Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 391 



through the rock of the Red Hills in Skye ; they are sharply defined 

 from the enclosing rock, as if the latter had already solidified before 

 their intrusion. 



With regard to the microscopic structure of some thin slices 

 prepared from these veins, Mr. Harker remarks that 'the material 

 of the veins is of a type intermediate between granophyre and 

 microgranite [6622, 6623]. The chief bulk is a finely granular 

 aggregate of quartz and felspar, the latter very turbid ; but in this 

 aggregate are imbedded numerous patches of micropegmatite, often 

 of perfect and delicate structure. These areas of micropegmatite 

 show some approach to a radiate or rudely spherulitic structure, 

 and, in some cases, are clustered round a crystal of felspar or quartz. 

 Some granules of magnetite and rare flakes of brown biotite are the 

 only other constituents of the rock. Although they must be of 

 somewhat later date, there is evidently nothing in the petrographical 

 characters of these fine-textured veins to separate them widely from 

 the ordinary granophyres of the region.' 



These veins may be compared with the spherulitic dyke which 

 traverses the granophyre of Meall Dearg at the head of Glen 

 Sligachan, and which, though undoubtedly somewhat later than the 

 rock that contains it, yet presents the very same structures as are 

 visible at the margin of that rock. 1 The material of this dyke, and 

 of the finer veins of St. Kilda and the Red Hills, probably belongs 

 to a later period of protrusion from a deeper unconsolidated portion 

 of the same acid magma as that which, at first supplied the general 

 body of granophyre. 



Undoubtedly the most interesting feature in the granophyre of 

 St. Kilda is its junction with the mass of basic rock to the west of 

 it. It requires no close search to find in situ the dark rock with 

 acid veins of which Macculloch found scattered fragments. The 

 line of junction between the basic and acid masses runs across the 

 island from the western side of the chief bay to the northern coast, 

 where it is exposed in a line of high cliffs. 



The beach to the west of the landing-place in the bay is strewn 

 with blocks of various dark, finely crystalline basic rocks, traversed 

 by pale veins of granophyre. At the western end of the shingle, 

 the rocks are met with in places forming a line of low cliff and a 

 rugged foreshore. The basic rock consists of various gabbros and 

 basalts of rather fine grain, profusely traversed with veins of white 

 granophyre. Some of these veins are 2 feet or more in breadth, 

 and, when of that size, show the distinctive granular texture and 

 drusy structure of the main part of the acid rock. But from these 

 dimensions they can be traced through every stage of diminution 

 until they become mere threads. When they are only an inch or 

 two broad they assume a finely granular texture, like that of the 

 veins which run through the body of the granophyre. 



The amount of injected material in the dark basic rocks is here 



and there so great as to form a kind of breccia (fig. 28, p. 392), 



which, from the contrast of tone between its two constituents, makes 



a conspicuous object on the shore. The enclosed fragments are of 



1 Quart. Journ. Geol. Soc. vol. 1. (1894) p. 220. 



