Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 553 



confirmed by the researches of Messrs. Dakyns & Teall among the 

 granitic rocks of Garabal Hill and Meall Breac, 1 and these rocks form 

 the continuation to the south-east of the Glen-Fyne granite. From 

 this interesting region the above-mentioned authors have described a 

 series of rocks, forming portions of a continuous mass, ranging from 

 peridotites to granite. They find that the first rocks formed were the 

 peridotites, and then followed diorites, tonalites, and granites in the 

 order of increasing acidity. The more basic rocks occupy a position 

 which is marginal with respect to the more acid. This would also 

 appear to be the case among the intrusions of the Ben-Bhuidhe area. 

 It may be said, generally speaking, for this area, that the more 

 distant any intrusion is from the granite, the more basic will be its 

 character, so that we find the kentallenites tending to occupy the 

 outer zone of eruptive activity. While, on the whole, more basic 

 material is invariably marginal in its position with regard to more 

 acid, it would not be possible, owing to local exceptions and variations 

 in the nature of the intrusions, to map out any of our Argyllshire 

 igneous areas into a series of concentric belts or zones showing an 

 uniformly progressive increase in basicity from centre to margin. 

 Kentallenite, for instance, may occur, as in the Ballachulish area, 

 nearer to the granite than diorite ; while, as the Ben-Bhuidhe area 

 again shows, granite may occur in the same zone with kentallenite. 

 Broadly speaking, however, it is true for each eruptive area 

 that the more basic intrusions have invaded the mar- 

 ginal portion, while the more acid material occupies 

 the centre. The phenomenon of the distribution of separate 

 intrusions of a common origin is exactly parallel to that of the 

 distribution of the material of a single intrusion varying in character 

 from centre to margin. The phenomena, too, of the dykes and 

 sills present a general parallelism to that of the more deep-seated 

 masses ; and we cannot but regard both as belonging to the same 

 series of events. 



From these considerations, and on the petrological grounds already 

 advanced, we do not hesitate to adopt the view that the various 

 related intrusions of the Ben-Bhuidhe and other igneous areas under 

 consideration, owe their origin to differentiation in a common parent- 

 magma. It would hardly, however, be within the scope of the 

 present communication to enter at any length into the speculative 

 physics bearing on the particular type of magmatic differentiation 

 of which they represent the final result. At the same time, it will 

 not be out of place to indicate briefly what we consider may have 

 been the main lines that such a process may possibly have followed. 



The phenomena which are illustrated in the Argyllshire eruptive 

 areas would not appear to differ to any great extent from those 

 already well known from other regions, and so admirably described 

 by many American and Continental penologists, such as Rosen- 

 busch, Brogger, ladings, G. H. Williams, Weed & Pirsson, and 



1 Quart. Journ. Geol. Soc. vol. xlviii (18S2) p. 104. 



