﻿Vol. 66.'] SKIDDAW GRANITE AND ITS METAMORPHISM. 131 



effects produced on them by a high grade of metamorphism, we may 

 take the well-known exposures of Eoughten Gill and Sinen Gill. 

 Between the district just described and the head of Sinen Gill there 

 is a large spread of peat with no exposures, and when next seen in 

 place the rocks are obviously much more highly altered. In fact, 

 the upper part of Sinen Gill may be taken as a typical locality for 

 the so-called 6 cordierite-gneiss/ and this gradually passes into the 

 rock commonly spoken of as mica-schist. Here also are to be 

 found gritty bands, but these seem to be quite subordinate, and are 

 neglected in the following descriptions. 



Cordierite- gneiss. — As a typical example of this rock, we may 

 select a specimen from a point in Sinen Gill about 180 yards above 

 the visible outcrop of the granite. To the naked eye it is a massive 

 dark-brown or bluish-grey rock, very hard and weathering out into 

 very large blocks. On a weathered surface it shows conspicuous 

 banding, owing to slight differences of hardness in successive 

 bedding-planes, and these lamination-planes are thrown into the 

 most extraordinary folds and contortions, so that a surface of 3 or 

 4 square feet often shows a most complicated series of sharp folds 

 and faults. From these it is evident that observations of dip are 

 of no possible significance. 



The microscopic structure of this rock is very simple. It con- 

 sists of cordierite, andaiusite, white and brown mica, with a few 

 grains of opaque black iron-ore, which is probably magnetite. The 

 cordierite occurs in large and well-developed crystals, often 2 or 

 3 millimetres in diameter, and roughly spherical. They show 

 the usual optical properties, a rather low refractive index, about 

 equal to that of quartz, and moderate birefringence (about *011). 

 The crystals are commonly twinned on the usual law, giving rather 

 complex inter penetration- twins, frequently in six sectors, of which 

 the opposite pairs extinguish together, so that the twins consist of 

 three individuals. These crystals have been very fully described 

 by Harker, 1 and for further details reference may be made to his 

 description. The andaiusite occurs almost exclusively as an aggre- 

 gate of irregular grains, showing no definite crystal outlines. Now 

 and then, however, fairly well developed crystals are seen, and these 

 often show, especially in their central parts, the very characteristic 

 rose-coloured pleochroism. Both brown and colourless mica occurs 

 in great abundance, and it is noticeable that the latter generally 

 shows much more definite outlines than the former. The two 

 are often intergrown in parallel position. The micas occur largely 

 as inclusions in the cordierite crystals, but the andaiusite tends rather 

 to concentrate in the spaces between them. In some parts of the 

 rock, however, the crystals of cordierite are continuous, and enclose 

 all the other minerals. 



A specimen collected about 40 yards from the granite is very 

 similar to the last, but shows occasional gritty bands, in which 



1 'The Naturalist ' Leeds, 1906, p. 121. 



