﻿400 METAMORPHISM AROUND THE ROSS OF MULL GRANITE. [Allg. IO/lO, 



hornblendic character, which usually showed on the margins, or 

 throughout, a variable degree of foliation and cataclastic deformation. 

 The early granites never showed sharp contacts, but almost in- 

 variably produced injection-structures in the adjoining gneisses. 

 The two sets of granites had evidently crystallized under very 

 different conditions of temperature and pressure, and to these 

 conditions might be ascribed the presence or absence of marginal 

 injection-structures. The earlier granites had penetrated both the 

 crystalline igneous gneisses and the gneisses of sedimentary origin, 

 and had produced brecciation and injection-structures of a type 

 similar to those now described by the Author. No evidence had 

 been found of the production of true banded igneous gneisses, as 

 the result of injection of sedimentary gneisses by granitic material. 

 The transformation was theoretically possible, but it must take 

 place at very great depths and under conditions of temperature and 

 pressure widely different from those which prevailed during the 

 intrusion of either the earlier or the later granites now exposed. 

 The intrusion of the granites in Nigeria appeared to have been 

 accompanied by little or no contact-metamorphism. 



Dr. J. W. Evans thought that the extent and character of the 

 metamorphism caused by granite intrusions depended greatly on 

 the depth below the surface, which affected the pressure, the amount 

 of volatile fluxes, and the duration of the action. Dr. Barrois had 

 shown this to be the case in Britanny, where subsequent erosion 

 had laid bare the junctions between intrusive granitic masses 

 (which were believed to be of the same age) and the surrounding 

 rocks to different depths in different localities. The speaker further 

 laid stress on the importance of the study of the trimorphic 

 minerals (andalusite, fibrolite or sillimanite, and kyanite), as indices 

 of the pressure and temperature of metamorphism ; and enquired 

 what were the relations between the andalusite and the sillimanite 

 in the metamorphic aureole, especially with regard to the order of 

 crystallization. He also referred to the phenomenon of the fel- 

 spathization of the metamorphosed rock, which Dr. Barrois believed 

 to occur by means of osmotic action where the intrusion took place 

 at great depths. Finally, he laid stress on the extreme fluidity 

 that the intrusive magma must have possessed, to enable it to 

 penetrate into the narrowest planes of division of the injected 

 rock, and considered that this was evidence of the presence of a 

 large amount of the elements of water and other volatile fluxes. 



The Author thanked the Fellows present for their reception of 

 his paper and for their criticisms ; and, in reply to Mr. Barrow. 

 said that, owing to the varied chemical composition of the different 

 beds of Moine rocks, the intricacy of the schist-and-granite junction, 

 and the obscuring of the ground by peat, there was no continuous 

 aureole at the surface to be mapped. Patches of the sillimanite 

 rock, however, had been indicated on the 6-inch maps. The 

 kyanite-tourmaline rocks in Mull could not be regarded as con- 

 nected with the granite. On the one hand were sillimanite, 

 andalusite, and cordierite, developed in various bands of 



