Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 393 



'The acid rock, though styled granophyre above, belongs to a 

 granitoid variety of that group of rocks, and has but little indication 

 of micrographic structures. Compared with the other granophyres 

 from St. Kilda, sliced and examined, these examples show a less 

 acid composition. This is expressed mineralogically in the presence 

 of a somewhat larger proportion of ferro-magnesian minerals and 

 of soda-lime felspar. These features might indeed be matched iu 

 many normal granophyres among the Western Isles, but in the 

 present case it can hardly be doubted that they are to be explained, 

 at least in some degree, by the acid magma having taken up a 

 certain amount of material from the basalt. Many of these Tertiary 

 granophyres have undoubtedly been modified by the ineorporation 

 of pieces of basalt and gabbro, and a collection made in the Strath 

 district of Skye will furnish examples for future study. Prof. Sollas's 

 description of similar phenomena in the Carlingford district has 

 already proved the importance of this kind of action. 1 In tho 

 present instance, both brown mica and hornblende occur plentifully 

 in the granophyre, and especially round the basalt-fragments. This 

 latter point is conclusive as to the derivation of the basic material, 

 and further proves a certain degree of viscosity in the acid magma 

 at the time of its intrusion.' 



On the northern side of St. Kilda the junction-line of the granophyre 

 runs up the cliffs, abundant pale veins of the acid material striking 

 off from the main body of the rock and traversing the dark gabbros. 



The testimony of the rocks of St. Kilda to the posteriority of 

 the granophyre to the gabbros and basalts is thus clear and emphatic. 

 It entirely confirms my published observations regarding the 

 order of sequence of these rocks in Mull, Kum, and Skye. But the 

 St. Kilda sections display, even more strikingly than can be usually 

 seen in these islands, the intricate network of veins which proceed 

 from the granophyre and the shattered condition of the basic rocks 

 which these veins penetrate. 



I have already alluded to the remarkable association of acid and 

 basic material in numerous dykes as well as in some sills in the 

 district of Strath in Skye. I formerly described some examples of 

 this association from that district, 2 and many more have recently 

 been observed and mapped by Mr. Clough and Mr. Harker during 

 the progress of the Geological Survey. The conjunction commonly 

 shows a central and thicker band of granophyre or spherulitic felsite 

 with two thinner parallel bands of some dark intermediate or basic 

 rock. This triple arrangement occurs both in dykes and sills. 



As an illustration of the association of the two kinds of rock in 

 dykes t may cite the example which appears on the southern edge 

 of the Market Stance of Broadford (fig. 29, p. 394). Here the charac- 

 teristic triple arrangement is typically developed. A central light- 

 coloured band, about 8 to 10 feet broad, consists of a spherulitic 

 granophyre in which the spherulites are crowded together and project 

 from the weathered surface like peas, though they do not hero 

 show the curious rod-like aggregation so marked in some othei 



1 Trans. Roy. Irish Acad. vol. xxx. (1894) pp. 477-512. 



2 Trans. Roy. Soc. Edinb. vol. xxxv. (1888) p. 174. 



