﻿140 THE SKIDDAW GRANITE AND ITS METAMORPHISM. [May I9IO,. 



of the weather and the roughness of the ground. I have received 

 valuable assistance from Dr. P. H. Hatch, Mr. P. Eigby of Christ's. 

 College, Mr. A. H. Noble of Queen's College, Cambridge, and. 

 especially from Mr. James Romanes of Christ's College. To all 

 these I return my most hearty thanks. 



EXPLANATION OF PLATE XIV. 



Geological sketch-map of the Skiddaw district, on the scale of 1 inch to the- 

 mile. The map is oriented north and south. Altitudes are indicated in 

 feet. 



Discussion. 



The President (Prof. W. J. Sollas) remarked that the supposed, 

 identification of a granitic intrusion with the core of an anticline 

 was of particular interest at a time when increasing scepticism 

 was felt as to the existence of a direct relation between granitic 

 masses and tectonic features. As a rule, granitic magmas seemed 

 to make their way upwards in independence of folding. 



Dr. J. W. Evans referred to the importance of pressure in- 

 determining the character of the metamorphism of sedimentary 

 strata by intrusive rocks, and especially in inducing the formation 

 of minerals with greater density and less molecular volume. The 

 presence of kyanite, the aluminium silicate with highest specific 

 gravity, would be an indication that the metamorphism occurred 

 under considerable pressure ; for under low pressure kyanite was. 

 converted into fibrolite at a high temperature. Andalusite was 

 also converted into fibrolite when strongly heated. The presence 

 of andalusite and the absence of kyanite and fibrolite in the meta- 

 morphic rocks described by the Author would, therefore, constitute 

 evidence that the granite was intruded at a comparatively low 

 temperature, and at the same time under quite moderate pressure. 



Mr. Postlethwaite welcomed the Author's confirmation of his 

 own opinion, that the extensive aureole of metamorphosed rock 

 indicated the presence of a large mass of granite at a little distance 

 beneath the surface. If the progress of denudation had been some- 

 what less, there would have been no exposure of the granite, and. 

 the existence of an igneous mass beneath could only have been 

 inferred from the altered character of the superincumbent strata. 



Mr. Lamplugh remarked that, some years ago, on visiting the 

 district described by the Author, he had been impressed with the 

 difference in the degree of metamorphism shown by the rocks of 

 Skiddaw in the neighbourhood of the granite, as compared with 

 that shown by the similar rocks in the Isle of Man near the Pox- 

 dale granite. At Poxdale the gradual shelving of the granite- 

 surface beneath the slates had been actually proved by mining 

 operations ; yet the alteration was often comparatively slight. In 

 the Isle of Man he was forced to surmise that there had been 

 some other active factor besides the heating of the sediments by the 

 igneous intrusion, namely, differential movement in strata that were^ 

 already heated nearly up to the critical temperature. 



