THE MINERAL FERTILIZERS 203
consist of carbonate of lime, which is altered to phosphate
of lime. The phosphate may at first form a thin impermeable
crust which may be broken by the collapse of solution cavities
in the underlying limestones, and the pieces are cemented into
phosphate breccia. In volcanic islands the phosphoric acid
produces phosphate of alumina, such as the phosphatized
trachyte of Clipperton Island in the Pacific Ocean off Mexico.
Phosphate of alumina is more expensive to convert into
superphosphate, and less valuable as a fertilizer than phos-
phate of lime; it is used for some special purposes, such as
the treatment of sewage.
The phosphate of Christmas Island south of Java includes
phosphatized volcanic rocks and coral reefs, and phosphatic
breccia; and these rocks are traversed by thin veins of
staffelite, a fibrous and concretionary variety of fluo-apatite,
which show that the phosphoric acid was introduced in
solution, doubtless as Andrews suggested from once overlying
guano
Nauru was discovered in 1798 and named Pleasant Island.
A block of rock from it, regarded as fossil wood, was used in
Sydney as a door weight. The recognition of its nature led
to the discovery that Nauru and Ocean Islands contain
about 100 million tons of high-grade phosphate. Cargoes are
shipped averaging 85 to 88 per cent. of tricalcic phosphate
(CagP,0g). The phosphate occurs in depressions between
pinnacles of limestone, and it does not pay to work deeper
than 20-30 feet. H. B. Pope (Austral. Indust. and Min.
Stand., 15 February, 1923) regards the phosphate as derived
from bird guano, though few birds live on the islands at present
(cf. L. Owen, 0.7.G.S., 1xxix, 1923, pp. 1-14).
Lagoon PHOspHATE—Some important beds of phosphate
of lime are formed in tropical and subtropical lagoons.
Dead animals are washed into them and their bones and fish
remains collect in patches, whence the current sweeps away
the finer sediment. The bones may be dissolved and re-
deposited in nodules of earthy phosphate of lime, which are
then known as coprolites.
The term coprolite was given to the fossil dung of reptiles
LC. W. Andrews, Monogr. Christmas Island, 1900, pp. 290-1. The
staffelite veins are described by the author, 77. G. Soc. Glasg., xvi, 1917,
p. 134, pl. VL.
