124 
THE TERTIARY VOLCANOES 
BOOK VIII 
and the Jurassic strata, through which they rise, are hardly to be seen 
at all. 
Among the districts where dykes of the gregarious type abound at a 
distance from any of the basalt-plateaux, reference should be made to the 
curious isolated tract of the central granite core of Western Donegal. In 
that area a considerable number of dykes rises through the granite, to 
which they are almost wholly confined. Again, far to the east another 
limited district, where dykes are crowded together, lies among the Mourne 
Mountains. These granite hills are probably to be classed with those of 
Arran, as portions of a series of granite protrusions belonging to a late part 
of the Tertiary volcanic period which will be treated of in Chapter 
xlvii. 
Though the dykes may be conveniently grouped in two series or types, 
which on the whole are tolerably well marked, it is not always practicable 
to draw any line between them, or to say to which group a particular dyke 
should be assigned. In some districts, however, in which they are both 
developed, we can separate them without difficulty. In the Argyleshire 
region above referred to, for example, which Mr. Clough has mapped, he 
finds that the abundant dykes belonging to the gregarious type run in a 
general X.W. or N.N.W. direction, and distinctly intersect the much 
scarcer and less basic dykes of the solitary type, which here run nearly E. 
and W. (Fig. 257). Hence, besides their composition, distinction in number, 
breadth, rectilinearity and persistence, the two series in that region 
demonstrably belong to distinct periods of eruption. 1 
The characteristic habit in gregarious dykes of occurring in crowded 
groups which are separated from each other by intervals of variable dimen- 
sions, marked by the presence of comparatively few dykes, is well illustrated 
in the district of Strath in Skye, which indeed may be taken as a typical 
area for this peculiarity of distribution. While the dykes are there 
singularly abundant in the Cambrian Limestone and the Liassic strata, 
they have been found by Mr. Clough and Mr. Harker to be comparatively 
infrequent in the tracts of Torridon Sandstone. It is not easy to under- 
stand this peculiar arrangement. As the Torridon Sandstone is the most 
ancient rock of the district, it probably underlies all the Cambrian and 
Jurassic formations, so that the dykes which penetrate these younger strata 
must also rise through the Torridonian rocks. Some formations appear to 
have been fissured more readily than others, and thus to have provided more 
abundant openings for the uprise of the basaltic magma from below. To 
the effect of such local differences in the structure of the terrestrial 
crust we have to add the concentration of the volcanic foci in certain 
areas, though there seems no means of ascertaining what part each of these 
causes has played in the distribution of the dykes of any particular 
district. 
1 Mr. Clough is inclined to suspect that the E. and W. dykes are older than the Tertiary 
series and may he later Palaeozoic. 
