﻿122 ME. E. H. EASTALL ON THE [May I9IO, 



aureole on its northern margin, but can scarcely be said to be 

 involved in it. Grits of a less coarse type are abundant in the 

 north-eastern and central parts of the aureole, and are well seen on 

 the western face of Skiddaw ; but there is no obvious reason for 

 assigning these to a pre-Cambrian age, since they appear to be 

 perfectly conformable to and continuous with, the slaty bands 

 containing fossils of Arenig age. 



In most parts of the area there are fairly rapid alternations of 

 lithological character, indicating variable conditions of deposition ; 

 but usually one type of sediment or another is dominant over a 

 considerable thickness of strata, so that in one part the rocks are 

 mostly grits, in another mostly shales, and so on. Around the 

 metamorphic aureole three fairly well-marked rock-types may be 

 distinguished : these, when unaltered, comprise pale grey grits of 

 different degrees of coarseness, rather soft sandy flags of varying 

 colours, and lustrous black slates. Between these types numerous 

 transitional forms occur. The alterations which have been brought 

 about in these rock-types by the granite intrusion form the main 

 subject of the present.paper. 



The general strike throughout the district is remarkably uniform, 

 varying but little on either side of an average of E. 15° N.- 

 W. 15° S. The boundaries of the principal lithological bands also 

 possess this direction, so that there is evidently not much fear of 

 complications from variations of lithological character along the 

 strike. When we come to consider dips, however, great difficulties 

 arise, and it has not hitherto been found practicable to work out the 

 tectonic structure of the district. The area is a very large one, 

 and over the greater part of it exposures are few and far between, 

 owing to a thick covering of drift and peat. Where exposures are 

 numerous, it is evident that the structure is of intense complexity : 

 the rocks often show the most violent folding and contortion on 

 a small scale, even in hand-specimens, and there is every reason 

 to believe that the folding on a large scale is equally well developed. 

 The rocks are so strongly cleaved and slickensided, that it is often 

 difficult, if not impossible, to determine the direction of the original 

 bedding. The unravelling of the structure of the Skiddaw Slates 

 would be a task of immense difficulty, and would probably not 

 repay the expenditure of the necessary time. 



Within and around the region affected by metamorphism, as be- 

 fore stated, three well-defined rock-types occur, and these show a 

 considerable amount of regularity, as follows : — the southern portion, 

 including most of the Saddleback massif, Lonscale Pell, and the 

 southern part of Skiddaw, consists of a rather soft black slate, 

 which is often very much crushed and slickensided. Certain bands 

 of this, which have escaped crushing, contain a few fossils, especially 

 tuning-fork graptolites, and Caryocaris. Next to this on the north 

 comes a fairly broad band of grey somewhat flaggy slate, which 

 stretches from the north side of Bowscale Pell past the north side 

 of Sinen Gill, and appears to pass below the surface on reaching 

 the higher ground south of the summit of Skiddaw. The middle 



