298 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



seldom exceed six or eight feet. But perhaps the best examples are 

 to be seen along the cliffs to the south of Eudh nan tri Chlach. At 

 this part of the coast, owing to the southward slope of the surface of 

 the tableland of Beinn Bhuidh, the greater part of the overlying ba- 

 salts is absent, and only the porphyrite and the underlying beds 

 form the capping of the cliffs above the Oolitic rocks. The section 

 (fig. 4, p. 297) represents the succession of rocks there to be seen, 

 and shows how the intrusive sheets may be intercalated either 

 with the Oolitic strata or with the older parts of the doleritic 

 series. 



y. Dykes and Veins. 



Another mode of escape to the pent-up molten rock was furnished 

 by long straight fissures and by irregular winding cracks — the former 

 giving rise to dykes, and the latter to veins. I reserve for a future 

 paper a full consideration of that remarkable feature of the Tertiary 

 volcanic rocks, the long parallel dykes. With regard to those which 

 occur in Eigg, I may remark that they are not remarkable for 

 numbers or other peculiarities, but that they exhibit many, of 

 the characteristic features of the dykes which range from the 

 basaltic plateaux of the Hebrides across Scotland and the north 

 of England. They run, as a rule, persistently from north-west 

 to south-east, varying in breadth from a few feet to a few 

 yards in breadth. They consist either of a close-grained ana- 

 mesite or of basalt, and sometimes contain large grains of olivine. 

 They cut across even the newest of the sheets of the plateau, as 

 may be seen along the terraced slopes that descend from the Scur. 

 But in some of the cliff-sections, as, for example, below Bideann 

 Boidheach and on the east side northwards from Kildonan, they may 

 be seen rising through the lower, but stopping short of the higher 

 beds of dolerite. That truncation may not indicate that where it 

 occurs the dykes are older than the interbedded flows which cover 

 them, but only that the fissures through which they rose did not 

 extend further upward, or at least did not receive an injection of 

 lava into their upper parts. At the same time, there can hardly 

 be any doubt that the dykes as a whole are contemporaneous with 

 the eruptions of the plateau, some of them belonging to earlier, 

 others to later stages in the long volcanic history. No dyke has 

 been observed cutting the pitchstone of the Scur ; but several are 

 covered unconformably by that rock (see fig. 10). 



The igneous veins by which the rocks of Eigg are traversed do not 

 differ in origin from the dykes ; but their smaller size and irregular 

 form enable us to group them by themselves, and to note among 

 them some characteristic features which are not found, or at least 

 found much less distinctly, among the dykes. The veins may be 

 arranged in two groups, according to their component rock, viz. : — 

 1st. Basalt veins ; and, 2nd. Pitchstone and Felstone veins. This 

 classification may be regarded as also a chronological one, since 

 there is reason to believe that the former group is older than the 

 latter. 



