﻿600 ME. E. B. BAILEY ON EECTJMBENT POLDS IN THE [Nov. I9IO,. 



III. Tectonics. 



(a) The Appin Core. 



The Appin Core has been proved to be a fold-core, and not a 

 mere stratigraphical intercalation, by the mapping of its component 

 groups, 1-6. The evidence is especially clear between Onich, 

 where the core is composed of all the groups from the Appin 

 Phyllites (2) to the Ballachulish Limestone (6), and Glen Nevis, 

 where the core consists of the Ballachulish Limestone (6) and 

 nothing else. Mapping also renders it evident that the fold-core is 

 of complex isoclinal structure, since the outcrops of the various 

 subdivisions zigzag in and out of one another, although the dips 

 observed are almost invariably directed towards the south-east at. 

 high or moderate angles. 



Another point firmly established is that the Appin Core is. 

 synclinal in form along the course of its outcrop in the present 

 district, since 



(1) its synclinal form is clearly exposed on the steep southern face of Glen 



Nevis, where the Ballachulish Limestone (6) emerges from beneath the 

 Ballachulish Slates (5) — see PI. XLIII, Section C ; and 



(2) the filling-up of the fold-core between Glen Nevis and Onich is apparently 



determined by a constant gentle south-westerly pitch, which is indicated 

 in the featuring of the hills everywhere between these two localities. 



The application of the terms 'anticline' and 'syn- 

 cline' in folded regions. — It is important to realize that the 

 Appin Core has been proved to be synclinal in form only, as has 

 been said above, along the course of its outcrop in the district here 

 described. At a distance from this outcrop it probably is sometimes 

 anticlinal and sometimes synclinal — for it is likely that the axial 

 plane of the fold is undulatory, so that sometimes the core will 

 gape downwards and sometimes upwards. This peculiarity of 

 recumbent folds in general is clearly illustrated in several of the 

 sections appearing in Pis. XLIII & XLIV : for instance, Section G. 



In describing a folded region, therefore, the terms 'anticline' 

 and 'syncline' must always be employed with a local 

 qualification, although this latter need not be explicitly stated,, 

 except in cases where otherwise it would not be clearly enough 

 implied in the context. As a larger and larger district comes 

 under observation the minor ups and downs of the recumbent folds 

 assume their true proportions, and appear almost as accidents in the 

 great scheme of mountain-structure ; but in the first instance the 

 local details are all important, especially when, as in the Highlands, 

 the work has to be done in the absence of fossils. 



The Port William Slide. — The regularity in the develop- 

 ment of the Appin Core between Onich and Port William fails at 

 once, as soon as we pass beyond the outcrop of the Ballachulish 

 Limestone (6). On the south-east this core is succeeded by a great 

 thickness of Leven Schists (7), while on the north-west it is flanked 



