THE COMPLETED ATOLL. 323 
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 fifty per cent to less than thirty per cent. 
The gypsum, or sulphate of lime, is usually soft and amor- 
phous, sometimes crystalline, and, at a depth of eighteen 
inches to 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 sul- 
phate 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, McKean’s, and 
other islands of similar structure, it contains a large deposit 
of 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 
lagoons have similar deposits of gypsum. ae 
PAs regards the distribution of these phosphatic guano 
deposits, I believe them, in this region of the Pacific, to be 
confined to latitudes very near the equator, where rain is 
comparatively of rare occurrence. In latitudes more remote 
from the equator than 4° or 5°, heavy rains are frequent, and 
this circumstance is not only directly unfavorable to the for- 
mation of guano deposits, but it encourages vegetation, and 
when an island is covered with trees and bushes, the birds 
prefer to roost in them, and 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 
