Blackwclder — The Geologic Bole of Phosphorus. 297 



is correct, it affords another illustration of the well-known 

 tendency of colloidal or amorphous minerals to assume the 

 crystalline state with the lapse of time. 



With certain minor exceptions the transformations of phos- 

 phorus that take place on the surface of the earth have now 

 been reviewed. It remains to trace the element downward 

 into the interior of the crust and at the same time more deeply 

 into the realm of inference. Phosphatic deposits of any of 

 the types already described may have sediments deposited 

 upon them until, in the course of geologic ages, they may be 

 buried thousands of feet below the surface. In that region 

 pressures are great, temperatures are much increased, and the 

 activity of solutions is greater than, or at least different from, 

 that ahove. In the past, some of the older phosphate beds 

 have in addition been subjected to overwhelming compressive 

 forces, the origin of which is still a debatable subject with 

 geologists, although we see plenty of evidence of their opera- 

 tion in the folded rocks of our mountain systems. It is now 

 well known that under these conditions of the interior of the 

 earth, minerals of various kinds undergo radical changes. 

 Some, like calcite, merely recrystallize in more compact form. 

 Others recombine to form new minerals, while still others are 

 metamorphosed by either losing or gaining constituents. 

 Although no case of this kind for phosphates has yet been 

 proven, it is entirely in harmony with the established prin- 

 ciples of rock metamorphism for us to suppose that the phos- 

 phatic sediments would, under these conditions, be reorganized. 

 The hydrous minerals and the carbonates characteristic of the 

 surface should become dehydrated and decarbonated. As a 

 result, collophaniteand staff elite as well as many other minerals 

 of less importance, should pass over into the anhydrous calcium 

 fluophosphate, apatite, in which the proportion of P 2 5 may 

 rise to 42-43 per cent, — which is apparently the maximum 

 concentration attainable in rocks. It was suggested many years 

 ago by Sir William Dawson* and others that some of the rich 

 apatitic beds that are intimately associated with the ancient 

 Grenville marbles and gneisses near Ottawa, Canada, are really 

 the highly metamorphosed representatives of phosphatic sedi- 

 ments which were once deposited on the bottom of the sea. 

 No weighty arguments against their hypothesis have been 

 advanced. 



There is still another fate that may befall the phosphatic 

 deposits either before or after recrystallization. Rocks of any 

 kind in the crust are liable sooner or later to be invaded by 



*Sir J. W. Dawson, Note on the Phosphates of the Laurentian and Cam- 

 hrian Rocks of Canada, Quart. Journ. Geol. Soc. of London, vol. xxxii, 

 pp. 285-291, 1876. 



