CHAP. XXXV 
CONNECTION OF DYKES WITH SILLS 
155 
to believe that such a deep cover, though now removed by denudation, once 
overspread the area in which they appear. It will be shown in the sequel 
that such horizons have been peculiarly liable to intrusions of igneous 
material of various kinds, and at many different intervals, during the 
volcanic period. A thick cake of crystalline rock seems to have offered such 
resistance to the uprise of molten material through it, that when the sub- 
terranean energy was not sufficient to rend it open by great fissures, and 
thus give rise to dykes, the lavas were either forced into such irregular 
cracks as were made partly in the softer rocks underneath and partly in the 
cake itself, or found escape along pre-existing divisional planes. In Ardna- 
Fig. 248. — Basic veins traversing secondary limestone and sandstone on the coast clitts, 
Ardnamurchau, 
murchan, round the Cuillin Hills of Skye, and in Hum, the overlying 
resisting cover now consists mainly of gabbro sheets. In the east of Skye, 
in Eigg, and in Antrim, it is made up of the thick mass of the plateau- 
basalts. 
14 . CONNECTION OF DYKES WITH SILLS 
Every field-geologist is aware how seldom he can actually find the vent 
or pipe up which rose the igneous rock that supplied the material of sills 
and laccolites. He might well be pardoned were he to anticipate that, in 
a district much traversed by dykes, there should be many examples ol 
intrusive sheets and frequent opportunities of tracing the connection of such 
sheets with the fissures from which their material might be supposed to 
have been supplied. But such an expectation is singularly disappointed by 
an actual examination of the Tertiary volcanic region of Britain. That 
there are many intrusive sheets belonging to the great volcanic period with 
