﻿128 ME. E. H. EASTALL ON THE [May 1910,. 



developed, showing the usual black cross. They seem to consist of 

 a sort of matted aggregate of very minute flakes of a micaceous- 

 mineral, and show none of the optical properties of ordinary 

 andalusite. There is no confirmation of the idea that the rounded 

 pale spots eventually develop into chiastolite — in fact, their general 

 appearance is suggestive of embryo crystals of cordierite. 



It so happens that in no case does this type of black slate 

 approach very closely to the granite exposures, so that it is not 

 possible to say what effects are produced in it by the higher grades- 

 of metamorphism. The reason for this will be obvious from tho 

 map : the northern limit of the black slate in the Glenderaterra 

 valley occurs about a quarter of a mile south of Sinen Gill, and the 

 other exposures of granite lie wholly in the bands of grey slate and 

 grey grit. Hence it appears that Clifton Ward's zones were not 

 originally all of the same lithological character. 



At certain localities on the eastern side of the Glenderaterra 

 valley, especially just above the high-level cart-track, there are to 

 be seen unusually large crystals of chiastolite or andalusite, often 

 an inch or even an inch and a half long. These possess a peculiar 

 metallic lustre, and strongly recall the well-known crystals of the 

 same mineral in the schistes macliferes of the Salles de Rohan 

 in Brittany. It is also frequently noticeable that these large 

 crystals are bent and distorted, so that there has evidently been 

 considerable dynamic metamorphism after or accompanying the 

 intrusion of the granite. 



As showing the low grade of metamorphism which suffices to> 

 produce well- developed crystals of chiastolite, the fact may be men- 

 tioned that on Gibraltar Crag, and elsewhere near the summit of 

 Skiddaw, there are to be found specimens showing both chiastolite 

 and graptolites. A similar association is also known in specimens, 

 from Brittany. 



Type II.— The Grey Flags. 



As already pointed out, the black slate is succeeded on the north 

 by a belt of rocks of a distinctly different character. The dominant 

 colour of these rocks when unaltered is grey or brownish, and 

 certain bands possess a very characteristic silvery lustre. This 

 belt varies somewhat in width according to the form of the ground,, 

 but possesses an average breadth of outcrop of about a mile. 

 Included in it are the well-known exposures of Roughten Gill and 

 Sinen Gill, which have been so often mentioned as types of the 

 Skiddaw metamorphism. The same rocks are again well exposed 

 at the head of the River Glenderamackin, in Bannerdale Crags, and 

 in some of the crags in the neighbourhood of Bowscale Tarn. On 

 the western side of the aureole the exposures are poor. A similar 

 belt occurs again on the north, running through the summit of 

 Skiddaw and Little Calva towards Coomb Height ; here again 

 exposures are not very frequent. Consequently the development 

 as seen in the northern half of the Glenderaterra valley may be 

 taken as typical of the metamorphosed grey flags, and thus may be 



