﻿Yol. 66.'] METAMORPHISM AROUND THE ROSS OF MULL GRANITE. 385 



1000 feet north-north-east of Knock-na-fenaig, at which latter 

 place the rock is seen to contain sillimanite. 



Similar felspathization (or else recrystallization) has been pro- 

 duced in the granulites : for, near the margin of the granite, well- 

 formed imgranulitized porphyritic crystals of felspar are present in 

 abundance. 



Conclusion. — The facts just described show how readily 

 pelitic schists may be impregnated with granitic matter in various 

 ways, to form banded gneisses of more acid character ; and possibly 

 similar processes of igneous impregnation have been employed, 

 again and again, on a grander scale, in the earlier stages of the 

 conversion of ancient sediments from the schistose condition into 

 the crystalline gneisses which underlie the oldest recognizable 

 sedimentary rocks. 1 



III. Contact-Metamorphism of the Pelitic Gneisses. 



In the cases which have been described above it is possible that 

 some recrystallization occurred among the rocks invaded by the 

 granite, but generally no minerals have been detected other than 

 those proper either to the schist or to the granite. 



There are, however, in many places near and within the granite, 

 masses of pelitic gneiss which have undergone intense contact- 

 metamorphism. 



Distribution of the Con tact- Altered Schist. 



At the surface there is no continuous aureole of contact-altered 

 schist such as can be mapped. This is partly because the rocks are 

 greatly obscured by peat, and partly because beds of micaceous 

 granulitic quartzite alternate with the beds of pelitic gneiss ; and it 

 is only where certain of these latter strike up to the granite that the 

 sillimanite-rock is found. Most patches of this altered rock are 

 actually within the granite area— some of them in the very middle 

 of it (for instance, between T6rr H6r and Cnoc Dubh). Those 

 patches which are outside occur close to the generalized boundary- 

 line, where schist-masses and granite-veins are inextricably mixed. 

 Some of the patches have been mapped on the 6-inch scale. On 

 the accompanying map (fig. 1, p. 378) a heavy dotted line has been 

 drawn : east of this line no schist, contact-altered as described below, 

 has been found outside the granite mass. 

 , Of the new minerals sillimanite is most abundant, and is 

 visible in the field as pink or greenish fibrous prisms, sometimes 

 exceeding an inch in length. Usually, except for the presence of 

 these prisms, the rocks have a normal appearance, being coarse and 

 often very quartzose ' muscovite-biotite-gneiss,' with or without 



1 See J. J. Sederholm, ' On Granite & Gneiss ' (English summary), Bull. 

 Comm. Geol. Finlande, No. 23, 1907. 



