Kneeland. ] 238 [April 16, 
phate, but mostly as a neutral phosphate, and a portion even as super- 
phosphate. 
The pure crust contains 
Lime | ¢)sshan 12. Ba) os nS AR gece 
Phosphoric Acid . pei “ 50.04 
SulplmrreyAcidyy tf) «0. fed), 2 1.63 
a nearly pure diphosphate of lime; or 90 per cent., with an excess of 
about 3 per cent. of phosphoric acid, even allowing the amount of 
lime required by the sulphuric acid. 
That resting on the coral formation contains. 
Lime . . ° . : . 42.17 
Phosphoric Acid . : : “ 34.01 
Sulphuric Acid . : - 3.06 
with a little magnesia and carbonic acid, 
These and similar guano islands in the Pacific, are, according to 
Prof. Hague, in latitudes near the equator where rains occur but 
rarely; in the rainless Peruvian islands, the true guano occurs. 
Where rains are very heavy, at 4° or 5° from the equator, the ma- 
terials are washed away and vegetation is encouraged; the birds then 
roost in the trees or bushes and no deposits of guano are accumulated. 
The beds of gypsum, or sulphate of lime, remain after the evapora- 
tion of the sea water, which holds it in solution, in the gradually dry- 
ing lagoon. . 
Wherever large numbers of sea fowl frequent islands for the pur- 
pose of laying their eggs, guano may be deposited; but rains, as we 
have seen, by washing away the organic matters produce a very dif- 
ferent substance, chemically, from true guano. 
Enderbury Island, from which these specimens came, is two and 
three quarter miles long, by one mile wide, trapezoidal in form, and 
about eighteen feet above the sea at low tide, this rising about six 
feet. The beach is about twelve feet high, the first eight with a 
regular slope of 30° to’ 35°, made up of sand, pebbles, and rounded 
stones of coral, with a few shells; it is then horizontal, according to 
Dana, for eighty to one hundred feet, and then comes a gradual rise 
of three or four feet; the corals are principally immense foliaceous 
madrepores, some twelve to fifteen feet in diameter; many fragments 
on the beach are four inches thick, with thirty square feet of surface. 
Above the crown of the beach is a sandy ridge which encircles the 
