﻿Yol. 66.~] SKIDDAW GEANITE AND ITS METAMORPHISM. 133 



nature is very difficult to make out. Brown mica and white mica 

 are almost equally abundant, though the crystals of the former are 

 somewhat larger and better developed ; its colour is a rather pale 

 yellowish-brown and the pleochroism fairly strong : pleochroic 

 haloes are quite common. They surround small crystals which are 

 presumably zircons ; but, owing to their minuteness, it has not been 

 possible to determine this with certainty. The crystals of white 

 mica are on the whole less regular in form and generally smaller, 

 although the two micas often occur intergrown in parallel position. 

 The small amount of interstitial matter is colourless and trans- 

 parent, with a rather low refractive index, and shows no very 

 definite optical properties. After a very careful examination, I am 

 inclined to think that it is, in part at any rate, slightly decomposed 

 •cordierite, which is in process of being converted into a kind of 

 shimmer-aggregate ; and this idea is confirmed by the fact that 

 other specimens show fairly well-defined patches of shimmer- 

 aggregate having the typical form of the cordierite crystals of the 

 kind previously described. 



The foregoing description applies only to the more argillaceous 

 types of sediment, in which little or no quartz appears to have 

 been originally present ; in the more gritty bands the development 

 is slightly different. An interesting specimen was obtained from 

 the contact in Sinen Gill, within an inch of the granite, on the 

 right-hand side of the exposure, as one faces up stream. 



This is a reddish-brown micaceous rock, containing conspicuous 

 garnets. These are usually 2 or 3 millimetres in diameter, 

 pale pink in colour, and showing w T ell-developed dodecahedral 

 faces. Under the microscope the rock is distinctly banded, con- 

 sisting for the most part of white and brown mica, with quartz 

 and cordierite, the last being partly represented by a micaceous 

 shimmer-aggregate. There are also a few shapeless crystals of 

 andalusite. Some bands are almost entirely composed of a mosaic 

 of quartz-grains. The garnets are colourless in thin slices, with 

 many minute inclusions, which appear to be quartz : except for 

 the inclusions, they are absolutely isotropic. Certain bands show 

 fairly large and rather shapeless patches of a mineral of a bright 

 yellow or orange colour, with a rather high refractive index and 

 very variable birefringence. Owing to the brilliant yellow colour 

 the latter property is difficult to determine. The mineral frequently 

 occurs as an aggregate, but sometimes as continuous crystals with 

 definite extinction. The precise identification of this mineral was 

 rather troublesome, and Dr. Bonney very kindly gave his assistance 

 in the matter. In his opinion, it is to be regarded as an early and 

 imperfect stage in the crystallization of staurolite. It shows the 

 closest possible resemblance to some specimens of typical staurolite 

 in Dr. Bonney's collection. This mineral is confined to particular 

 bands, and it only occurs within a very short distance ,of the 

 granite. 



