38 MANUFACTURE OF FERTILIZING MATERIALS 



grains to 1 in. in diameter, averaging possibly a 

 little over J in. They are hard and have usually 

 a matrix of phosphate clay and sand. The per- 

 centage of phosphate of lime which the pebbles 

 contain is from 75 to 80, but the average material 

 as mined would not reach this standard. This ore 

 is mined open cut, by dredging, and by hydraul- 

 icking; it is then washed, dried, and shipped to 

 fertilizer works, mostly abroad. 



River-pebble phosphates are found as bars in 

 the rivers of southern Florida, and with them are 

 found the fossil remains of vertebrates. The 

 river pebble is blue or black, varying from 1 in. 

 downward in size, and frequently occurring as the 

 hardened casts of small molluscs. The percentage 

 of phosphate of lime in the river pebbles is between 

 58 and 68; at present river dredging for phos- 

 phate pebbles is not as active as the mining opera- 

 tions for land pebbles. 



Florida phosphate beds are covered with soil to 

 varying depths, which is removed by means of 

 steam shovels or hydraulic nozzles. If the deposits 

 are below water level, traction dredges working on 

 the land, or floating dredges, are used. 



The overburden of land deposits is stripped 

 from the ore by steam shovels, a cut being taken 

 the full swing of the shovel boom, and the shovel 

 moved forward a short rail length when this is 

 finished. 



The capacity of the dipper or shovel varies from 

 1 to 2 1 cu.yd., and the boom and dipper arm is 



