part 2] THE SOUTH-WEST HIGHLAISTDS OF SCOTLAIS'D. 105 



Appin, cream-coloured, pink or dark- striped, dolomitic Limestone; 

 sometimes in two well-marked subdivisions separated by flaggy qviartzite 

 and phyllite. 



Appin grey Phyllites (or Mica-Schists), often with a large propor- 

 tion of flaggy quartzite. 



C u i 1 - B a y black graphitic Slates. 



Original order of deposition. — The local evidence of 

 relative age is much less conclusive in the Ballappel Foundation 

 than in the preceding cases. I venture, however, to suggest that 

 the order of statement in the previous section, with Eilde Flags 

 at the top and Cuil-Bay Slates at the bottom, corresponds with 

 the original order of deposition. My reasons are as follows : — 



(1) Stratigraphical correlation between the great structural subdivisions of 

 the Highlands is at the present time tentative. Still, there is much to be 

 said for the commonly accepted view that the Ballachulish Slates, Easdale 

 Slates, and, in a broad sense, the Tayvallich Slates are on one and the same 

 horizon (p. 106). If so, then, since the Easdale and Tayvallich Slates can 

 both be shown to be younger than their associated pebbly quartzites, it 

 follows that the Ballachulish Slates are younger than the Appin Quartzite. 



(2) Such a belief is in accord with the interpretation of the best-known 

 slides of the district as thrusts (p. 90). 



(3) Such a belief leads also to the conclusion that the great recumbent 

 folds of the Ballappel Foundation originated during south-eastward movement 

 in conformity with the phenomenon that took place at higher levels, as 

 evinced by the Ben-Lui and Carrick-Castle Folds. 



Structure. — The stratigraphical characteristics of the Bal- 

 lappel Foundation are wonderfully helpfuL The groups are, for 

 the greater part, both distinctive and constant. This renders 

 mapping easy, and the reading of tectonic features correspondingly 

 accurate and full. The more important results are as follows : — 



(1) Great recumbent folds are characteristic of the district.^ Two main 

 examples, both of them closing south-eastwards, have been named the 

 Ballachulish and Appin Folds respectively. 



(2) The development of these great folds has been accompanied by very 

 extensive sliding "■^ : in any particular fold, slides are not necessarily restricted 

 to one limb — they may occur in both ; some of them are thrusts, some lags. 

 The Ballachulish Slide, which occurs in the lower limb of the Ballachulish 

 Fold, is interpreted by Mr. Macgregor and myself as having a displacement of 

 more than 24 miles (1912 b, p. 174; the irreducible minimum is 9 miles, 1916, 

 p. 83). 



(3) The recumbent folds and associated slides have been subjected to 

 extensive secondary folding, often thousands of feet deep and isoclinal in 

 character. A pronounced example is the Glen-Crer'an Syncline (fig. 1, p. 84), 

 which may be regarded as a partial northward continuation of the Loch- Awe 

 Syncline ; the Glen-Creran Syncline is separated from the Ben-Lawers 

 Syncline by a comparatively gentle anticline named after Glen Orchy (1912 b). 



Althougli slides of two kinds occur, it is i-easonable to suppose 

 that the dominant slides of the district are of the nature of thrusts, 

 not lags. In my judgment the two most important slides are 

 the Ballachulish and Fort-William Slides, which characterize the 



^ See also 1922, Report A, par. 5 ; Report B. 

 ^ Ibid. Report A, par. 5. 



