672 MESSES. CLOTJGH, HATJEE, AND BAILEY ON [Nov. IQ09, 



notable portion reveals the operation of considerable mechanical 

 stresses analogous to those which affected the granite of Criffel. 1 



'No line can be drawn between the two phases, the normal structure of a 

 fine-grained porphyritic rock, resembling porphyrite or andesite, appearing 

 gradually to pass into a structure indistinguishable from that of a thoroughly 

 foliated rock. . . . The planes of schistosity in the foliated portion are vertical 

 or almost so, and lie parallel to the general line of elongation of the mass.' 2 



"We have re-examined Mr. Kynaston's slides, and the evidence 

 of intense shearing accompanied by complete recrystallization 

 is extremely interesting. It is certainly possible that the 

 mass represents a complex ' early ' fault-intrusion, sheared and 

 contact-altered during the later subsidence which accompanied 

 the introduction of the main mass of the Cruachan Granite. 



But, if there is good evidence for regarding the Cruachan Granite 

 as the infilling of a subterranean cauldron, the same arguments 

 apply in the case of the Starav Granite which occupies the central 

 portion of the boss. Mr. Kynaston has shown that this granite is 

 later than that portion of the Cruachan intrusion which has been 

 preserved as a rim surrounding it on almost every side. As a rule, 

 the boundary between the two is sharp, and the Starav Granite 

 sends small veins into its companion, treating it in this respect as a 

 solid rock. It is probable, in fact, that a considerable interval of 

 time separated the intrusion of the two masses : for, as previously 

 shown, the injection of many, if not quite all, of the porphyrite 

 dykes was accomplished in the interval. 



Now, Mr. Kynaston's mapping shows that the boundary of the 

 Starav Granite is a steep or vertical plane. 3 The core of Cruachan 

 Granite which it replaces must, therefore, either have gone up, or 

 gone down. That there has been some movement of the kind is 

 rendered extremely probable, by the fact that 



'in many parts of Glen Kinglass [that is, along the south-eastern edge of the 

 Starav boss] . . . the Cruachan granite assumes a well-marked foliated or gneissic 

 structure close to the margin of the coarser, more acid type.' 4 



That the movement was downward, we may readily believe from 

 the analogy of the Glen Coe subsidence lying so short a distance 

 away to the north. 



Finally, support is given to the conception of the Etive boss as a 

 subterranean cauldron when attention is paid to the relations of 

 the narrow band of aplitic granite stretching between Stob Dubh 

 and Meall Odhar, which, as already said, is believed to be inter- 

 mediate in age between the Cruachan and the Starav Granites. 



1 See J. J. H. Teall, 'The Silurian Rocks of Great Britain : vol. i— Scotland ' 

 Mem. Geol. Surv. 1899, pp. 609, 610; see also J. B. Hill, for analogous 

 examples in Cornwall, ' Geology of Falmouth & Truro, &c.' Mem. Geol. Surv. 

 1906, p. 59. 



2 H. Kynaston, ' Geology of the Country near Oban & Dalmally ' Mem. 

 Geol. Surv. 1908, p. 96. 



3 We have since made certain of this point in the field. 



4 H. Kynaston, op. cit. p. 86; see also footnote, p. 641 of this paper. 



