porcelain, but is 

 usually colored 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 phosphoric acid and lime, but, owing to the variable 

 amount of sulphate of lime with which it is mechanically mixed, 

 there is a lack of uniformity in different samples. Hence 

 the percentage 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 color, and where it underlies 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 centains a large deposit of 



