48 PEOCEBDINGS OF THE aEOLOGICAL SOCIETY. 



ridges, which clearly show that their rounded form was not due to 

 mechanical wearing. In the specimens of decomposed granite 

 which I have examined in greatest detail, the larger grains of 

 quartz have a somewhat opaque surface, as if corroded, and the 

 angles are rounded. This rounding is relatively much greater in 

 the case of the smaller grains, which is the reverse of what is met 

 with in w^orn sand. On the whole the facts seem to indicate that 

 the quartz has been more or less corroded and dissolved by the 

 action of the alkaline silicates set free by the decomposition of the 

 felspar. The contrast between its corroded surface and the glassy 

 fractures of broken quartz is very great. 



The internal structure of the detached grains can be easily 

 studied when they are mounted in Canada balsam, since its index 

 of refraction is so nearly the same as that of quartz. High powers 

 can easily be used, if we employ lenses of small angle of aperture 

 having their focal point as remote as possible from the front lens. 

 The shape of the grains, however, is better seen when they are in 

 water under thin glass, since they can then be made to turn round 

 so as to exhibit their form in more directions than one. 



The quartz of thin-foliated gneiss and mica-schist differs from 

 that of granite in having a far less simple optic structure, and 

 in being often more or less flattened in the plane of foliation. 

 Instead of the larger portions of quartz being made up of a few 

 comparatively large crystals, they are frequently composed of very- 

 many, closely dovetailed together, as if formed in situ. Though 

 the outline of these separate crystals is sometimes shown by more 

 or less faint lines of impurity, they are often in such close con- 

 tact that they cannot be separately distinguished without using 

 polarized light. Such quartz, when broken up, would usually give 

 rise, either to comparatively fine sand, or to larger grains showing 

 a more or less compound structure. Besides this, in very thin- 

 foliated rocks containing much mica, a large part of the quartz 

 occurs as plates, flattened between the parallel flakes of mica ; and 

 when such a rock is broken up, -\ve obtain flattened grains of quartz, 

 with tolerably smooth surfaces, thus differing materially from the 

 more irregular grains with rougher surfaces derived from granite. 



Passiug now to the internal microscopical structure of the indi- 

 vidual grains, I must say that less can be learned from the fluid- 

 cavities than might have been expected. Their number in the 

 Scotch schists is, indeed, far less than in the Cornish granites, but 

 not sensibly less than in some of the granite associated with them. 

 I do not remember to have ever seen any cubic crystals of alka- 

 line chlorides in the fluid-cavities in the quartz of any granite or 

 schist from Scotland ; but they occur abundantly in some Cornish 

 granites. Hence, clearly enough, the evidence derived from fluid- 

 cavities is of very limited value, except in particular cases. Par 

 more characteristic evidence is furnished by the glassy and stony 

 enclosures, when they are present. The quartz-crystals of some 

 rhyolites contain small imperfectly rhombic or six-sided enclosures 

 of coloured obsidian-like glass, with accompanying bubbles, even 



