﻿Yol. 66.~\ SKIDDAW GKANITE AND ITS METAMORPHISM. 137 



^andalusite is not an important constituent of the metamorphosed 

 rocks, probably on account of a deficiency of alumina, but in 

 certain other localities this mineral occurs abundantly, even under 

 a high grade of metamorphism. In these more highly altered 

 rocks the andalusite is fresh, whereas the chiastolite crystals of 

 the outer zone seem to be always represented by decomposition- 

 products. The best development of such large andalusite crystals 

 occurs in a transition-zone between the grey flags and the grey 

 grits, and they are well seen in Blackhazel Beck, on the north, side 

 •of Saddleback. 



A hand-specimen of this rock shows large prismatic crystals of 

 andalusite, which are commonly clear and colourless, with a glassy 

 lustre, when absolutely fresh ; sometimes they are grey or white, 

 ■and in these the dark cross can often be distinguished. The ground- 

 mass is fine-textured and generally very dark in colour, often 

 almost black. Under the microscope this rock does not differ much, 

 from the altered grits before described, except in the presence of the 

 large andalusite crystals. The ground-mass consists of small 

 grains of quartz, with abundant flakes of a rather deep brown 

 •mica ; in some specimens there are in addition rounded crystals of 

 cordierite. There are also occasionally rather large rounded spots 

 stained a bright yellow ; the true nature of these was not determined, 

 hut they do not seem to be of much importance. 



The andalusite crystals are clear and colourless, with a fairly 

 high refractive index, and birefringence a little higher than that of 

 quartz (y — a = '012 approximately). The mineral shows very well- 

 marked cleavage and straight extinction. The black cross or some 

 other symmetrical figure is very conspicuous, and inside it there is 

 frequently to be seen a core of clear andalusite which often shows 

 pink pleochroism. The outlines of the crystals are quite sharp, and 

 they do not enclose other minerals. 



The contact of the grits with the granite is well exposed in the 

 'bed of Blackhazel Beck, but the rock-types here seen are not of 

 'much interest. The grits seem to have been exceptionally free 

 from aluminous matter, so that andalusite and cordierite are 

 almost absent. The latter is probably represented by a few small 

 patches of shimmer-aggregate. The greater part of the rock 

 •consists of clear grains of quartz, with a varying amount of both 

 white and brown mica : close to the granite the latter is a good 

 •deal bleached, as at Grainsgill. No garnets or staurolite could be 

 -found. 



Y. General Conclusions. 



From the preceding detailed account of the metamorphism 

 produced by the Skiddaw granite in different parts of its aureole 

 certain general conclusions can be drawn ; in the first place, it is 

 evident that a considerable variety of rock-types are involved, and 

 that a description taken from one area may be inapplicable to 

 others. Furthermore, it follows from this that the descriptions 



