314 
THE TERTIARY VOLCANOES 
BOOK VIII 
wav y ribbons of basic material cross many of these veins, while the later 
north-west dykes cut sharply through everything older than themselves. 
As a natural section for the study of the phenomena of intrusion in many 
oi their most characteristic phases, I know no locality equal to the northern 
coast-line of the Sound of Soa, unless it be the cliffs of Ardnamurchan. 
hut the Skye cliffs, though less imposing than those of the great Argyllshire 
headland, have this advantage, that instead of being exposed to the full roll 
of the open Atlantic, they form the margin of a comparatively sheltered 
strait, and can thus be conveniently examined. 
Following still the western seaboard of Skye, we meet with other strik- 
ing examples of sills at a distance of some eight miles in a straight line 
i 
c « « « d <1 (Ida 
Fig. 323. Section of the base of the Basalt-plateau with sill and clykes, Sound of Soa, Skye. 
a a, Torrklon Sandstone; 6, Bedded basalts ; c, Sill ; d d, Dykes. 
eastward where, between Lochs Slapin and Eishort, the prominent headland 
of Suisnish juts out into the sea. This promontory has long been known 
to geologists from the section of it given by Macculloch as an instance of 
the connection between overlying rocks and dykes. I have already alluded 
to it in that relation, and refer to it again as an example of one of the 
thicker intrusive sheets of the Inner Hebrides. Denudation has here also 
proceeded so far that the whole of the volcanic plateau has been stripped 
off, only some of the underlying sills being left, together with the plat- 
form of older rocks between which and the vanished basalts they were 
injected. Most of these sills consist of granophyres belonging to the acid 
group of rocks to be afterwards described. But basic sheets occur not in- 
frequently interposed between the granophyres and the subjacent Lias, and 
sometimes even intercalated in the former rock. Though at first shdit it 
might be thought that these sills had insinuated themselves after the & erup- 
ti°n of the granophyre, and there are instances where this cannot be shown 
not to be the case, I have obtained so many proofs of the invasion of the 
basic by the acid rock that I have no doubt the former is, as a general 
rule, the older of the two. 
The Suisnish headland exhibits the structure represented in Fig. 249. 
For about 300 feet above the sea-level the steep grassy slope shows 
outcrops of the dark, sandy shales and yellowish brown, shaly sandstones 
of the Lias which form the range of cliffs to the eastward. These gently 
