300 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



dency to segregate or separate itself from mixtures containing it. 

 Hence we should expect to find that even when it formed only a 

 small proportion of the constituents of the rock in which a cavity 

 occurred, it would fill it up to the complete or partial exclusion of 

 the more abundant minerals. We do not know the precise nature 

 of the conditions which have caused the elimination of individual 

 minerals from the country rocks, and their deposition in the veins 

 which traverse them, but from the above considerations it is easy to 

 conceive that the phosphates might be separated out into cavities 

 from the enclosing rocks, in which the mineral is now but sparingly 

 difiiised, especially when we consider that apatite is soluble in heated 

 waters holding alkaline silicates, whereas the felspars and pyroxene 

 are not thus soluble. 



In the formation of the apatite masses, or of any other vein-like 

 deposits, it is not necessary to suppose that the whole space which 

 they now occupy was open at once like an empty cavity, or indeed 

 that it was ever open at all to any appreciable extent, but only that 

 where the slightest vacancy occurred from movement in the wall- 

 rock, it was immediately filled by particles of one or more of the 

 minerals of the parent rock, for the transference of which the condi- 

 tions were for the time favorable. 



The general form of the apatite deposits as seen in a section across 

 any of the courses of the primeval jointing approximates what we 

 should expect to find if the above hypothesis be correct. A vein in 

 descending, after following an ancient vertical joint past several 

 nearly horizontal branches, may suddenly jog ofi" to another parallel 

 joint to which the original opening had been transferred by a lateral 

 movement on the plane of one of the horizontal joints. The hori- 

 zontal branches which ai'e sometimes as large as the veins themselves, 

 when exposed in place, constitute the "floors" or "beds," and 

 they are as likely to be cut off" by throws along the planes of the 

 vertical joints as are the veins by throws along the horizontal joints. 

 In this way either vei'tical or horizontal masses of apatite may be 

 cut off" suddenly all round or they may pinch out gradually or irregu- 

 larly. The latter would result from the disturbance of the blocks of 

 the country rock all separated from each other by the three sets of 

 joints during the movemements which took place while the strata 

 were in a plastic condition. Both the veins and " floors " of apatite 

 are sometimes observed to curve or gradually change their dip. This 



