70 ORIGIN AND OCCURRENCES OF PHOSPHATE ROCK 



other organisms. These latter are again eaten by sea birds, 

 which excrete much of the phosphoric acid in their droppings. 

 Phosphates ]n sedimentary rocks may therefore be derived 

 from (1) bird, seal or fish excrements ; (2) bones of fishes 

 and other animals ; (3) shells of molluscs, molluscoids, 

 Crustacea, corals, foraminifera, etc. ; (4) remnants of swamp 

 vegetation. Phosphoric acid, like carbon, is constantly 

 changing its place bet\\een the organic and inorganic world. 



MODE OF OCCURRENCE AND ORIGIN. 



Apatite occurs in all rocks in small amount, and, being 

 the most soluble mineral in igneous rocks, it easily passes 

 away into sedimentary formations and the sea to be absorbed 

 into organic life when the rock decomposes. But apatite 

 of this kind is of no commercial value, the percentage in 

 igneous rocks being too small for profitable separation. In 

 some cases, however, large dykes and sheets of pure apatite 

 occur in igneous rocks and in metamorphic sedimentary 

 rocks in proximity to great plutonie intrusions. The most 

 notable instances are the Canadian and Norway occurences. 

 In such cases profitable mining is possible. The origin of 

 these dykes and intrusive sheets is due to vapour action in 

 the period of cooling of the great igneous masses with, which 

 they are conected. Vapours or hot waters containing 

 phosphoric, hydrochloric and hydrofluoric acids have 

 circulated in the cracks and joints formed by the cooling of 

 the magma, or formed in the adjoining sedimentary forma- 

 tions by the thrust of the intrusion. In these passages the 

 phosphoric acid has been precipitated in combination with 

 calcium. 



Phosphorite occurs associated both with igneous and 

 sedimentary rocks. It may occur either as veins in igneous 

 rock, or bedded in sedimentary or metamorphic rocks, or 

 in lenticular bodies in-filling former caverns in limestones, 

 or as an alteration product of limestone underlying guano 

 deposits. In igneous rocks it may be either a primary 

 product of vapour action like the apatite described above, 

 or a secondary product formed by the alteration of calcite 

 veins by solutions containing phoshporic acid from over- 

 lying guano beds. 



Secondary phosphorite owes its phosphoric acid to the 

 decomposition of organic remains. Guano, bone breccia, 

 decaying animal and vegetable matters have the soluble 



