THE MINING DISTRICTS OF CORNWALL. 331. 



is also frequently present. Two differently-coloured micas are dis- 

 seminated throughout these rocks in the form of imperfectly crystal- 

 lized flakes. The first, which is black, or dark brown, may be either 

 muscovite or lepidomelane ; the second, which is pearly white or 

 pink, is lepidolite. 



The quartz in granite seldom occurs in the form of distinct crys- 

 tals, but is usually granular, and is more or less irregularly disse- 

 minated through the rock. It is generally either transparent and 

 colourless or white ; but it has sometimes a bluish or greyish tint. 

 Numerous microscopic cavities, partially filled with liquid, are 

 observable in the quartz of all Cornish granites ; some, on the 

 contrary, are apparently full, while others are entirely empty. 



The preceding Table (p. 330) gives the composition of three diffe- 

 rent specimens of unweathered Cornish granite, the figures being in 

 each case the mean of two separate analyses. 



I. From Carn Brea Hill, Redruth. — This is a somewhat coarse- 

 grained granite in which the proportion of glassy quartz is consider- 

 able ; it contains two varieties of mica, the one nearly black, and 

 the other white or slightly tinged with pink. A microscopical 

 examination of this rock shows that it contains a large amount of 

 monoclinic felspar, but that plagioclase is also present in considerable 

 quantities. The outlines of the felspathic crystals are not in all 

 cases sharply defined, and they are frequently rendered slightly 

 cloudy by greenish flocculent microliths, which may perhaps be 

 the result of metamorphism. Two varieties of mica are distinctly 

 visible, which, together with a little tourmaline, are enclosed alike 

 in quartz and felspar ; a few minute crystals of either apatite or 

 beryl are also present. The quartz, which does not form well- 

 defined crystals, is much fissured, the sides of the cracks being some- 

 times coloured by hydrated oxide of iron. It contains numerous 

 fluid-cavities enclosing bubbles, which, in some of the smaller ones, 

 are observed to be in continual motion. 



II. From Botallack. — This is a greyish granite with a mode- 

 rately fine grain, and appears to consist of a nearly equal mixture 

 of transparent or milky-white felspar and glassy quartz, through 

 which are disseminated a few small flakes of mica, and minute 

 crystals of schorl. 



Under the microscope the felspar is seen to be of two kinds ; and 

 both it and the quartz are in places stained by ferric oxide. The 

 mica chiefly belongs to some white variety ; and the fluid- cavities 

 in the quartz, although less numerous, are larger in size than those 

 in the granite from Botallack. 



III. From Chywoon Morvah. — This granite is composed of a 

 mixture of brownish red felspar and milky quartz, the first in the 

 form of distinct crystals, and the second in that of disseminated 

 crystalline grains. The amount of mica present is very small ; and 

 the mineral is usually much decomposed. The proportion of quartz 

 is apparently slightly less than in the generality of Cornish granites. 



Under the microscope the felspar is seen to be monoclinic, and is 

 partially replaced by patches of a greenish dust-like mineral ; the 



