344 J. J. ET. TEALL ON THE METAMORPHOSIS OE 



Scourie dykes, I feel that it would not be right to allow this paper to 

 pass without pointing out that the ideas expressed in it have been 

 to a very great extent suggested by the lessons he taught me in the 

 Durness-Erribol area during the summer of 1883. 



EXPLANATION OF PLATE II. 



Fig. 1, Dolerite (diabase) section showing lath-shaped felspars, crystalline 

 plates of augite, titaniferous magnetic iron-ore, secondaiy hornblende, 

 and green decomposition-products. Magnified 24 diameters. 

 2. Hornblende-schist Tiewed with polarizer only. Plane of vibration at 

 right angles to scbistosity. Hornblende, colourless crystalline grains 

 (quartz and modified felspar), and titaniferous iron-ore partially 

 surrounded with sphene. On rotating stage through 90°, all the 

 yellowish -green grains change simultaneously to deep green. Magni- 

 fied 50 diameters. The hornblende is more abundant in the portion 

 of schist here represented than is usually the case. 



DlSCTJSSIOlJ^. 



The PEEsroEisTT pointed out that while others had suggested the 

 relations in certain eases between igneous and metamorphic rocks, 

 to the author belonged the merit of having demonstrated this in a 

 particular instance. He agreed with him that the sehistosity of 

 the rock of the dyke could not have been produced during the 

 cooling of the mass. He had always thought that some of the 

 hornblende-schist of the Lizard might similarly have been derived 

 from basic igneous masses ; but when working there he had failed 

 to obtain any proof of it. 



Mr. Baiteeman remarked on dykes in the older Archaean rocks 

 of South America, which soimetimes run along planes of foliation 

 and at others cut across them. He thought that in such cases the 

 dykes were probably not very different in age from the gneiss 

 which they traverse. 



Prof. Seeley said that the interest of Mr. Teall's work was in 

 the direct conversion of a volcanic texture into a schistose texture ; 

 but though the fact was new, it did not need any new views in 

 metamorphism to explain it. We could only conceive of the change 

 taking place after the dyke had cooled and cracked, because the 

 rock could not otherwise have offered the resistance under which 

 the crystals would extend themselves at right angles to pressure in 

 the foliated part. He thought that the change had been an ex- 

 tremely slow one, due to the temperature of the water in the 

 cracked rock being raised by pressure, so that it had slowly dis- 

 solved the material of the dyke, which had as slowly recrystallized 

 in a schistose form. Mr. Teall's account was conclusive ; but such 

 origins for hornblende-schist are necessarily local. 



Dr. HiCES thought that the case cited in this paper, where one 

 crystalline rock was changed into another and into closely allied 

 minerals, probably by infiltration along minute fissures, lent no 



