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ME. E. BATTEBSBY BAILEY ON 



[vol. lxxii, 



reveal any bed of the kind at the neighbouring margin of the 

 main conglomerate, I am disposed to regard them as interstratifted 

 members of the limestone group. 



Mr. Wilkinson has traced the outcrop of the Islay Limestone, 

 in a curve like a horseshoe, from near Bowmore to the Midi of 

 Oa 1 ; and has thus demonstrated quite clearly that a fold-axis 

 runs up Central Islay. It is true that the limestone-outcrop, as 

 drawn upon the map, is discontinuous ; but many of the inter- 

 ruptions are due to the masking by Drift of portions of the 

 district. 



(3 d) Portaskaig Conglomerate. 



I cannot do better than quote the words with which Dr. Peach 

 concludes his description of the conglomerate as exposed in the 

 Garvellachs : — 



' The study of the rocks on this group of islands tends to confirm Mac- 

 culloch's sagacious and far-seeing correlation of this boulder-bed with that of 

 Islay and Schiehallion.' 



A very interesting feature of much of the Portaskaig Con- 

 glomerate in Islay is its extremely glacial aspect. Large portions 

 of it are imbedded boulder-olay charged with far-travelled boulders. 

 Thomson has urged that it must be of glacial origin, and states 

 that he found a typical striated boulder of quartzite embedded in 

 it [3, p. 211]. I was not equally fortunate, however, although 

 I searched the conglomerate carefully with the same end in view. 

 It is not clear why striated boulders should be difficult to find if 

 the boulder-clay is of glacial origin ; for, although the matrix is 

 cleaved, the boulders themselves are often quite unaltered by later 

 movement. There is another difficulty to be faced by the glacial 

 theory : much of the conglomerate is a stratified deposit, and is 

 interbedded with layers of quartzite and dolomite, from which 

 latter it has derived fragments as a result of obviously non -glacial 

 ' contemporaneous erosion.' In the light of these sections one 

 cannot help wondering whether it is necessary to invoke glaciation 

 in order to account for any part of the conglomerate. 



Islay. — Splendid exposures of the conglomerate occur both 

 north and south of Portaskaig. The base of the deposit is not 

 seen at the coast in this neighbourhood, but inland sections about 

 Loch Lossit make good the deficiency. Here the grey dolomitic 

 topmost bed of the Islay Limestone, previously mentioned, is 

 overlain by a few feet of dark shale or slate with quartzose ribs, 

 and these by the conglomerate itself. The strata are lying at very 



1 The coast exposure near the Mull of Oa is disappointing, for the lime- 

 stone is bleached and reddened in the vicinity of a curious breccia, which, as 

 Dr. Peach suggests, may be a miniature Triassic outlier. Inland exposures, 

 however, even that of the raised-beach cliff, show the grey limestones in 

 characteristic form, and Mr. Wilkinson has recorded oolitic beds as far south 

 as the farm Coillabus. 



