100 ME. E. B. BAILEY ON THE STEUCTTJEE OP [vol. Ixxvili, 



sequence of the Iltaj Nappe as developed in the South- West 

 Highlands — that is, so long as one is restricted to the use of local 

 evidence. I am delighted to say that it is possible to arrive at the 

 result more simpl}^ along a quite independent line of enquiry. 



The same long stratigraphical sequence occurs in the South- 

 Central Highlands, under conditions which render it relatively 

 easy to read. Once read, its application to the South- West is a 

 matter involving fairly obvious correlations. All this was realized 

 many years ago by Mr. George Barrow (1913 h, p. 306 ; 1917, 

 p. 160), whose sequence for the South-Central Highlands can be 

 found on turnino- to Sir Archibald Geikie's first Presidential 

 Address to this Society (1891, Proc. p. 74). It is, however, only 

 fair to remind the reader that in the Central Highlands, as in the 

 West, grave differences of opinion exist as to the interpretation of 

 local successions. The question at issue is whether the Perthshire 

 (Islay) Quartzite is part of the general sequence, or whether it is 

 an unconformable unit. Mr. Barrow adopted the former alterna- 

 tive, and, after careful examination of the Pitlochry and Blair- 

 Atholl districts, I have no doubt that he is justified by the nature 

 of the evidence. As to the possibility of detailed correlation 

 between the Central and the Western Highlands, there has long 

 been sufficient excuse for optimism. Macculloch did well, when he 

 wrote in 1819 of his discovery of a schistose conglomerate in the 

 Garvellach Isles, ' which,' he said, ' will be seen hereafter to occur 

 in Isla, and which I have also observed in Schihallien ' (1819, 

 vol. ii, p. 159). This conglomerate is characterized by its abundant 

 nordmarkite-boulders, its frequent close resemblance to tillite, and 

 its well-defined stratigraphical associates. 



We may pass now to another important feature of the Iltay 

 Nappe. Clough directs special attention to the recurrence of 

 * green beds ' on widely different horizons in the Cowal succession 

 (1897, p. 89). It is impossible to account for all these scattered 

 outcro2:>s by assigning them to a single group, reduplicated by 

 folding — as a matter of fact, good examples occur on both sides of 

 the Loch-Tay Limestone. Accordingly, Clough was impressed by 

 the evidence which the re^Dctition of this peculiar type of sediment 

 affords of continuous deposition, from what he took to be the mar- 

 ginal portion of the ' Ardrishaig Phyllites.' well nigh to the Dunoon 

 boundary of the Ben-Ledi Grits and Schists. On cjuite other 

 grounds I have had to transfer some of Clough's ' Ardrishaig 

 Phyllites ' to the Ben-Lawers Group (p. 121) ; and, in default of 

 more precise local evidence, I have been guided in mapping the line 

 of the separating thrust by the improbability of ' green beds ' 

 continuing in Cowal across the boundary of the Iltay Nappe. 

 At the same time, I think that Clough rather exaggerates the 

 peculiarities of the ' green beds.' He compares them with the 

 epidotic grits, etc., of the Lower Torridonian of Skye ; he might 

 have extended the comparison, I believe, to certain Ordovician 

 sediments (for instance, the Tappins Group) figuring prominently 

 in the Southern Uplands. 



