﻿Yol. 66.~] SKIDDAW GKANITE AND ITS METAMOKPHISM. 129* 



compared with the little-altered rocks of the Bowscale district or 

 Bannerdale. 



The metamorphism of the rocks of this belt possesses certain 

 very distinctive features, of which the most important are a strong 

 tendency to the development of spots, and the formation of 

 cordierite and mica. Andalusite is usually subordinate, though 

 present in considerable quantity. 



As a type of the unaltered rocks we may take those exposed oid 

 Bowscale Fell to the east of the tarn. These are, for the most part,, 

 well-cleaved slates with a distinctive silvery lustre, very different 

 from the black slates hitherto described. When weathered these- 

 slates frequently possess a yellowish or brown colour, doubtless due 

 to oxidation of the iron, but when fresh the characteristic colour is 

 grey. 



An average specimen of this rock from the neighbourhood of 

 Bowscale Tarn, when examined under the microscope, is seen to< 

 consist for the most part of small flakes of a colourless mica, with 

 minute dark grains of indeterminable nature, probably either iron- 

 ore or carbon, and a considerable number of crystals of the ottrelite 

 mineral before described. The mica-flakes are often arranged with 

 radiate or stellate grouping, and the whole texture of the rock is 

 very spongy. The foregoing description relates to the unaltered 

 rock. 



On approaching the metamorphic region spots begin to develop,, 

 and quickly become very abundant. The effects produced by slight 

 alteration are very similar to those seen in the black slate. As an 

 example of moderate alteration we may take specimens from the 

 head of the Eiver Glenderamackin, where exposures are very 

 frequent. In the upper part of the valley, north-east of the 

 summit of Saddleback, the field-relations of the rocks are very 

 clearly displayed, and appear to be as shown in the accompanying 

 diagram (fig. 2, p. 130). At this point the valley runs almost exactly 

 across the strike, so that several different lithological types are 

 exposed. The outer limit of the aureole is here in the black-slate 

 band, and this is succeeded by a belt of spotted slate ; still farther 

 north this passes into a rock somewhat harder and characterized by 

 cordierite, showing a higher grade of metamorphism. The cleavage 

 is approximately at right angles to the bedding, and the outer edge 

 of the aureole appears to be distinctly curved in a vertical plane 

 (see fig. 2). 



When examined microscopically, a thin slice of the spotted flags 

 is seen to consist of a very fine-textured base including numerous 

 spots and a few crystals of chiastolite. The spots are of two 

 kinds : the smaller ones are ragged and ill-developed crystals of a 

 rather pale brown biotite, which usually have a definite orientation 

 in the rock. The axes of maximum absorption almost invariably 

 lie transversely to the cleavage of the rock, so that the cleavages 

 of the mineral do not, as might be expected, lie parallel to the.- 



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