IV.] ROCK PHOSPHATES 117 



Carolina. There in many places the subsoil is a sandy- 

 deposit full of coprolitic pebbles, which can readily be 

 separated by screens or washing ; the beds of the rivers 

 and creeks, again, are wholly composed of the same 

 pebbles, which are recovered by dredging. The land 

 phosphate contains some oxide of iron and alumina, and 

 is chiefly sold in the United States, but the river 

 deposits have been particularly valued in Great Britain 

 for superphosphate making, because though they only 

 contained about 60 per cent, of phosphate of lime they 

 were specially free from iron and alumina. About 

 1 50,000 tons per annum used to be imported, but of late 

 years the supply has been falling off. The various 

 phosphate deposits in North America yielded in 1901 

 nearly 1,600,000 tons, of which more than half was 

 exported to Europe. 



Just as it is impossible to draw a line between the 

 recently formed true guanos and the weathered deposits 

 which have practically become phosphate rock, so again 

 no real distinction can be made between the guanos and 

 coprolites of known origin and the phosphate-bearing 

 strata which are to be found in many countries and at 

 all geological horizons. Many of these may have 

 originated in guano beds, others are coprolitic, others 

 again are due to solution of phosphate of lime, originally 

 diffused through a great mass of rock, and its concentra- 

 tion in a single layer. In all cases, however, the material 

 has been of animal origin, whatever processes of solution 

 and redeposition it may have suffered since. In the older 

 rocks the phosphate has often become crystalline, forming 

 the hard mineral known as apatite, which is mined on a 

 small scale in Canada and Norway. The Estremadura 

 deposits were perhaps the first of the rock phosphates 

 to be described, though they were not much worked 

 until the seventies of the last century. 



