204 ECONOMIC GEOLOGY

and fish. True coprolites are usually cylindrical or ovoid
in form, and have a spiral mark impressed by the wall of
the intestine. The bulk, however, of the coprolites used by
the phosphate industry are concretionary nodules which
may derive their phosphoric acid from either bones or dung.
Such lagoon and nodular deposits are represented by the
coprolite beds of the East of England, and among the phosphates
 of Florida, and South Carolina, on the coast of the
Unites States. Some of the American coastal phosphates
may have been formed by the alteration of limestone reefs
or islands from deposits of guano.
Some lagoons were underlain by limestone which has been
phosphatized by acid from above. This phosphate usually
occurs in isolated masses and hummocks, owing to the irregularity
 with which the phosphoric solutions percolated
through the overlying deposits or penetrated the limestone.
The American coastal phosphates in 1925 yielded 3,000,000
tons.
GRANULAR PHOSPHATES AND PHOSPHATIC CaaLr—Granular
phosphatic limestones and phosphatic chalk represent another
important source of phosphate. The phosphoric acid in
sea-water may act upon the microscopic shells of dead organisms
 and convert them into phosphate of lime either in their
fall through the sea-water or while lying on the sea-floor.
In most marine deposits the proportion of phosphatized
shells is small, but on shoals and parts of the sea-bed swept
by strong currents, the calcareous organisms may be broken
up and removed, while the harder and less soluble phosphatic
shells remain as layers of granular phosphate. Layers of
phosphatic chalk occur in the English chalk as at Taplow ;
in the North of France and Belgium this process has produced
 larger beds which supported the phosphate industries
of those countries. Some of the more massive occurrences, as
in the Somme valley, have been attributed to mineral springs.
In warmer seas the process has formed thick beds of limestone
charged with granular phosphate in Algeria, Tunis, and parts
of Egypt. The Carboniferous Limestone of the Rocky
Mountains belong to this group, and include large reserves
of granular phosphates which are too remote from fields
where fertilizers are required to be worked profitably at
oresent,