150 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



(Marsco, Glen Sligachan) there are isolated roughly idiomorphic crystals, of a white, less 

 turbid orthoclase, which belong to a slightly earlier consolidation than that of the more 

 kaolinised felspar of the rest of the rock. 



With the Tertiary volcanic series of the West Highlands, I have little doubt that the 

 granite of the island of Arran should be classed. In 1873 I gave my reasons for believing 

 this rock to be of so recent date,* and subsequent microscopic examination has tended to 

 confirm this inference by showing the occasional presence in the Arran granite of the 

 same granophyric structure so characteristic of the acid rocks of Skye and Mull.t The 

 granite of the Mourne Mountains has been shown by the Irish Geological Survey to be 

 younger than some of the basic dykes of the south-east of Ireland, and older than 

 others.^ And microscopic evidence in this case also links that rock with the granophyres 

 of the west of Scotland. 



§ 2. Types of Structure. 



In the history of opinion regarding the relative position of the Tertiary eruptive rocks, 

 no point has struck me more than the universal acceptance of what I must now term the 

 misconception regarding the place of the acid protrusions. In tracing this mistake to its 

 source, we find that it probably arose from the fact that along their line of junction the 

 granitoid masses generally underlie the basic. This order of superposition, which would 

 usually suffice to fix the age of two groups of stratified rocks, is obviously not of itself 

 enough to settle the relative epochs of two groups of intrusive rocks. Yet it has been 

 assumed as adequate for this purpose, and hence what can be proved to be really the 

 youngest has been placed as the oldest part of the Tertiary volcanic series. 



Macculloch, who showed that his "syenites" and "porphyries" had invaded the 

 Secondary strata of the Inner Hebrides, and must therefore be of younger date than these, 

 left their relations to the other igneous rocks of the region in a curiously indefinite 

 position. He was disposed to regard them all as merely parts of one great series ; and 

 seems to have thought that they graduate into each other, and that any attempt to dis- 

 criminate between them as to relative age is superfluous. Yet he evidently felt that the 

 contrasts of topography which he described could hardly fail to raise the question of 

 whether rocks so distinct in outward form did not differ also in relative antiquity. But 

 he dismissed the question without answering it, remarking that if there is any difference 

 of age between the two kinds of rock, " there appears no great prospect of discovering 

 it." § He records an instance of a vein of " syenite " traversing the " hypersthene rock " 

 in the valley of Coruisk. " If this vein," he says, " could be traced to the mass of 

 syenite, it might be held a sufficient ground of judgment, but under the present circum- 

 stances, it is incapable of affording any assistance in solving the difficulty."! Instead, 



* Trans. Edin. Geol. Soc, vol. ii. part 3. 



t See Mr Teall's British Petrography (1888), p. 328. 



X Explanation to Sheets 60, 61, and 71, Geo!. Survey, Ireland, pp. 16, 30. 



§ Western Islands, i. p. 368 ; see also pp. 488, 575, 578. 



|| Op. cit., p. 37 ). 



