88 ME. E. B. EAILET OX THE STETJCTFEE OF [vol. Ixxviii, 



The strike of folds is very conspicuous in the case of small-scale 

 steep folds, but is very elusive in the case of big-scale recumbent 

 folds. With the latter, all one can do is to determine the more or 

 less sinuous ^ line, along which each particular fold closes upon 

 some well-marked constituent member serving locally as its core. 

 While recognizing the difficulty of the subject, I suggest that the 

 greater folds of the Highlands seem to run roughly north-east and 

 south-west in agreement with the lesser folds, b}^ which they are 

 so conspicuously accompanied and sometimes affected. Three 

 examples j)ointing to conformity of strike will be noticed he]-e : — 



(1) As already set forth, Clongh accounts for the contrast on the two sides 



of the Cowal Anticline by suggesting that this anticline arches a 

 sequence profoundly influenced by big- scale folding of earlier date 

 (figs. 1 & 4 & PL I). As the difference referred to persists across 

 the Highlands, it would seem that Clough's early and late folds 

 must have had very similar lines of strike. This is the more likely, 

 since he recognizes countless small-scale ' pre-anticlinal ' folds 

 conforming in strike to the line of the anticKne, and associated 

 with contemporaneously-developed stretching and rodding approxi- 

 mately at right angles to the same line (1897, pp. 16, 17). 



(2) In like manner, I attribute the marked differences met with on the 



two sides of the Loch- Awe Syncline at levels below the outcrop of 

 the Loch- Awe Nappe to early flat folding and thrusting,- which 

 here again must have agreed faii'ly closely in strike with the rela- 

 tively late folding that developed the syncline (figs. 1, 4, & PL I). 



(3) There is much in the gape of the Ballachulish Fold, as exposed along 



the north-western hmb of the relatively late-formed Glen-Creran 

 Syncline between Lairigmor and Loch Creran (1910 a, pi. xhi), 

 which does not reappear to the south-east either in Glen Coe or in 

 the Windows of Etive. 



How direction along line of movement is recognized. — 

 Once the line of movement is known, the next question is the 

 actual direction along this line. Trustworthy vertical structures 

 are rare in undisturbed rocks, else their deformation would serve 

 as an invaluable index. Let us consider for a moment the section 

 below the Moine Thrust, as exhibited at the base of the Stack of 

 Grlen Coul in Sutherland. An outcrop of Cambrian quartzite 

 occurs there, resting unconformably upon Lewisian Grneiss and dip- 

 ping beneath the Moine Xappe with the same inclination as that 

 which characterizes the intervening thrust-plane. In the quartzite 

 are numerous annelid-tubes — the well-known ' pipes,' as they are 

 called. In the undisturbed foreland these pipes are always at 

 right angles to the bedding of the quartzite. In this particular 

 exposure they have been sheared into a very oblique position, and 



^ Without entering into detail, I may point out that I suspect that the 

 west-north-westward close of the Beinn-Udlaidh Fold (1912 b,pp. 168, 169) is 

 a misleading local feature of • a fold which, viewed in its entirety, closes 

 towards the south-east. 



2 See also 1922, Report A, par. 6. 



