146 EEV. E. HILL AITD PKOF. T. G. BONNET ON THE 



' crystalline schist ' may be produced, and feel some confidence that 

 by patient working we shall be able, as a rule, to distinguish the 

 one from the other ; ^ though often we may not be able to say 

 more than that pressure has been the last agent of alteration, and 

 has obliterated any earlier record. Very probably also important 

 changes^ may be produced, as an indirect rather than a direct 

 result of pressure (that is, by modification without crushing), 

 through the recrystallization of the mineral constituents of a rock. 

 But, with my present knowledge, I incline to consider this a sub- 

 ordinate cause of change. 



DiSCTJSSION. 



Major-General McMahon congratulated the Authors on the result 

 of their examination of the Sark rocks. It was gratifying to find 

 that their work at Sark confirmed the results arrived at by them 

 and himself at the Lizard. With reference to a fact stated by 

 the Authors regarding the genesis of biotite in some of the Sark 

 rocks, he might mention that he had observed several instances in 

 the Himalayas where the contact-action of granite had converted 

 diorite into a mica-trap in which biotite was very abundant. 



Prof. Jtjdd congratulated the Society on the new light thrown on 

 the geology of the Channel Islands by the researches of the 

 Authors. 



Mr. Htjdleston, after referring to the excellent work of Mr. Hill 

 on the Channel Islands, said it was evident that his former paper 

 on Sark left some open questions which seemed to invite further 

 investigation. The origin of the hornblende-schists both there and 

 at the Lizard had long excited attention, and good petrologists had 

 held that they might have been basic tuffs. At Sark they were not 

 associated with serpentines, &c., to any extent. The application of the 

 new jjhilosophy seemed to point to alterations in a plutonic complex, 

 and the suggestion that the lowest in the series might not be the 

 oldest in date gave rise to speculations of much interest. Unless it 

 could be shown from their geognostic relations on the mainland that 

 these rocks were of Archgean age, their petrology alone could scarcely 

 be held to prove it. 



Mr. Baeeow drew attention to the strong resemblance of the 



^ I have not referred to ' contact-metamorpbism ' because the effects pro- 

 duced by it ou sedimentary deposits can be readily distinguished, as a rule. 

 For instance, a ' inica-schist ' from Skiddaw, Britanny, Normandy, &c., produced 

 by a large mass of granite, differs much from such a mica-schist as we find in 

 the ' Upper Grroup ' in the Alps. So far as I have been able to judge, no very 

 important changes are produced bj the intrusion of molten masses into rocks 

 already crystalline, probably because their constituents are already in a stable 

 Qondition. 



2 I refer to such changes as the replacement of labradorite by scapolite, to 

 which Prof. Judd has called attention in a suggestive paper (Min. Mag. vol. viii. 

 (1889) p. 186), certain cases of the formation of urahte, &c., but not to the 

 ordinary processes of decay, hydration, and even renovation. Heat, pressure, 

 and water are the chief agencies of metamorphism, and we must be content 

 at first if we can distinguish and classify the more striking and common 

 phenomena which have been produced by them. 



