rilE SHAP GRANITE AND ASSOCIATED ROCKS. 295 



whiter and more evidently crystalline appearance, aud stands out 

 like pebbles on a weathered surface, often enclosing a kernel of 

 hornblende. At the same time the dull grey ground of the rock 

 becomes blacker and more compact, and often contains greenish 

 crystalline streaks of hornblende, or more rarely a pyroxenic 

 mineral in its place. Pyrites is of common occurrence in these 

 streaks and in the interior of some of the vesicles. The dark 

 colour of the rock is due to the development of mica, and nearer to 

 the granite this mineral imparts a brown or purplish-brown sheen 

 to the rock, and eventually becomes apparent to the eye. IS^ear the 

 contact the vesicles lose something of the distinctness of their 

 external boundaries, their contents being to some extent merged in 

 the general recrystallization of the rock. 



The microscope brings out more clearly the nature of the 

 transformations undergone by these rocks. The chloritoid substance 

 which we identify with delessite is found to have disappeared 

 completely in the thoroughly metamorphosed specimens. It is 

 replaced most frequently by a deep brown, intensely dichroic mica, 

 which is disseminated through the rock in very minute flakes, with 

 occasionally a few larger ones in clusters and patches. This 

 mineral occurs almost universally, but is not uniformly distributed. 

 Instead of it in some parts of the sections we find a green horn- 

 blende, in crystalline grains of varying size, showing the prismatic 

 cleavage-traces, and giving the usual absorption-formula : 

 y, grass-green ; /3, a slightly less deep green ; a, pale yellow-brown ; 



y^/3>>rt. 



With the hornblende, or locally replacing it, is seen occasionally a 

 green fibrous actinolite in characteristic sheaf-like bundles. These 

 amphibole-minerals are very generally confined to streaks varying 

 from a very narrow width to half an inch or an inch, so that a 

 slide may show hornblende as the characteristic mineral in one 

 half of the field and brown mica in the other. Less common is a 

 pyroxenic mineral, colourless in thin slices and having the general 

 characters of monoclinic augite. It occurs in well-cleaved crystal- 

 line grains with the hornblende, and less frequently a vein is seen 

 consisting entirely of a mosaic of crystalline augite [759]. 



Magnetite is a very" common mineral, usually building minute 

 but rather perfect octahedra. It is associated more frequently with 

 the hornblende than Avith the brown mica, though the latter 

 mineral sometimes encloses a few grains of magnetite also. 

 Pyrites * is another mineral having the same association. It forms 

 little cubical crystals or irregular patches. More remarkable is the 

 very frequent occurrence of sphene, almost always in those parts of 

 the altered rocks which contain hornblende. The sphene is in 

 little rounded grains or clusters of minute granules, less commonly 

 in rather imperfect crystal forms. It exhibits unusually strong 



* Judging from the colours in reflected light, both pyrites and pyrrhotite 

 occur, but it would ba very difficult to isolate the little granules for examina- 

 tion. 



