494 ■ C. E. Tilley — Metainorpliism of 



entering the constitution. At the periphery and along cracks or 

 cleavages it is changing to actinolite. The potash felspar is 

 present in considerable amount. The type of extinction is that 

 characteristic of certain microclines in which a submicroscopic 

 twinning is suggested. 



Scapolite is plentifully distributed in brilliantly polarizing grains, 

 whose double refraction is greater than '035, and it has the high 

 refractive index corresponding to a scapolite of the meionite end of 

 the series. It is uniaxial and optically negative. The rectangular 

 cleavages are well developed : they are not pinakoidal cleavages, 

 but prismatic, and the extinction of sections, which are not quite 

 basal is in accord with this. This apparently is the case for the 

 scapolites of the Grenville series of Ontario as described by Adams 

 and Barlow.^ The mineral separates with plagioclase from the rock 

 powder in a bromoform-benzol solution of density 2'72. It dissolves 

 in a mixture of strong hydrochloric and hydrofluoric acids without 

 any effervescence. 



In thin section the interference colours are as high as third order 

 greens when the diopside gives second order yellows. 



Plagioclase is present in small amount, with the usual twinning 

 lamellae. Separated from the rock powder, it has a specific gravity 

 greater than 2"72. The refractive index is greater than r56, but 

 only a few sections show a R.I. greater than 1"57. yS is just under 

 1*57. Most sections give a negative sign, but a few show positive 

 birefringence. The symmetrical extinctions of albite lamellae 

 reach 40 degrees. The plagioclase is therefore a bytownite, and 

 positive sections indicate a transition to labradorite. The grains 

 are sometimes bordered by a scapolite fringe, and in other cases the 

 grain is broken up and converted into a confused aggregate of 

 scapolite, with isolated sectors of plagioclase. 



The remaining constituents are subordinate in amount. Strongly 

 pleochroic actinolite borders the pyroxene, and in a few cases 

 appears to be intergrown with a small amount of biotite. A mineral 

 regarded as sphene is sparsely distributed through the rock. It is 

 usually in more or less rounded grains of a brown colour. Occasionally, 

 however, traces of prism and pyramid faces can be seen, and in such 

 cases the extinction is distinctly oblique. Pleochroism is often 

 noticeable, the change being from a light to a deeper brown. 



The axial angle is not large to judge by the curvature of the 

 isogyres. The birefringence is positive, and a strong axial dispersion 

 is to be observed p > v. 



Graphite is present in sjDongy and irregular flakes, apatite in 

 prisms and grains embedded in microcline, or associated with 

 scapolite. Quartz is quite localized, and is probably part of a 

 veinlet which cuts the rock. 



In another rock (216) plagioclase takes the place of scapolite in 

 the type already described. The plagioclase is of the same 



1 F. D. Adams and A. E.' Barlow, Mem. Geol. Surv. Can., No. 6, 1910, p. 102. 



