Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/438

 seldom exceed six or eight feet. But perhaps the best examples are to be seen along the cliffs to the south of Rudh 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 basalts 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 anamesite 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.