﻿Vol. 66.] SKIDDAW GRANITE AND ITS METAMORPHTSM. 127 



rounded or rectangular in form, as noted by Clifton Ward. Some- 

 times a darker spot can be seen in the centre, and it is a curious 

 fact that this spot is more noticeable in photographs of specimens 

 of this rock than in the specimens themselves. The presence of 



Fig. 1. 



1 = Granite ; 2 = Chiastolite-zone ; 3 = Outer spotted zone. 



,'[This figure illustrates the variations in width of the outer spotted zone, 

 according to the form of the ground. It is purely diagrammatic, and 

 does not exactly represent any particular line of section.] 



this spot and the angular outline suggest that the spots are in 

 effect embryo crystals of chiastolite ; but this cannot always be 

 the case, since the light spots are often abundant in specimens which 

 contain well- formed and large crystals of chiastolite. 



(6) The chiastolite-slate. — It is perhaps hardly necessary 

 to enter into much detail as to the structure aud composition of this 

 very well-known rock-type. Chiastolite is one of the most charac- 

 teristic products of a low grade of metamorphism of argillaceous 

 rocks, and is found in many localities. 



Chiastolite, in the strict sense of the term, is largely developed 

 wherever argillaceous rocks have been metamorphosed. The 

 same mineral under a slightly different form (andalusite) occurs 

 largely in the more gritty portions, and the distinction between 

 andalusite and chiastolite is purely artificial. Most commonly, 

 when the mineral is white and opaque in its appearance to the 

 naked eye owing to decomposition, it is called chiastolite; and 

 when clear, fresh, and glassy, it is called andalusite. 



An ordinary specimen of chiastolite-slate in most cases differs 

 •only from the unaltered slate, or from that of the outer spotted zone, 

 in the presence of more or less well-developed crystals of chiastolite. 

 Sometimes these appear among the whitish spots of the outer 

 spotted zone, and in other cases the chiastolite comes on at the 

 outer margin of the aureole without any preliminary spotting. In 

 all essential respects the character and composition of the ground- 

 mass remains unchanged, while the ottrelite mineral is commonly 

 present and is frequently enclosed in the chiastolites. A typical 

 specimen of chiastolite-slate from the Glenderaterra valley has a 

 highly micaceous ground-mass, with a good deal of carbon and very 

 little quartz. It shows some rather indistinct round or oval 

 spots, which seem to differ from the rest only in the partial loss 

 of the black colouring-matter. The chiastolites are large and well 



Q. J. G. S. No. 262. k 



