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 
diffused, 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 off 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 are 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 vertical or horizontal masses of apatite may be 
cut off suddenly all round or they may pinch out gradually cr 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 
