396 
THE TERTIARY VOLCANOES 
BOOK VIII 
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quartz-felsite or rhyolite forms its southern houndary, runs as a broad 
dyke-like ridge from the head of the Scarrisdale Water north-eastward across 
Loch Ba’ (Fig. 352), 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 a different 
„ s „ period of extravasation. 
t The geologist, who 
■g. approaches this district 
g jjn| from the north-east, has his 
“ 2 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 
seaward 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 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. 352). 
^ a 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. Of all the junction-lines between the acid bosses 
and the lavas of the plateaux, those exposed on these Mull hillsides are 
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