Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/450

 isolated fragment of the great volcanic plateau which stretches in broken masses from Antrim through the Inner Hebrides.

3. These interbedded sheets are traversed by veins and dykes of similar materials, the dykes having the characteristic north-westerly trend with which they pass across the southern half of Scotland and the north of England. Veins of pitchstone and felstone, and intrusive masses of quartziferous porphyry, like some of those which in Skye traverse or overlie the lias, likewise intersect the bedded basalt- rocks of Eigg.

4. At least two widely separated epochs of volcanic activity are represented by the volcanic rocks of Eigg. The older is marked by the bedded basalts and by the basalt veins and dykes, which, though, strictly speaking, younger than the bedded sheets which they intersect, yet probably belong to the same continuous period of volcanic action. The later manifestations of this action are shown by the pitchstone of the Scur. Before that rock was erupted, the older basaltic lavas had long ceased to flow in this district. Their successive beds, widely and deeply eroded by atmospheric waste, were here hollowed into a valley traversed by a river, which carried southward the drainage of the wooded northern hills. Into this valley, slowly scooped out of the older volcanic series, the pitchstone and porphyry coulees of the Scur flowed. Vast, therefore, as the period must be which is chronicled in the huge piles of volcanic beds forming our basalt-plateaux, we must add to it the time needed for the excavation of parts of those plateaux into river-valleys, and the concluding period of volcanic activity during which the rocks of the Scur of Eigg were poured out.

5. Lastly, from the geology of this interesting island we learn, what can be nowhere in Britain more eloquently impressed upon us, that, geologically recent as that portion of the Tertiary period may be during which the volcanic rocks of Eigg were produced, it is yet separated from our own day by an interval sufficient for the removal of mountains, the obliteration of valleys, and the excavation of new valleys and glens where the hills then stood. The amount of denudation which has taken place in the Western Highlands since Miocene times will be hardly credible to those who have not adequately realized the potency and activity of the powers of geological waste. Subterranean movements may be called in to account for narrow gorges, or deep glens, or profound sea-lochs ; but no subterranean movement will ever explain the history of the Scur of Eigg, which will remain as striking a memorial of denudation as it is a landmark amid the scenery of our wild western shores.

DESCRIPTION OF PLATE XIV.

Fig. 1. View of the Scur of Eigg from the east.

2. View of the Scur from the south.

3. View of the precipice of the Scur to the south-west of the Loch a Bhealaich.