﻿Vol. 66.'] SKIDDAW GRANITE AND ITS METAMORPHISM. 119 



felspars show a considerable range of composition ; they include 

 both alkali-felspar and plagioclase, and the alkali-felspar is as a 

 rule distinctly more turbid and decomposed than the plagioclase. 

 The large phenocrysts, when present, consist of a somewhat coarse- 

 textured variety of perthite, and the great majority of the smaller 

 individuals of alkali-felspar also show the perthitic structure with 

 greater or less clearness. This perthite is probably to be referred 

 to the secondary type. 1 



The plagioclase possesses a low extinction-angle, usually not 

 more than 10°, and generally less, while its index of refraction, 

 when tested by the Becke bright-line method, is lower than that of 

 quartz. It may, therefore, be referred to albite-oligoclase or oligo- 

 clase. Both white and brown micas are present, and in some 

 specimens from near Grainsgill the white mica is the more abundant. 

 This is correlated with the fact that, as noted by Harker, the 

 granite of Grainsgill is more acid than that of the other localities. 

 The two micas are commonly intergrown in parallel position, and 

 there is no reason to doubt tbe original nature of the large flakes of 

 muscovite. They show no evidence of a secondary origin due to 

 greisening in the southern part of the exposure, although the 

 greater part of the colourless mica in the northern part of the same 

 mass (the Grainsgill greisen) is certainly secondary. The biotite is 

 of a peculiarly brilliant foxy-red colour, and strongly pleochroic ; 

 it is, however, often more or less completely converted into green 

 chlorite and epidote. The only other mineral present is a very 

 small quantity of magnetite. The microscopic structure of the rock 

 on the whole is granitic, although some specimens show a distinct 

 tendency to the graphic. There are to be seen rough micropeg- 

 matitic intergrowths of quartz and alkali-felspar (perthite) and 

 ■also in one or two cases intergrowths of felspar and muscovite, a 

 much less common phenomenon. 



The granite of Sinen Gill differs from that of the Grainsgill 

 •district in texture rather than in composition ; it is of finer grain, 

 and more distinctly porphyritic. The mutual relations of the quartz 

 and felspar are variable : sometimes the quartz occurs as rounded 

 grains, and sometimes interstitially ; in many places there is a rude 

 graphic intergrowth. The constituent minerals are precisely the 

 same as in the case last described, and here also muscovite is 

 abundant, though subordinate to biotite. The latter is for the most 

 part converted into green chlorite. 



The more distinctly porphyritic character of this rock is precisely 

 what might have been expected, in view of its mode of occurrence 

 as a comparatively small apophysis of the main mass ; and, taking 

 into account both microscopic structures and field-relations, it would 

 l)e most correctly identified as a granite-porphyry, whereas the 

 €aldew and Grainsgill rocks should be called porphyritic granites. 



The rock of the largest exposure, which can be well seen in 

 the neighbourhood of Wiley Gill, varies a good deal in character. 



1 A. Harker, 'The Natural History of Igneous Kocks ' 1909, p. 258. 



