108 DR OEIRIE ON THE HISTORY OF VOLCANIC ACTION 



its original dimensions and its relations to the surrounding rocks, especially to the bedded 

 basalts, are much obscured. It can be followed continuously from the lower end of Loch 

 Kilchrist along the southern slopes of Beinn Dearg Bheag round to the western roots of 

 Beinn Dearg Mhor — a distance of more than two miles in a straight line, and from 

 Kilbride to the flank of Beinn na Caillich above Coire-chat-acban — a direct distance of 

 two miles and a quarter. A similar rock, possibly a portion of the same mass, appears in 

 Creagan Dubha, on the north side of the Red Hills. If the whole of this agglomerate 

 forms part of one originally continuous mass, the vent must have been upwards of two 

 miles in diameter. There may, however, have been two or three closely adjacent vents. 

 The Beinn na Caillich patch, for example, appears to belong to a different area, and that 

 of Creagan Dubha may also be distinct. But there seems no reason to doubt that the 

 mass which forms Cnoc nam Fitheach, and all the long declivity on the southern flank of 

 Beinn Dearg Bheag, occupies part of the site of a single volcanic funnel, which was 

 almost two miles in diameter. 



This agglomerate is a coarse tumultuous assembage of blocks and bombs, embedded 

 in the usual dull, dirty-green matrix. Among the stones, scoriaceous, vesicular, and 

 amygdaloidal basalts are specially abundant ; also pieces of various quartz-porphyries, 

 among which a black felsite like that of Mull may often be recognised. In some places, 

 large masses of altered Lower Silurian limestone and quartzite are included ; in others, 

 pieces of yellow sandstone and dark shale (Jurassic). The rock is wholly without strati- 

 fication or structure of any kind. On the north-west side of Loch Kilchrist, indeed, it 

 weathers into large tabular forms, the parallel surfaces of which dip to S.W. ; but this is 

 probably due only to jointing. Here and there, dykes of basalt cut the rock in a general 

 north-westerly direction, but their number is remarkably small when compared with the 

 prodigious quantity of them in the limestone at the bottom and opposite side of the 

 valley, some of which may possibly mark the fissure of the vent. More abundant and 

 extensive are the masses of granophyre that rise more particularly along the outer margin 

 of the vent. These are doubtless connected with the great boss that forms the Red 

 Hills, of which further details will be given in a subsequent section of the paper. 



The important question of the relation of this huge vent to the plateau-basalts does 

 not admit of satisfactory treatment, owing to destruction of the evidence by the 

 intrusion of the granophyre and likewise to enormous denudation. Nevertheless, some 

 traces still remain to indicate that the basalts once stretched over the site of the vent, 

 which probably rose through them. Looking westward from the flanks of Beinn Dearg 

 Bheag to the other side of Loch Slapin, the geologist sees the bold basalt-escarpment of 

 Strathaird presenting its truncated beds to him at a distance of only two miles. That 

 these beds were once prolonged eastwards beyond their present limits is obvious, and that 

 they stretched at least over these two intervening miles can hardly be doubted. But we 

 can still detect relics of them on the flanks of Beinn Dearg. As we follow the agglomerate 

 round the margin of the granophyre that mounts steeply from it, we lose it here and 

 there under beds of amygdaloidal basalt. The rocks next the great eruptive mass of the 



