part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLA^^D. 123 



(B) When the problem was approached by the Geological Survey, it was 

 with the preconception that the Ben-Lui Schists were older than the Ben 

 Lawers-Ardrishaig Complex, which latter was readily accepted as a strati- 

 graphical unit. Moreover, large- scale slides were not at that time recognized 

 as a j)henomenon of the Southern Highlands. Accordingly, both the suc- 

 cessions (1) & (2) were interpreted as of later date than the Ben Lawers- 

 Ardrishaig Complex, and, in a general sense, as equivalent the one to the 

 other. As regards details, the Islay and Crinan Quartzites were correlated, 

 and local erosion was invoked to account for the continuous absence of 

 Easdale Slates at the junction of the Crinan Quartzite and the Ardrishaig 

 Phyllites. An attempt was also made to minimize the importance of the 

 contrast of (1) & (2) by pointing to the Tayvallich Slates as widespread 

 relics of the Easdale Slates. It is true that the Tayvallich Slates do not 

 occur at the Crinan -Ardrishaig junction ; but in the initial stages of the 

 enquiry it was easy to postulate underground continuations of Ardrishaig 

 Phyllites forming unexposed cores to imaginary anticlines wherever black 

 slate showed itself in the quartzite area ; also, it must be admitted, Dr. B. N. 

 Peach mistook Loch-Avich Slates (1913 h, p. 290) for Ardrishaig Phyllites. 

 In course of time, it has become apparent both to Dr. Peach and to myself 

 that the Crinan Quartzite structurally intervenes between the Ardrishaig 

 Phyllites, below, and the Tayvallich Slates, above (1911, Chap, v; for 

 Mr. Hill's criticism of this change of front, see 1911, p. 61 ; 1913 f>, p. 306). 

 The progress of research has thus tended to emphasize the lithological 

 dissimilarities of the successions (1) & (2). 



But the reader may well ask whether the magnitude of the folding does 

 not in itself afford a sufficient explanation. As i^ointed out already, the 

 Easdale-Islay succession completely underlies the Ardrishaig Phyllites, while 

 the Crinan- Tayvallich succession completely overlies the same. Accordingly, 

 a correlation of the Easdale-Islay and Crinan- Tayvallich successions involves 

 a recumbent fold with a core of Ardrishaig Phyllites not less than 20 miles in 

 length, and, with so large a fold, a marked difference of facies might well be 

 expected in the lower and upper limbs respectively. It is, however, un- 

 necessary to develop this anticipated criticism, since it will be shown in the 

 succeeding paragraph that the particular large-scale fold here contemplated 

 is flatly contradicted by the local evidence. 



(C) Fig. 8 illustrates the type-area for the union of the Ben-Lawers and 

 Ardrishaig oixtcrops. It also shows clearly that the Ardrishaig Phyllites, 

 where they extend westwards between the Islay-Easdale and Loch-Awe 

 Assemblages, cannot be interpreted as a 20-mile fold-core. So massive and 

 extensive a fold would have a recognizable core of Ben-Lui Schists for 

 some part of its course, and such assuredly does not exist. The weight of 

 this negative evidence will be better appreciated on consideration of the 

 Ben-Lui Fold close at hand. Here, Ben-Lawers Schists are folded into the 

 heart of the Easdale Schists ; but they only extend a couple of miles west of 

 the termination of their associated Ben-Lui Schists. 



Enougli has been said to show that the original interpretation 

 of the district cannot support our present-day knowledge without 

 extensive reconstruction. I shall now pass to the consideration of 

 the new interpretation, which is the only one that I have been 

 able to devise to take the place of the old.^ 



^ [When in the summer of 1921 I revisited the ground with a small party 

 of geological friends, I found myself criticized for not having explained in the 

 text of my paper (here printed) how impressive a phenomenon the Ben-Lui 

 Fold really is : I could only answer that, as I had in this instance added 

 nothing material to the data collected by my predecessors, I thought brevity 

 justified. At the same time, it is perhaps well to state that the Ben-Lui 

 Fold is spectacular, according to South- West Highland standards — albeit 



