280 Geological Society : — 



Among the ideas set forth in 1874, of which Dr. Geikie now 

 announces his acceptance, and to which, indeed, he supplies valuable 

 support and confirmation, from his own observations and those of 

 various members of the Geological Survey, are the following : — 



(1) The perfect transition between the plutonic rocks of the 

 district (granites and gabbros) and the lavas ("felstones" and 

 basalts), and the dependence of each variety of texture exhibited by 

 them — from the holocrystalline to the vitreous — on the conditions 

 under which solidification took place. 



(2) The presence of great masses composedof volcanic agglomerates, 

 breccias and tufis, with numerous intrusive bosses, sheets, and 

 dykes, at five well-marked eruptive centres, namely Mull, Ardna- 

 murchan, Rum, Skye, and St. Kilda, and the subaerial character of 

 the ejections at these five centres. 



(3) The Tertiary age, not only of the lavas, but also of the gabbros 

 and granites found associated with them at these difi"erent centres. 



The conclusions to which exception is taken are as follows : — 



(1) That the ejection of the "felstone" lavas and the intrusion 

 of the granites preceded the appearance of the basalts and gabbros. 



(2) That the five centres of eruption mark the sites of as many 

 great volcanic cones, now ruined and dissected by denudation. 



The view that the acid rocks were, as a whole, older than the 

 basic ones, was originally put forward by Prof. J. D. Forbes and 

 Dr. F. Zirkel, and is supported in the memoir of 1874. Dr. Geikie 

 admits that around several of the centres indicated basalts may 

 frequently be seen resting on more acid rocks ; but the latter he 

 regards as being, in every case, of an intrusive character ; he also 

 allows that the tuff's intercalated with the basalts often contain 

 fragments of felsite, but he does not accept this as a proof that the 

 felsites must have been erupted before the basalts. Much of the 

 divergence of opinion that has arisen appears, however, to be due to 

 the circumstance that Dr. Geikie classes as basalt many of the dark- 

 coloured lavas (augite-andesites &c.) which were, in the original 

 paper, grouped under the name of " felstones." In these " felstones " 

 the granites and gabbros alike were shown to be intrusive ; and it 

 was also admitted that there were many intrusions of acid rocks of 

 later date than both the " felstones " and the basaltic lavas. 



With respect to the existence of great volcanoes in the district, 

 Dr. Geikie, while confirming most of the statements which were 

 made in 1874 as to the several centres of eruption, prefers to refer 

 the origin of the great plateaux of basaltic lava to " fissure-erup- 

 tions." He maintains that the numerous basic dj'kes of the district 

 mark the actual cracks through which the lavas in question rose 

 up and welled out at the surface. 



In opposition to this view, it was pointed out that the numbers and 

 dimensions of the Tertiary dykes are not such as would warrant us 

 in inferring that they formed the conduits through which the enor- 

 mous masses of lava forming the plateaux were erupted ; and the 

 absence of all proofs of contact-metamorphism at their sides, and of 

 evidence that the majority of them ever reached the surface at all, 

 was commented upon. In 1874 it was pointed out that some of these 



