10 



The classification of natural phosphates of lime is, as remarked 

 by Dr. Penrose in Bulletin No. 46 of the U. S. Geological Survey, "a 

 matter attended with many difficulties, not only on account of the great 

 variety of forms in which phosphate of lime occurs, but also because 

 many varieties blend into one another, thus often rendering it uncertain 

 to which class a special deposit should be referred," and he adopts the 

 following classification, based mainly on the chemical composition of 

 the deposits, and grou|)ed under the headings thus : — 



, ,. , T^i 1 ^ ( Apatites f Fluor-Apatites 



Mmeral Phosphates < j^l , . i r-^ i \ ..-^ 



' I Phosphorites ( Chlor-Apatites 



/ . , J 1 f Loose nodules. 



1 Amorphous nodules ^, . j / 1 4. \ 



' '■ ' Cemented (conglomerates) 





,, , T^, 1 , I Phosphoric limestone beds 



Rock Phosphates < ' / o 1 ui 



' I ^ Soluble guanos 



I Guanos ■[ j , f 



I ( Leached gU;:.nos 



VBone beds. 



We shuU recognise as we proceed with the study of the various 

 phosphatic deposits, formed during the different geologic periods, that 

 by tar the greater part owe their origin to animal or organic remains, 

 and we shall see that as soon as the organic compounds of a guano, 

 for example, are dissipated and resolved into their elements, we may 

 consider that the residual products, to all intents and purposes, revert 

 to the mineral state, in accordance with the iamiliar expression " earth 

 to earth." 



We pass over, for the present, the guano of various localities, which 

 however will be observed to lie mostly within 10 to 20 degrees of the 

 equator. 



We should remember, however, that this product has attained its 

 zenith, both as to quality and quantity, and must cede its commercial 

 importance ultimately to the mineral resources of phosphoric acid, which 

 are before us for our more particular consideration. 



We shall find the diagram on the wall which shows the approximate 

 geological position or age of the different phosphate deposits, very use- 

 lul to our present purpose, and we will commence with the more recent- 

 ly formed or mineralized products. 



