170 DR' GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Similar evidence may be gathered from the area of the great granophyre cones 

 further north. The profusion of basalt-dykes in the surrounding rocks stops short at the 

 margin of that area. The comparatively few dykes which cross the boundary pursue a 

 general N.W. course through the granophyre, and as already remarked, from their dark 

 colour, greater durability and straightness of direction stand out as prominent ribs on 

 the flanks of the pale cones which they traverse. 



d. St Kilda. — I have not personally visited this remote island, of which the only 

 geological account we have is that by Macculloch. But through the kindness of my 

 colleague, Mr John Horne, F.E.S.E., I have had a series of specimens submitted to me 

 which were collected by Mr A. Ross of Inverness. These prove that in St Kilda, not 

 only are the rocks of the mountainous parts of Skye and Mull repeated, but that their 

 relative order of appearance is likewise the same. The olivine-gabbros and the granitoid 

 and granophyric rocks are precisely those of the Inner Hebrides. One of the specimens 

 shows a vein of fine granite traversing an ophitic dolerite, and another is a piece of altered 

 fine dolerite or basalt, from near the junction with the acid rocks, and weathers with the 

 white crust so characteristic of similar rocks in the districts above described. 



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Fig. 56. — Section showing the Truncation of a Basalt Vein (5), in Lower Silurian Limestone (a), 

 by the Granophyre (c) of Beinn na Dubhaich, Skye. 



e. Antrim. — In the volcanic region of the north of Ireland the areas of acid rocks are 

 comparatively few in number, and small in size. They nowhere assume the form of 

 prominent hills, and indeed would never attract attention from anything conspicuous in 

 their topography. The largest of them covers a space of about 10 square miles in the 

 heart of the basalt-plateau to the north-east of the town of Antrim, rises to about 1000 

 feet above the sea, and forms a few featureless hills, some of which are capped with basalt. 

 The best known localities in this tract are Tardree and Carnearny. The rock is chiefly a 

 quartz-trachyte, but here and there pieces of pitchstone and pearlstone may be picked up, 

 which, though I could not find them in place, I have no doubt form part of the main 

 trachyte mass. 



Owing to the cover of soil and turf, the junction of this rock with the basalts of the 

 plateau cannot be so clearly seen as in the sections of the Inner Hebrides, and hence the 

 stratigraphical relations of the two groups are apt to be misunderstood. What is actually 

 seen is represented in fig. 57. It has been supposed that the trachyte forms the summit 



