306 
THE TERTIARY VOLCANOES 
BOOK VIII 
of the Fair Head sheet, 
geological structure than 
they present a greater variety of rock and of 
is to he found in Antrim. They are specially 
developed at the base of the 
thick, overlying, basalt-plateau 
— a platform on which such 
a prodigious quantity of erup- 
tive material has been in- 
jected. Part of this material 
consists of basic rocks in the 
form of dykes, veins, or sills ; 
part of it is included in the 
intermediate and acid groups, 
and comprises veins, sheets, 
A & and bosses of granitoid, fel- 
sitic, rhyolitic, trachytic, and 
pitchstone rocks. One of the 
peculiarities of the Skye sills 
is the occurrence among them 
of compound examples, where 
sheets of basic and acid 
material have been injected 
along the same general plat- 
form. These will he more 
specially referred to in Chapter 
xl vi i i. With regard to the basic 
sills (dolerites, basalts, etc.), I 
would remark that while in 
Western Scotland the Antrim 
type of short, thick intrusions, 
or laccolites, is also found, 
the vast majority of the sheets 
are much thinner, more per- 
sistent, and less easily dis- 
tinguishable from the bedded 
basalts. 
In describing the sills of 
Skye I shall take first those 
of the eastern and then those 
of the western side of the 
island. Along the east coast, 
from Loch Sligachan to the 
most northerly headlands and 
islets the sills play a notable 
part in the scenery, inasmuch 
as they cap the great sea-cliff 
of Trotternish and run as a line of ridges parallel to the trend of the coast. 
