13 



The exhausted guano then becomes phosphatic in distinction to 

 being n'trogenous and aramoniacal ("leached"), and the subjacent 

 limestone undergoes a metamorphosis by a double decomposition, 

 into phosphate of lime. If the absorbing limestone is pure, the 

 phosphae of lime formed thereby will be correspondingly pure ; and 

 on the other hand, if the calcareous base is intermixed with clay or 

 sand or ferruginous material, the newly formed product will contain 

 alumina, silica, oxide of iron, etc., in like proportions. 



Such has been the undoubted origin of the deposits of Aruba 

 Rock phosphate, samples of which are on the table, and which are 

 typical of this kind of metamorp.iosis and will serve to illustrate many 

 similarly formed deposits, notably those of Curasao, Sombrera, 

 Navassa and Redondo (in which latter case the subsoil must have 

 been aluminous, since the mineral is a phosphate of alumina). 



In some cases the phosphatic principle may have been derived 

 from animal debris^ such as bones. 



The composition of animal bones varies somewhat, according to 

 the animal furnishing them, and even with the particular part of the 

 same animal, but the following analysis, expressed in loo parts, may be 

 taken as an average : — 



Green Bones. Bone Ash. 



Moisture 



^ • I "W (gelatine) 



Organic matter ) oo \r> 



Phosphate of lime 56 I 70 7c;°/ 



Phosi)hate of Magnesia 3 J ' "'^^" 



Carbonate of Calcium 3 



Alkaline Salts 4 

 Silica 



The bones of birds are even richer in phosphoric acid than those 

 of animals, but bones of amphibia and fish conta'n less thari those 

 of birds and animals. 



Amongst other animal organisms rich in phosphoric acid or 

 phosphate of lime may be mentioned certain shell fish, or rather their 

 shell remains, notably the shells ot Lmgula and Oibiaila, which 

 consist for the greater part of phosphate of lime, and are found in 

 accumulated beds in the Lower Silurian rocks, being thus described by 

 Sir Wm. Logan (Geology of Canada, 1863) : 



