DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



177 



parallel to the walls, may be seen on the west front of Ben Meabost. In this case, the 

 veins are three miles and a half from the granophyre mass of Strath-na-Creitheach to the 

 north, four miles from that of Beinn-an-Dubhaich to the north-east, and nearly three 

 miles from that of Coire Uaigneich at the foot of Blath Bheinn. 



A special place must be reserved for the pitchstone-veins. Ever since the early 

 explorations of Jameson and Macculloch, the west of Scotland has been noted as one 

 of the chief European districts for these vitreous rocks. From Skye to Arran, and 

 thence to Antrim, many localities have furnished examples of them, but always within 

 the limits of the Tertiary volcanic region. That all of the pitchstones are of Tertiary 

 age cannot, of course, be proved, for some of them are found traversing only Palaeozoic 

 rocks, and of these all that can be absolutely affirmed is that they must be younger than 

 the lower part of the Carboniferous system. But, as most of them are unquestionably 

 parts of the Tertiary volcanic series, they are probably all referable to that series. Not 

 only so, but there is, I think, good reason to place them among its very youngest 

 members. It is a significant fact that they almost always occur either in or close to 

 granophyre bosses, the comparatively late origin of which has now been proved. The 

 first pitchstone observed in Skye was found by Jameson on the flanks of the great 

 granophyre cone of Glamaig. Another rises on the side of the porphyry mass of Glas 

 Bheinn Bheag, in Strath Beg. A third occurs at the foot of Beinn-na-Caillich. 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 inter- 

 sect the plateau-basalts, but in their near neighbourhood 

 lie the masses of quartz-porphyry already alluded to. 

 In Ardnamurchan also, the pitchstone has been injected 

 into the plateau-basalts, but there are many small veins 

 of close-grained felsitic or rhyolitic rocks in the vicinity. 

 In Antrim, pitchstone occurs in the midst of the trachyte 

 of Tardree. The only marked exceptions to the general 

 rule, with which I am acquainted, are those of the island 

 of Arran. Most of the pitchstone-veins in that district 

 traverse the red sandstones which lie at the base of the 

 Carboniferous system, or belong to the Old Bed Sand- 

 stone. 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 granite boss itself, the probably Tertiary date of 

 which has been already referred to. 



This common association of pitchstone-veins with the Tertiary eruptive bosses of acid 

 rocks can hardly be a mere accidental coincidence. It seems to me to prove a renewed 

 extravasation of acid material, now in vitreous form, from the same vents that 



Fig. 61. — Pitchstone Vein traversing the 

 Bedded Basalts, Rudh an Tangairt, 



