part 2] THE SOUTH-WEST HIGHLAE^DS OE SCOTLAIS^D. 121 



Ben-Lui Fold and the Thrust at the Base of the 

 Loch-Awe Nappe.^ 



Before discussing the Ben-Lui Fold, I wish to express my in- 

 debtedness to Mr. J. B. Hill and to the late H. Kynaston for their 

 mapping of that part of the district which, in fig. 8 (p. 122) lies 

 south of the outcrop of the quartzite of the lltay Nappe. The more 

 northerly portion of fig, 8 is taken, practically as it stands, from 

 pi. X of the account given by Mr. Macgregor and myself (1912 h) ; 

 but the southern two-thirds of the map are based, with trifling 

 changes, on Hill & Kynaston's work published in Sheets 45 & 16 of 

 the Geological Survey 1-inch map. At the same time, it should be 

 clearly understood that I alone am responsible for two important 

 matters of interpretation expressed in the southern part of fig. 8 : 

 one, the distinction of the Ben-LaAvers Schist from the Ardrishaig 

 Phyllites ; the other, the recognition of the thrust-plane at the 

 base of the Loch- Awe Nappe. 



In 1891, Mr. Hill traced the Ben-La wers and Ardrishaig Groups 

 into contact with one another in the neighbourhood of Ben Lui, 

 and thus, it was thought, established their stratigraphical identity 

 (1892 5, p. 385). The correlation is, of course, supported by a 

 close general lithological resemblance of the two groups ; but there 

 has always been a difficulty : the Ben-Lawers-Ardrishaig Complex, 

 considered as a unit, has three persistently different margins. The 

 successions outwards from this complex are as follows : — 



(1) Easdale Black Slates, leading on to Islay Qnartzite ; the local evidence 

 points to the slates as later than the quartzite (p. 101). 



(2) Crinan (Loch-Awe) Quartzite, approached through Shira Limestone, 

 and followed by Tayvallich Black Slates, Limestones, and Lavas ; the local 

 evidence points to the slates as later than the quartzite (p. 97). 



(3) Ben-Lui Garnetiferous Mica-Schist, succeeded by Loch-Tay Limestone ; 

 the local evidence points to the limestone as later than the schist (p. 101). 



Exposures are far from continuous, but it seems certain that 

 each of the three successions holds true for more than 50 miles 

 along curving lines of outcrop. Manifestly normal faulting cannot 

 be the explanation. In fact, only two alternatives are feasible : 

 the first is to interpret the successions (1) & (2) as in some sense 

 equivalent ; the second to admit that extensive fold-faulting (or 

 sliding, to use the shorter term) has been the determining factor 

 of the triple margin. Three independent reasons (A-C) for 

 adopting the second of these two alternatives are outlined below: — 



(A) Succession (1) cannot be directly equivalent to Succession (2), since in 

 (1) the rock-groups met with on leaving the margin of the Ardrishaig 

 Phyllites are increasingly old, while in (2) they are increasingly young. This 

 age-relation — coupled with the fact that the lithology of succession (3) is 

 admittedly too dissimilar to be correlated with either (1 ) or (2) — obviously 

 dema.nds a slide. At the same time, age-relations are notoriously difficult to 

 establish in unfossiliferous rocks, and therefore it is well that other evidence 

 is available. 



^ See also 1922, Report A, pars. 5 & 7 ; Report B. 



