93 



111 some crystaliine rocks we find apatite, or crystalline calcic phosphate 

 so abnndant that it can easily be recognized under the microscope. It 

 is needless to say, however, that the percentage of phosphate present 

 in ordinary rock masses is quite too small to suit them to be used as 

 fertilizers for exhausted soils. We must have recourse to some richer 

 sources of supply, and the concentration of phosphates in nature is 

 generally found to have been brought about by organic agency. Of 

 these concentrated deposits of phosphatic matter we may first glance at 

 those known as Guanos. These are essentially composed of excrements 

 of sea birds. Extensive accurauhitions of this character can occur 

 only in dry climates, for though formed wherever sea birds congregate 

 in great numbei-s, the rainfall is usually sufficient to remove them 

 before they reach important dimensions. 



Guanos are naturally divided into two classes, though between the 

 extremes of these there are many intermediate varieties. These 

 classes have been named respectively nitrogenous and phosphatic 

 Those of the first class occur in exceptionally dry climates, such as are 

 found on the coast and adjacent islands of Peru, Bolivia and Chili, 

 where rain seldom or never falls In these the nitrogenous constituents 

 of the organic matter — converted by decomposition into ammonia 

 salts — remain as a part of the mass. In phosphate guanos, on the 

 contrary, the rainfall has been sufficient to remove the whole or nearly 

 the whole of the very soluble ammonia salts, while not enough to wash 

 away the phosphatic material. Guanos of this class are of common 

 occurrence in the West Indian islands, and in some of these in which 

 the subjacent coral rock is penetrated by caverns, only such i)arts of the 

 phosphatic accumulations are preserved as have been washed into these 

 subterranean hollows through fissures, or have penetrated to them in 

 solution through the j)orous coral rock. 



In the Ardennes region of the south of France, phosphatic deposits 

 occur which, in my opinion, are very similar in origin to those just 

 alluded to. These, however, are very much older and in fact include 

 fossils of Tertiary age, and so far as known, none of modern forms. 

 They fill irregular cavernous fissures which traverse the surface of 

 plateaus composed of Jurassic limestone, and it would appear that the 

 higher parts of these i)lateaus have at one time formed an archipel-- 



