90 



ME. E. B. BAILEY ON THE STETJCTUEE OF [vol. Ixxviii, 



one limb of an isoclinal fold.^ Clouo-h was fully conscious of 

 this difficulty, so far as small-scale folding was concerned, but 

 he was not deterred thereby from arriving at a positive conclusion 

 in the particular case just considered. 



In the Ballachulish district I found the phenomenon of eddy- 

 ing forward motion illustrated on a big scale : that is, I found 

 important slides of two kinds, both thrusts and lags (1910«). Of 

 the two I should expect the thrusts, as a class, to be predominant, 

 and on this account (among others) I have for many years inclined 

 to the view that the Ballachulish and Fort- William Slides are 

 thrusts rather than lags (1910 &). If these two slides are thrusts, 

 then here again the movement has been towards the south-east, 

 as shown by the direction of close of the associated folds. 



Fig. 2. 



-ClougVs criterion of south-eastward movement during 

 ^ ^re-anticlinaV times in Coival. 



It Avill be noticed that little or no attention has been directed in 

 this discussion to the inclination of thrust-planes as a guide to the 

 direction of movement. With the ' double fold ' of "the Glarus, 

 and numberless other instances, in our minds, we are well advised 

 to preserve a very cautious attitude in this matter. Where 

 schuppenstruktur is clearly developed, as in the Xorth-West 

 Highlands so faithfully portrayed by Dr. B. N. Peach & Dr. J. 

 Home, the upward forward inclination of the individual scales, 

 relative to the major thrusts above and below, affoixis a much 

 more trustworthy indication . Schuppenstruktur is, moreover, 

 a packing phenomenon appropriate to a compressed uninverted 

 sequence. It differs in this respect from the attenuation -pheno- 

 menon illustrated in fig. 2. One might hope, therefore, in a 

 region of recumbent folds to find some analogue of schuppen- 

 struktur jDredominant in one set of limbs and indications of 



^ Another difficulty is that attenuation is in part a solution effect, for 

 Clough found it commonly more marked in regard to the quartz than to the 

 mica of a deformed band (1897, p. 22). 



