Vol. 52.] BASALT-PLATEAUX OF SOUTH-WESTERN EUROPE. 



38a 



that in this remote island a similar difference in age and in petro- 

 graphical character may be made out between two series of dykes, 

 one older and the other younger than the granophyro. 



The pale colour of the precipices in which the St. Kilda grano- 

 phyre plunges into the sea gives marked prominence to the dark 

 ribbon-like streaks which mark the course of the basalt-dykes 

 through that rock. Moreover, the greater liability of the material 

 of the dykes to decay causes them to weather into long lines of 

 notch or recess. Four or five such dykes follow each other in 

 nearly parallel bands, which slant upward from the sea-level on the 

 eastern face of the hill known as Conacher to a height of several 

 hundred feet (fig. 27). 1 



Fig. 27. — Basalt-veins traversing granophyre. St. Kilda. 





Dykes abound in the Faroe Islands, where they cut the basalt- 

 plateau in the same way as they do that of the Inner Hebrides. 

 On the whole, however, they do not play, in these northern isles, 

 the important part which they take in the geology and scenery of 

 the West of Scotland. I have not had sufficient opportunity to 

 ascertain whether there is a general direction or system among the 

 Faroe dykes. In the fjords north of Thorshaven, and again along 

 the western side of Stromo, many of them show an east-and-west 

 strike or one from E.N.E. to W.S.W. 



Numerous examples of compound dykes, where a central band of 

 granophyre or spherulitic felsite is flanked on each side by one of 

 basic material, have recently been met with in Skye by Messrs. 

 Clough and Harker in the course of the geological survey of that 

 island. They will be further noticed in the Vllth section of this 

 paper. 



1 This relation of the later dykes to the granophyre was observed here by 

 Macculloch, ■ Description of the Western Islands,' vol. ii. (1819) p. 55. 



