﻿Vol. 66.~] SKIDDAW GRANITE AND ITS METAMORPHTSM. 121 



These are not, however, the only minerals showing graphic inter- 

 growths in the Skiddaw granite. A. parallel intergrowth of 

 alternate lamellae of muscovite and biotite is common. This is a 

 feature often observable in similar rocks, and probably indicates a 

 close relationship between the two minerals, with incomplete 

 isomorphism. 



The most interesting and unusual case is that of a well-developed 

 graphic relationship of the type usually associated with eutectics 

 between muscovite and a perthitic felspar, which is by no means 

 uncommon in this rock. From its general appearance there is 

 every reason to believe that the muscovite is original, and not a 

 secondary product of greisen-formation : the white mica formed in 

 this way in the greisen-modification of certain parts of the granite 

 mass shows a very different development, and is always distinctly 

 scaly and fibrous ; whereas in the graphic patches many distinct 

 sections embedded in the felspar show a uniform cleavage and 

 optical orientation over a large area. All are obviously parts of 

 one and the same crystal. In one or two instances quartz also 

 takes part in this structure, so that we have here an instance of a 

 eutectic of four components. 



The complete list of mineral molecules recognizable in this rock 

 is six : namely, quartz, orthoclase, albite, anorthite, muscovite, and 

 biotite ; of these the albite is divided between perthite and plagio- 

 clase, so that in practice the number is reduced to five, namely, 

 quartz, perthite, plagioclase, muscovite, and biotite. Hence the 

 eutectic last mentioned contains nearly all the components, and 

 approaches the theoretical eutectic of the magma. 



From these considerations it appears to follow that the order of 

 crystallization was that which finally led to a eutectic composition 

 which expressed itself as a graphic intergrowth of a varying number 

 of components. The large and comparatively pure phenocrysts 

 represent the excess of certain components over this eutectic 

 ratio. 



III. The Metamorphic Aureole. 



So far as is known, all the rocks comprised within the area 

 affected by metamorphism belong to the Skiddaw Slates. This 

 term, however, is a somewhat vague one, and there is no doubt 

 that, as commonly employed and indicated on the maps of the 

 Geological Survey, the name Skiddaw Slate includes rocks of greater 

 antiquity than the Skiddavian division of Dr. Marr, and also some 

 rocks of younger date, Lower and possibly even Middle Llandeilo 

 (Glenkiln). The Skiddaw Slates, as at present denned, also include 

 sediments of Cambrian age, and there is reason to believe that some 

 at least of the coarser grits may be pre-Cambrian, corresponding 

 to the Torridonian and Longmyndian Systems, and showing a 

 very strong lithological resemblance to the coarse grits of Ingleton. 

 A large exposure of coarse grits of this type is just touched by the 



