27Q ME. R. H. EASTALL ON THE [May 1906, 



that the nearest exposures of the Eskdale Granite do not exhibit any 

 marginal characters : — 



' If, on further study of the neighbour!) ood of Easthwaite, this proves to be the 

 case, it would point to one of two things : (a) Ennerdale rock intrusive in, and 

 therefore later tban, the granite; or (b) a faulted junction.' 



Further work is required before anything definite can he said on 

 the subject. I have much pleasure in expressing my thanks to 

 Dr. Dwerryhouse for this interesting note. 



V. Geanophyric Structure. 



As before stated, a detailed study of a large number of slices 

 from different parts of these intrusions has shown that there exists 

 a good deal of variation in different parts of the mass, and these 

 variations seem to be arranged according to certain definite laws. 

 Prom the field-evidence, it is clear that the intrusion, as a whole, 

 forms a series of laccolites of varying size, which are sharply 

 marked off one from the other. If we examine the distribution of 

 the different rock-types in any one laccolite, we find a certain 

 regularity of distribution, as follows : — Near the margin of the 

 laccolite the rock shows a structure nearly approaching that of the 

 normal acid plutonic rocks, with scarcely a trace of graphic inter- 

 growth ; a short distance from the margin, graphic structure begins 

 to make itself manifest, at first of a very imperfect and irregular 

 type; and, as we approach the centre of the visible mass, this inter- 

 growth becomes continuously finer in texture, and of an increasingly- 

 perfect micropegmatitic structure. In some specimens from near the 

 head of Ennerdale Water in particular, this intergrowth becomes 

 so intimate in character as to be scarcely capable of resolution by 

 the ordinary powers of the microscope, and no doubt it also exists in 

 a degree of fineness beyond the limits of visibility. Some of these 

 finer types resemble certain forms of perthite, while others show 

 a very strong likeness to photomicrographs of alloys of metals. 

 This may also be expressed by saying that the structure in the 

 inner parts of the laccolites approaches more and more closely to a 

 molecular mixture of quartz and felspar, which may also be regarded 

 as a solid solution. 



The micropegmatite is of several different types. In one very 

 common structure, large and more or less porphyritic crystals of 

 felspar are surrounded by a sort of ring or fringe of micropegmatite, 

 and the felspar of this fringe is in optical continuity with the 

 large crystal. Structures of this sort seem to occur in connection 

 with both orthoclase and plagioclase. Mr. Harker l has noted that, 

 in some cases, crystals of plagioclase are surrounded by a narrow 

 zone of orthoclase, and that by micropegmatite, and the felspar of 

 the micropegmatite has the same orientation as this zone of ortho- 

 clase. However, this is not always the case, and many crystals of 

 plagioclase are surrounded by a fringe of this nature without any 

 intervening orthoclase. If the central crystal is small, the struc- 

 ture becomes somewhat spherulitic ; but, unless a nucleus of this 



1 Quart. Journ. Geol. Soc. vol. li (1895) p. 128. 



