﻿Vol. 66.~] SCHISTS OP THE SCOTTISH HIGHLANDS. 593 



well have proved impossible, had it not been for the wealth of 

 connecting exposures. The limestones suffer most from this type 

 of metamorphism, and are altered to calc-silicate hornfelses for 

 distances ranging from a quarter to 2 miles, measured from the 

 various granite margins. 



The rocks included in Groups 1-6 occupy the gape of a great 

 fold which has been traced from the River Spean to Appin. This 

 fold may be conveniently termed the Appin Fold, and the rocks 

 specified may be described as constituting the Appin Core. It 

 will be realized at once that the definition of a fold-core by refer- 

 ence to any particular group of rocks is essentially arbitrary, but 

 the convention adopted here will be found very serviceable in the 

 description of the structure of this complicated region. 



The lower limb of the Appin Fold is in large measure replaced 

 by a very important fold- fault, or slide, as it may be called for 

 brevity, which constitutes the north-western boundary of the Appin 

 Core from Spean Bridge to Onich, where it goes out to sea. The 

 evidence for this slide is most clearly displayed in the neighbourhood 

 of Fort William, and the structure will accordingly be named the 

 Fort William Slide. 



A word of explanation is necessary, perhaps, to justify the 

 introduction of this new term ' slide.' If there were only one type 

 of fold-fault present in the district, one might be bold and call 

 every fold-fault a thrust. But, as a matter of observation, it is 

 sometimes found that complementary fold-faults have developed in 

 the two limbs of one and the same recumbent fold (cf. the Fort 

 William Slide and Meall a' Bhuirich Slide, PI. XLIII, and several 

 other examples, PI. XLIV). The fold-faults, therefore, are some of 

 them thrusts, some lags, 1 and here at present our knowledge ends. 

 Hence a non-committal general term is unavoidable. ' Fold-fault ' 

 itself is too cumbrous for constant repetition, and accordingly 

 * slide ' has been introduced to take its place. 



Farther south-east, rocks included in Groups 2-6 are again en- 

 countered, where they form what may be termed the Ballachulish 

 Core. The Ballachulish Fold, to which the core belongs, is in 

 every sense analogous to the Appin Fold. Like the latter it gapes 

 towards the north-west and closes towards the south-east, while 

 its lower limb is in large measure replaced by a very powerful 

 slide, known in this case as the Ballachulish Slide. The 

 Ballachulish Core is structurally on a higher level than the Appin 

 Core (see fig. 1, p. 594). It has a much more varied and interesting 

 outcrop, and can be shown to behave as a highly persistent layer 

 extending south-eastwards from its gape at Ballachulish as far at 

 least as Allt Coire an Easain, a distance of 14 miles. 



1 Thrust is here employed in the sense of a fold-fault replacing the lower 

 limb of an overturned anticline. Lag, based on the term lag -fault in- 

 troduced by J. E. Marr [31] and A. Harker, is employed in the sense of a 

 fold-fault replacing the upper limb of an overturned anticline. Lags have 

 been shown in some sections illustrating the structure of Alpine regions, 

 cf. L. W. Collet [32]. 



