The Rev. T. G. Bonney on the formation of 'Cirques.' 317 



sire masses of quartziferous porphyry, like some of those which in 

 Skye traverse or overlie the lias, likewise intersect the bedded dole- 

 rites and basalts 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 dolerites 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 do- 

 leritic 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 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. Yast, therefore, as the period must be 

 which is chronicled in the huge piles of volcanic beds forming our 

 dolerite 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 hills then stood. The amount 

 of denudation which has taken place in the Western Islands 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 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. 



3. " On the formation of ' Cirques,' and their bearing upon theo- 

 ries attributing the excavation of Alpine Yalleys mainly to the 

 action of Glaciers." By the Rev. T. G. Bonney, M.A., E.G.S. 



The paper described a number of these remarkable recesses, which, 

 though not restricted to the limestone districts of the Alps, are best 

 exhibited in them. The author gave reasons why he could not 

 suppose them to have been formed either as craters of upheaval, or 

 by the action of the sea, or by glacial erosion. With regard to the 

 last he showed that, even if glaciers had been the principal agents 

 in excavating valleys, there were some cirques which could not have 

 been excavated by them ; and then went on to argue, from the fact 

 that glaciers had occupied cirques, and from the relation between 

 them and the valleys, that they could not be attributed to different 



