Yol. 65.] IN THE NEIGHBOURHOOD 01? ESKDALE. 69 



referred to on p. 57. It consists of allotriomorphic quartz in masses 

 at least ten times the size of those ordinarily found in the marginal 

 mosaics. Strain-shadows are to be seen in nearly every crystal. 

 Felspar and mica are entirely absent, and the crystals of quartz are 

 fimbriated along their margins and closely interlocking. Further- 

 more, they contain numerous minute specks of a mineral possessing 

 a high refractive index and strong double refraction, but which I 

 have been unable to identify. In the hand-specimen this vein is 

 precisely similar to those so commonly found in the Skiddaw Slates, 

 and I do not consider it to be of igneous origin. It is quite different, 

 both in appearance and in structure, from the marginal varieties of 

 the granite. 



Some Theoretical Considerations. 



The fact that the granite-mass becomes more acid toward its 

 margin is of considerable interest from a theoretical point of view. 



The normal granite is more acid than the average, containing as 

 it does about 75 per cent, of silica ; and this I consider to be the 

 explanation of the unusual marginal phenomena. It is a well- 

 known fact that in the case of substances which form eutectic 

 mixtures, where either of the constituents is present in excess of 

 the quantity necessary to form the eutectic, that constituent sepa- 

 rates out first, until the molten residue reaches eutectic proportions. 



It may be considered as demonstrated that quartz and orthoclase 

 are capable of forming an eutectic mixture, and that this eutectic 

 plays an important part in the formation of igneous rocks, appearing 

 as graphic and micropegmatitic structures. 



That this is not the only eutectic that occurs in magmas seems 

 to be indicated by the occurrence of similar micropegmatitic inter- 

 growths of other minerals ; and I am inclined to regard certain 

 other intergrowths, which do not exhibit the same type of structure 

 in their solid form, as being due to the existence of an eutectic 

 mixture of their constituents in the magma at a certain stage of its 

 consolidation. Thus, the parallel intergrowths of biotite with 

 muscovite may be due to this cause ; and there seems even greater 

 reason for supposing the ' perthite ', so common in the granite 

 under consideration, to be a solidified eutectic. 



The difference in the texture and structure of these various inter- 

 growths may be explained on the hypothesis that the structure is 

 controlled by the crystalline form of the constituents. Thus, in 

 the parallel intergrowth above mentioned both minerals belong to 

 the same system. In the case of perthite, where there is also a 

 certain parallelism, the systems of the two constituents are not 

 greatly dissimilar, being monoclinic and triclinic respectively ; 

 while in the quartz-orthoclase micropegmatite, where anything in 

 the nature of parallelism is exceptional, the crystalline systems of 

 the constituents are widely removed from each other as regards 

 symmetry, being monoclinic and hexagonal. 



