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



given by earlier writers, and especially their definitions of zones, do- 

 not hold, since the rock- types described were originally different. 



In the broadest sense, three well-marked rock-types can be 

 recognized in the Skiddaw district : black slate, grey flags, and 

 grey grits. The chiastolite-slate of Clifton Ward and others is 

 chiefly derived from the black slate, which is never seen to undergo 

 the higher grades of metamorphism. The best examples of this 

 type are to be seen in the Glenderaterra valley on the south and 

 at Dead Crags, near Dash, on the north. The cordierite-rocks of 

 Sinen Gill and the central parts of the aureole generally are 

 derived from the grey flags, as is also the so-called ' mica-schist' of 

 Sinen Gill. In these the dominant minerals are cordierite and 

 biotite, andalusite being quite subordinate and occurring chiefly in 

 the more argillaceous bands ; garnet and staurolite are developed 

 only in immediate contact with the grauite. The grey grits occupy 

 all the central parts of the aureole, and both of the larger ex- 

 posures of granite are seen in contact with them. In the more 

 siliceous members of this group metamorphism is much less marked 

 than in the other types, and many examples resemble impure 

 quartzites. By far the most abundant mineral, besides quartz, is. 

 brown mica, but in the neighbourhood of the granite and greisen 

 of Grainsgill this is, for the most part, represented by pale-green 

 chlorite ; the difference is probably due to the bleaching and, 

 reducing action of pneumatolytic vapours. Here and there in the' 

 grey grit series are to be found more aluminous bands, and these 

 have given rise to cordierite and andalusite- bearing rocks, much 

 like those of the grey flags. Here also garnet is only found in 

 immediate contact with the granite. 



With regard to the chemical and mineralogical changes which 

 have taken place as a result of the intrusion, it is rather difficult 

 to say anything definite. In the first place, it is clear that the 

 Skiddaw Slates had undergone very extensive dynamic meta- 

 morphism previous to the intrusion of the granite, since the' 

 cleavage, foliation, and folding are still clearly visible, although 

 the divisional planes have been sealed up by the recrystallization 

 induced by heat. Now intense pressure itself sets up well-marked 

 mineralogical changes, so that when the heating process began the 

 rocks were no longer in their original condition, but had doubtless, 

 undergone a considerable amount of alteration. The most impor- 

 tant of these changes appear to have been the abundant formation 

 of secondary mica, and of the mineral which has been somewhat 

 doubtfully identified as ottrelite. These are characteristic of the 

 Skiddaw Slates in their normal condition ; and, for all practical 

 purposes, we may consider that the principal constituents of the 

 slates before alteration by the granite were quartz, mica, oxides of 

 iron, and carbon. 



The leading minerals produced by metamorphism are cordierite, 

 andalusite, biotite, and muscovite, all of which are aluminous 

 silicates : hence it follows that the rocks were originally rich in 

 alumina; the formation of abundant cordierite indicates the. 



