DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



153 



porphyritic and felsitic masses, which pass into it, and are more specially observable along 

 its border. An exceedingly compact black quartz-felsite forms its southern boundary, 

 runs as a broad dyke-like ridge from the head of the Scarrisdale Water, north- 

 eastward across Loch Ba' (fig. 43), and spreads out eastward into a mass more than a 

 mile broad on the heights above Kilbeg in Glen Forsa. The sharp line of demarcation 

 of this felsite, and its mass and extent, point to different periods of extravasation in the 

 Loch Ba' boss. 



The geologist, who approaches this district from the north-east, has his attention 

 arrested, even at a distance of several miles, by the contrast between the outer and inner 

 parts of the hills that lie to the south-west of Loch Ba'. He can readily trace from 

 afar the dark bedded basic rocks rising terrace above terrace from the shores of Loch 

 na Keal to form the sea- ward faces of the hills along the southern side of that fjord. 

 But he observes that immediately behind these terraces the mass of the rising ground 



Fig. 43. — View of the hills on the south side of the head of Loch 11a Keal, showing the junction of the Granophyre and the 

 Bedded Basalts. One bird, the bedded basalts of the Gribon plateau; two birds, the bedded dolerites and basalts of Beinn 

 a' Chraig adhering to the northern slope and capping the hill ; three birds, summit of Ben More, with A'Chioch to the 

 left and the top of Beinn Fhada appearing in the middle distance between them ; four birds, the granophyre slopes of 

 Beinn a' Chraig with the great dyke-like mass of felsite on the left. 



obviously consists of some amorphous rock which weathers into white debris. Nothing 

 can be sharper than the contrast of colour and form between the two parts of the hills. 

 The bedded plateau-rocks lie as a kind of wall or veneer against a steep face of the structure- 

 less interior (fig. 43). Seen from the other or hilly side, the contrast is perhaps even 

 more striking. But the astonishment with which it is beheld at a distance becomes 

 intensified when one climbs the slopes, and finds that the sheets of dolerite and basalt 

 (which from some points of view look quite level, yet dip towards the north-east at a 

 gentle angle), are immediately behind the declivity abruptly truncated by a mass of 

 granophyre. So little disturbed are they, that one's first impulse is to search for pebbles 

 of the granophyre between the basalts, for it seems incredible that the inner rock should 

 be anything but a central core of older eruptive material, against and round which the 

 younger basic rocks have flowed. But, though the granophyre is so decomposing 



