iS6 
THE TERTIARY VOLCANOES 
BOOK VIII 
which I am now dealing, I shall endeavour to show in the sequel. But it 
is quite certain that though these sheets have of course each had its sub- 
terranean pipe or fissure of supply, they can only in rare instances he 
directly traced to the system of dykes. On the other hand, the districts 
where great single dykes are most conspicuous, are for the most part free 
from intrusive sheets, except those of much older date, like the Carboniferous 
Whin Sill of Durham and those of Linlithgowshire, Stirlingshire and Fife. 
Yet a few interesting examples of the relation of dykes to sheets have 
been noticed among British Tertiary volcanic rocks. The earliest observed 
instances were those figured and described by Maccullocb. Among them 
one has been familiar to geologists from having done duty in text-books of 
the science for more than half a century. I allude to the diagram of “ Trap 
and Sandstone near Suishnish.” 1 In that drawing seven dykes are shown 
as rising vertically through the horizontal sandstone, and merging into a 
thick overlying mass of “ trap.” The author in his explanation leaves it an 
open question “ whether the intruding material has ascended from below 
and overflowed the strata, or has descended from the mass,” though from the 
language he uses in his text we may infer that he was inclined to regard 
the overlying body as the source of the veins below it. 2 3 * * * 
The section given by Maccullocb, however, does not quite accurately 
represent the facts. The narrow dykes there drawn have no connection 
Fig. 249. — Section showing the connection of a Dyke with an Intrusive Sheet, Point of Suisnisli, Skye. 
< 7 , Granopliyre of Cam Dearg ; /, similar rock, which appears eastward under the “sill” (d); c, intrusive sheet of 
fine-grained “ basalt” ; d , intrusive sheet or sill of coarse dolerite, 200 feet thick at its maximum, and rapidly 
thinning out; c, dyke or pipe of finer grain than d ; b, yellowisli-brown slialy sandstones, and a, dark sandy 
shales (Lias). 
with the overlying sheet, hut are part of the abundant series of basaltic 
dykes found all over Skye. The feeder of the gabbro sill was presumably 
the broad dyke which descends the steep bank immediately on the southern 
front of Carn Dearg (636 feet high). The accompanying figure (Fig. 
249) shows what seemed to me to be the structure of the locality, but the 
actual junction of the dyke and sheet is concealed under the talus of the 
slope. 8 I shall have occasion in a later Chapter to refer again to this 
1 W estern Islands of Scotland, pi. xiv. Fig. 4. 
2 Op. cit. vol. i. pp. 384, 385. 
3 In more recently surveying tliis ground, Mr. Harker lias been led to regard the coarse sill as 
independent of the other intrusions, and as almost certainly later than the basalt-sheets of the 
same locality. When it reaches the base of these sills it turns so as to pass beneath them as a 
gabbro-sill, which is conspicuous near the summit of Carn Dearg. It runs westward for some 
