ARTIFICIAL AND CONCENTRATED MANURES. 87 



since which time very great progress has been made in 

 developing the deposits. These deposits occur in a num- 

 ber of forms, first, " soft phosphate," a whitish product, 

 somewhat resembling clay, and largely contaminated with 

 it ; second, " pebble phosphate," consisting of hard peb- 

 bles, occurring both in river-beds and upon the land, and 

 mixed with other materials ; and third, " rock," or " bowl- 

 der phosphate," which occurs in the form of stony masses, 

 or bowlders, both large and small. These three forms 

 also differ widely in composition, both in reference to 

 their content of phosphoric acid and in respect to the 

 presence of other minerals. 



The soft phosphate is the poorest in phosphoric acid : 

 it is easily prepared, and is largely used directly upon 

 the land ; it is also the most variable in composition, 

 ranging from eighteen to thirty per cent. The pebble 

 rock is also variable in composition, though, when washed 

 free of sand and clay, it is richer in phosphoric acid 

 than the soft variety; good samples contain as high as 

 forty per cent and over of phosphoric acid. The bulk 

 of the "Florida Phosphate" is believed to exist in the 

 pebble form. 



The rock or bowlder phosphate, though apparently 

 much less in amount, is more uniform in composition, 

 and is much richer than either of the other forms. The 

 clean, dry bowlder phosphate often contains as high as 

 forty per cent phosphoric acid, far exceeding in richness 

 the South Carolina rock superphosphate. 



Canadian Apatite. This material is a crystallized 

 rock of true mineral origin, and occurs associated to a 

 greater or less extent with other materials. It is, there- 



