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Rh way becomes phosphatized; others are igneous, consisting of trachyte or basalt, and these rocks are also phosphatized on their surfaces but are not so valuable, inasmuch as the presence of iron or alumina in any quantity renders them unsuited for the preparation of artificial manures.

The leached guanos and phosphatized rocks, which are grouped with them for commercial purposes, have been obtained in great quantities in many islands of the Pacific Ocean (such as Baker, Howland, Jarvis and McKean Islands) between long. 150° to 180° W. and lat. 10° N. to 10° S. In the West Indies from Venezuela to the Bahamas and in the Caribbean Sea many islands yield supplies of leached guanos; the following are important in this respect Sombrero, Navassa, Aves, Aruba, Curagoa. Christmas Island has been a great source of phosphates of this type; also Jaluit Island in the Maldive Archipelago, Banaba or Ocean Island, and Nauru or Pleasant Island. On Christmas Island the phosphate has been quarried to depths of 100 ft. To these leached guanos and phosphatized limestones the name sombrerite has been given. It has been estimated that 500,000 tons of phosphate were obtained in Aruba, 1,000,000 tons from Curagoa since the deposits were discovered in 1870, and Christmas Island in 1907 yielded 290,000 tons.

In the older formations the phosphates tend to become more and more mineralized by chemical processes. In whatever form they were originally deposited they often suffer complete or partial solution and are redeposited as concretionary lumps and nodules, often called coprolites. The “Challenger” and other oceanographic expeditions have shown that on the bottom of the deep sea concretions of phosphate are now gathering around the dead bodies of fishes lying in the oozes, consequently the formation of the concretions may have been carried on simultaneously with the deposition of the strata in which they occur.

Important deposits of mineral phosphates are now worked on a large scale in the United States, the annual yield far surpassing that of any other part of the world. The most active operations are carried on in Florida, where the phosphate was first worked in 1887 in the form of pebbles in the gravels of Peace river. Then followed the discovery of “hard rock phosphate,” a massive mineral, often having cavities lined with nearly pure phosphorite. Other kinds not distinctly hard and consisting of less rich phosphatic limestone, are known as “soft phosphate”: those found as smooth pebbles of variable colour are called “land pebble-phosphate,” whilst the pebbles of the river-beds and old river-valleys, usually of dark colour, are distinguished as “river pebble-phosphate.” The land pebble is worked in central South Florida; the hard rock chiefly between Albion and Bay City. In South Carolina, where there are important deposits of phosphate, formerly more productive than at present, the “land rock” is worked near Charleston, and the “river rock” in the Coosaw river and other streams near Beaufort. The phosphate beds contain Eocene fossils derived from the underlying strata and many fragments of Pleistocene vertebrata such as mastodon, elephant, stag, horse, pig, &c. The phosphate occurs as lumps varying greatly in size, scattered through a sand or clay; they often contain phosphatized Eocene fossils (Mollusca, &c.). Sometimes the phosphate is found at the surface, but generally it is covered by alluvial sands and clays. Phosphate mining began in South Carolina in 1868, and for twenty years that state was the principal producer. Then the Florida deposits began to be worked. In 1892 the phosphates of Tennessee, derived from Ordovician limestones, came into the market. From North Carolina, Alabama and Pennsylvania, also, phosphates have been obtained but only in comparatively small quantities. In 1900 mining for phosphates was commenced in Arkansas. In 1908 Florida produced 1,673,651 tons of phosphate valued at 11 million dollars All the other states together produce less phosphate than Florida, and among them Tennessee takes the first place with an output of 403,180 tons.

Algeria contains important deposits of phosphorite, especially near Tebessa and at Tocqueville in the province of Constantine. Near Jebel Kouif, on the frontier between Algeria and Tunis, there are phosphate workings, as also in Tunis, at Gafsa. The deposits belong to the Lower Eocene, where it rests unconformable upon the Cretaceous. The joint production of Tunis and Algeria in 1907 was not less than a million tons. Phosphates occur also in Egypt, in the desert east of Keneh and in the Dakla oasis in the Libyan desert.

France is rich in mineral phosphates, the chief deposits being the departments of the Pas-de-Calais, Somme, Aisne, Oise in and Meuse, in the north-east, and another group in the departments of Lot, Tarn-et-Garonne and Aveyron, in the south-west: phosphates occur also in the Pyrenees. The deposits near Caylus and in Quercy occupy fissures and pockets in Jurassic limestone, and have yielded a remarkable assemblage of the relics of Tertiary mammals and other fossils. Phosphates occur in Belgium, especially near Mons, and these, like those of north-east France, are principally in the Upper Chalk. Two varieties of phosphate rock are recognized in these districts, viz. the phosphatic chalk and the phosphate sand, the latter resulting from the decomposition of the former. Large and valuable deposits of the sand have been obtained in sinks and depressions on the surface of the chalk. The production is on the whole diminishing in Belgium (180,000 tons in 1907), but in France it is still large (375,000 tons in 1907).

In the Lahn district of Nassau (Germany) there are phosphate beds in Devonian rocks. The deposits were rich but irregular and local, and were much worked from 1866 to 1884, but are no longer of economic importance. In northern Estremadura in Spain and Alemtezo in Portugal there are vein deposits of phosphate of lime. As much as 200,000 tons of phosphate have been raised in these provinces, but in 1906 the total production of Spain was only 1300 tons. Large deposits of phosphate occur in Russia, and those in the neighbourhood of Kertch have attracted some attention, it is said that the Cretaceous rocks between the rivers Dniester and Volga contain very large supplies of phosphate, though probably of low grade.

Phosphatic nodules and concretions, with phosphatized fossils and their casts, occur at various geological horizons in Great Britain. Bands of black nodules, highly phosphatic, are found at the top of the Bala limestone in North Wales; beds of concretions occur in the Jurassic series; and important deposits are known in the Cretaceous strata, especially in the Lower Greensand and at the base of the Gault. The Lower Greensand phosphates have been worked, under the name of “coprolites,” at Potton in Bedfordshire and at Upware and Wicken in Cambridgeshire. The Cambridge Greensand, rich in phosphatic nodules, occurs at the base of the Chalk Marl. The chalk occasionally becomes phoshatized, as at Taplow (Bucks) and Lewes (Sussex). At the base of the Red Crag in East Anglia, and occasionally at the base of the other Pliocene Crags, there is a “nodule bed,” consisting of phosphatic nodules, with rolled teeth and bones, which were formerly worked as “coprolites” for the preparation of artificial manure Professor R. J. Strutt has found that phosphatized nodules and bones are rich in radioactive constituents, and has brought this into relation with their geological age.

.—For American phosphates see The Phosphates of America, by Francis Wyatt (5th ed., New York and London, 1894); the Annual Reports on Mineral Resources of the U.S. (U.S. Geol. Survey), including some valuable reports by C. W. Hayes, also those in Rothwell’s Mineral Industry; “Nature and Origin of Deposits of Phosphate of Lime,” by R. A. F. Penrose, Jun., ''Bull. U.S.'' ''Geol. Survey, No 46 (1888); Florida, South Carolina and Canadian'' Phosphates, by C. C. Hoyer Miller (London, 1892); and The Non-metallic Minerals, by G. P. Merrill (1904). Many of the above include descriptions of mineral phosphates in other parts of the world. For a general discussion of the origin of the phosphates, see “The Natural History of Phosphate Deposits,” by, J. J. H. Teall, ''Proc. Geol. Assoc.'' xvi. 369 (1900). Consult also Étude complète sur les phosphates, by A. Deckers (Liége, 1894).

PHOSPHORESCENCE, a name given to a variety of physical phenomena due to different causes, but all consisting in the emission of a pale, more or less ill-defined light, not obviously due to combustion. The word was first used by physicists to describe the property possessed by many substances of themselves becoming luminous after exposure to light. This property has been noticed from early times. Pliny speaks of various gems which shine with a light of their own, and Albertus Magnus knew that the diamond becomes phosphorescent when moderately heated. But the first discovery of this property which apparently attracted scientific attention seems to have been that of the Bologna stone (barium sulphide), which was discovered