310 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



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 intru- 

 sive 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 inter- 

 sect, 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 ba- 

 saltic lavas had long ceased to flow in this district. Their succes- 

 sive beds, widely and deeply eroded by atmospheric waste, were here 

 hollowed into a vaUey 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 pUes of volcanic beds forming our 

 basalt-plateaux, we must add to it the time needed for the excava- 

 tion 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 hiUs 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 ade- 

 quately 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 sub- 

 terranean movement wiU 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. 



DESCEIPTION OF PLATE XIV. 



Fig. 1 . View of the Scm* of Eigg fi-om the east. 

 2. View of the Sciir from the south. 



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

 Bhealaich. 



