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 phos- 
phates 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 ir- 
regularity 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 organ- 
isms 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 pro- 
duced 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,