CHAP. XLVIII 
THE ACID DYKES AND VEINS 
445 
they almost always occur either in or close to granophyre or granite bosses, 
the comparatively late origin of which has now been proved. 
The first pitchstone observed in Skye was found by Jameson on the 
Hanks of the great granophyre cone ot Glamaig. Another rises on the side 
of the porphyry mass of Glas Bheinn Bheag, in Strath Beg. Several occur 
at the foot of Beinn na Callich. In Bum, 
I found a pitchstone vein traversing the 
western slopes of the wide granophyre 
boss of Orval. In Eigg, the well-known 
veins of this rock intersect the plateau- 
basalts (Fig. 381), but they are accom- 
panied, even within the same fissure, with 
granophyre, and in their near neighbour- 
hood lie the masses of this rock already 
alluded to. 1 In Antrim, pitchstone and 
obsidian occur in the midst of the rhyolite. 
The only marked exceptions to the general 
rule, with which I am acquainted, are 
those of the island ol Arran. Most of 
the pitclistone-veins in that district tra- 
verse the red sandstones which may be 
Permian. But none of them are far 
removed from the great granite boss of the 
northern half of the island, while large masses of quartz-porphyry, which 
strikingly resemble some of those of Skye and Mull, lie still nearer to 
them. ° It is also worthy of notice that pitchstone-veins rise through the 
Arran granite boss itself, the probably Tertiary date of which has been 
already discussed. 
This common association of pitchstone-veins with the Tertiary eruptive 
bosses of acid rocks can hardly be a mere accidental coincidence. It seems 
to prove a renewed extravasation of acid material, now in vitreous form, 
from the same vents that had supplied the granitoid, granophyric, porphy- 
ritic and felsitic varieties of earlier protrusions. We must remember that 
the pitchstone-veins are not mere local glassy parts ol the larger bodies of 
granophyre or granite in which they lie. Their margins are sharply 
defined ; they are indeed in all respects as manifestly intruded, and there- 
fore later masses, as are the basalt-dykes. Their occurrence, therefore, 
within the acid bosses proves them to be younger than these members of 
the Tertiary volcanic series. Whether they are also later than the latest 
basalt-dykes cannot yet be decided, for I have never succeeded in finding 
an example of the intersection of these two groups of veins and dykes. 
But with this possible exception, the pitchstones are the most recent of all 
the eruptive rocks of Britain. 
As a rule, the intrusive pitchstones occur as veins which cannot be 
traced far, and which vary from a few yards to less than an inch in width. 
1 For an account of the pitchstone veins of Eigg, see Quart. Journ. Geol. Soc. xxvii. p. 299. 
