]44 ME. ALFRED HAEKEE ON THE CAEEOCK FELL [May 1 895, 



greisen, and finally quartz-veins containing only local aggregations 

 of white mica or bordered by a narrow seam ricb in that mineral. 

 These quartz-veins, as at Grainsgill, traverse the other types of 

 rocks. 



Returning to the Cumbrian area, we note that the greisen occurs 

 on the north side of an outcrop of granite, which is probably in 

 subterranean continuity with the other outcrops to the south-west 

 and south. In other words, it forms a fringe on the northern border 

 of the large body of Skiddaw granite, though whether such fringe 

 occurs on the northern edge only is a point on which we have no 

 positive evidence. Eegarding it as a modification of the granite, 

 we see that, apart from subsequent chemical transformations, it 

 must be due in the first place to some process of differentiation 

 radically unlike any which have been discussed above. The cause 

 of this differentiation was probably that advocated by Mr. Barrow 

 in the case of his pegmatites, namely, mechanical force operating 

 on the granite-magma when cr} T stallization had already proceeded in 

 it to a certain stage. 



The Skiddaw granite, including the greisen, 1 has produced folia- 

 tion on the cleavage-planes of the adjacent slates, and therefore 

 belongs to the later group of igneous intrusions, to which also the 

 other Lake District granites, those of Eskdale and Shap, must for 

 various reasons be referred. From a general study of the district 

 we are led to believe that all these intrusions, though in great part 

 later than the cleavage, are intimately connected with the great 

 post-Silurian crust-movements of which the cleavage-structure is 

 only one manifestation. The forces by which these movements 

 were produced were directed from south to north, and it is in this 

 direction that the fringe of greisen- (or pegmatite-) magma seems 

 to have been thrown out from the partially consolidated Skiddaw 

 granite. If, when the zircon, the biotite, and part of the felspar 

 had already crystallized, a portion of the still fluid ' mother-liquor' 

 was squeezed out, it is clear that the partial magma thus forced 

 northward would be of more acid composition than the granite as 

 finally consolidated from the remainder, and would give rise by its 

 crystallization to a rock of different type. How far the chemical 

 difference between the granite and the greisen is thus accounted for, 

 or how far that difference must be ascribed to subsequent chemical 

 modification of the fringing rock, is a question not easily solved. 



It may be remarked in passing that any connexion established 

 between crust-movements and differentiation in rock-magmas must 

 have a most important bearing on the problem of the geographical 

 distribution of different types of igneous rocks in ' petrographical 

 provinces ' often divided by mountain-ranges. 2 



1 The greisen seems to have given rise to more intense and widespread 

 metaniorphisni than the granite. This extreme metaniorphism produced by a 

 very acid intrusion, rich in white mica, is in accord with what is known in 

 other areas. 



2 See ' The Evolution of Igneous Kocks,' in ' Science Progress,' vol. i. (1894) 

 pp. 152-165. 



