vol. 65.] in the neighbourhood of eskdale. 79 



Discussion. 



Dr. Teall said that the paper was evidently an important one, 

 but that, in view of the demonstration which was to follow, he 

 would only make one remark : the fact that the Eskdale Granite 

 had a very acid margin appeared to furnish another illustration 

 of the general law, that those substances which crystallize first 

 from the average magma — an acid one in this case — tend to con- 

 centrate in the cooler parts. 



Mr. Barrow, referring to the use of the word ' eutectic ' by the 

 Author, suggested that the highly quartzose material, on the outer 

 margin of the intrusion, was really an eutectic compound of low 

 consolidation-point, formed of quartz and water, the latter probably 

 in the form of vapour or steam. 



Mr. J. F. K. Green said that the Author's work in disentangling 

 the relative ages of these intrusions would be of the greatest value 

 to others working in the Lake District. So far as the southern 

 edge of the Eskdale mass was concerned, he could confirm the 

 Author's observations ; but he considered that, as the relations of 

 the faults on the map shown seemed to indicate, the Eskdale 

 Granite was older than the main crust-movements, whereas the 

 Shap Granite was younger. 



Dr. J. W. Evans called attention to the similarity between the 

 inclusions of quartz in the felspars, and those occurring in the 

 rocks allied to granulites described by him from the Rio Madeira, 

 and by Prof. Weinschenk from Ceylon. 



Mr. Cosmo Johns commented on the value of the Author's 

 observations, and said that the very acidic margin might be 

 explained by assuming that the excess substance had separated 

 from some eutectic ; yet, there was the possible alternative that 

 two liquid phases, one much more acid than the other, existed in 

 the magma and had partly separated, owing to their difference in 

 specific gravity, before intrusion took place. He thought the 

 Author's explanation of the presence of structurally free quartz and 

 felspar, where micropegmatite would be expected, most important. 

 It had been demonstrated experimentally in the case of certain 

 alloys, that if the cooling of the mass be arrested at the point where 

 the eutectic separates out, for a sufficiently long period, the two 

 constituents would segregate and be structurally free. That 

 particular structure, once thought to be characteristic of the eutectic 

 mixture, was therefore but the accident of a particular rate of 

 cooling, and this rate might differ for each eutectic. The usual 

 absence of the structure from rocks where it might be expected to 

 occur could be explained : it was its occasional presence that created 

 the difficulty. The Author's conclusion that the temperature of the 

 mass when intruded was comparatively low seemed well founded. 

 It did not appear that any satisfactory determination of the fusion- 

 point of quartz had been made. The high temperatures given for 

 the fusion-point of silica referred to tridymite. The temperature 

 at which the transformation of quartz into tridymite occurred was 



