64 ME. A. HAEKER ON THE GEOLOGICAL [Feb. I906, 



TV. Concluding Considerations. 



The question of the true relations of the pitchstone of the Sgurr 

 has been discussed above in the light of such direct evidence as can 

 be adduced. Other considerations, which may legitimately find a 

 place here, arise when we regard the petrographical characters of 

 the pitchstone and its probable place in the long sequence of British 

 Tertiary igneous rocks. 



The pitchstone itself has been described by Prof. Judd, 1 and 

 analysed by Mr. Barker North. The analysis, reproduced in 

 column I (below), proves that it is of subacid composition, and not 

 essentially different from some of the Arran rocks. Pitchstones, 

 rhyolitic, trachytic, and dacitic, referred to the Tertiary suite, occur 

 as sills and dykes in Arran, Ardnamurchan, Eigg, Rum, and Skye, 

 and in Counties Down and Donegal. There are also, in Arran, 

 Eigg, and elsewhere, rocks of the same general characters but 

 devitrified, and these are often intimately associated with the glassy 

 varieties. Most of these occurrences are of small dimensions, but 

 the sheet of devitrified pitchstone which forms the upper parts of 

 Ashval and Sgurr-nan-Gillean, in the Isle of Rum, exceeds in extent 

 and thickness the pitchstone of the Sgurr of Eigg. This Rum 

 occurrence and one or two others belong to an earlier epoch of 

 the hypabyssal phase ; but all the rest are to be assigned to the 

 very latest manifestations of igneous action in the British area. 

 Wherever evidence is obtainable, these rocks are clearly intrusive. 



I. II. III. 



SiO, 65-81 63-34 66-03 



Al o 3 14-01 ... 12-55 



Fe;d 3 4-43 ... 2-75 



MgO 0-89 ... 2-33 



CaO 2-01 ... 280 



Na 4-15 4-56 5'02 



K 2 6-08 4-50 4-13 



Loss on ignition... 2 - 70 ... 4'20 



10008 99-81 



I. Porphyritic pitchstone, Sgurr of Eisg : anal. Barker North, Quart. 



Journ. Geol. Soc. vol. xlvi (1890) p. 379. 

 II. The same : partial analysis by Dr. W. Pollard. 

 III. Pitchstone, Tormore, Arran : anal. M. M. Tait, Bryce's ' Geology of 

 Arran & Clydesdale ' 3rd ed. (1865) p. 185. 



The only case (omitting the Sgurr of Eigg) in which this state- 

 ment is not one of common agreement is that of Beinn Hiant in 

 Ardnamurchan. The sheets of andesite and pitchstone which form 

 that hill were regarded by Prof. Judd 2 as lava-flows, but Sir 

 Archibald Greikie 3 has given good reasons for treating them as 

 intrusive sills. The rock of the Sgurr of Eigg, then, if an intrusive 



1 Quart. Journ. Geol. Soc. vol. xlvi (1890) p. 380. 



2 Ibid. pp. 373-76. 



3 Trans. Eoy. Soc. Edin. vol. xxxv (1888-90) pp. 118-19. 



