﻿vol. 66.'] around the ross of mull granite. 397 



Discussion. 



The President (Prof. W. W. Watts) remarked that the paper 

 was not only of great interest in itself, but it was an indication of 

 the great interest which the members of the Geological Survey felt 

 in their work, and a testimony to the freedom of thought and 

 discussion which characterized that body. If the Author had 

 succeeded in discriminating between the effects of regional and 

 thermal metamorphism in the district under discussion, he would 

 have made a most important contribution to knowledge. 



Mr. G. Barrow congratulated the Author on his paper, and 

 especially on the discovery of the sillimanite crystals, which were 

 much finer than any that had been met with in the South-Eastern 

 Highlands. In this area the speaker had traced the outer (southern) 

 limit of the sillimanite-bearing gneisses for 100 miles. In them 

 green spinels were quite common, and the gneisses, like those shown 

 by the Author, were always far coarser in texture than the silli- 

 manite-bearing rocks that occurred in the aureoles of metamorphism 

 round any undoubted posfc-Torridon granite. He also congratulated 

 the Author on the clearness with which he had unconsciously shown 

 that the marginal phenomena of the granite were exactly those 

 characteristic of the older Archaean granites, and never of the newer 

 intrusions. Fortunately, geologists were able to satisfy themselves 

 of the justice of this observation by examining on the ground ■ the 

 facts shown on the published 1-inch Geological Survey Maps of 

 Scotland. In Sheet 66 the north-eastern margin of the great 

 Kincardineshire granite, belonging to the newer group, cut across 

 one of the more coherent patches of the older (Cairnshee) granite. 

 The margin of the newer intrusion had a fine-grained and quickly- 

 cooled edge, with no trace of veins or apophyses. The older rock 

 here could rarely be mapped, although the whole country, along a 

 belt fully 30 miles long, was flooded by it in the form of minute 

 intrusions. The ground on which the junction between the two 

 intrusions occurred was especially easy of access, and could bf 

 visited at any time. 



The permeation of the granitic magma along both the folding- 

 planes and the strain-slip cleavage was characteristic of the oldei 

 Duchray Hill gneiss, shown on the published maps. In Sheet 56, 

 on the east side of Glenshee, a large mass of diorite of Newer Granite 

 age was seen clearly intrusive through the far older Duchray Hill 

 gneiss. The junction was clear and bare on the south side of the 

 diorite, and here again the strong contrast between the Archaean 

 granite and the far newer one was well shown. 



If the Author still claimed that this intrusion was of Newer 

 Granite age and that it re-metamorphosed rocks previously altered, 

 the speaker challenged him to publish on the official 1-inch map a 

 line showing the outer limit of this new alteration. Long accounts 

 had been published by the Geological Survey of Scotland of the new 

 metamorphism produced by the Newer Granites in the Highlands ; 

 but no one had yet ventured to show the aureole of alteration on a 



