﻿140 



ME. E. BATTEBSBY BAILEY ON 



[vol. lxxii, 



well described by Mr. Wilkinson, who points to the ' drawing- 

 out' of the quartzite along lines varying between west 30° north 

 and north 10° west. The ' drawing-out,' where I have seen it, 

 is of the nature of striatum upon shear-planes ; at Loch Skerrols 

 it runs north 30° west. 



The existence of the Loch Skerrols Thrust can be demonstrated 

 on quite other grounds. It is the purpose of the present paper to 

 show that the Islay Quartzite is folded in an anticline overturned 

 north-westwards. In the heart of the anticline are the Portaskaig 

 Conglomerate, Islay Limestone, etc. ; on the south-eastern flanks 

 of the anticline lie the Port Ellen Phyllites. The absence of the 

 last-named group along the west side of the anticline, where 

 the quartzite directly overlies the Bowmore Sandstone, can only be 

 accounted for by invoking a thrust. The north-westward over- 

 turning of the anticline above the thrust-plane strongly suggests 

 that the movement along the Loch Skerrols Thrust is in the same 

 sense as that along the Moine and other well-known thrusts 

 farther north. The possible equivalence of the Loch Skerrols and 

 Moine Thrusts has already been touched upon in the previous 

 section. 



(3) Hocks above the Loch Skerrols Thrust, as far 

 East as Luing. 



The metamorphism of the rocks overlying the Loch Skerrols 

 Thrust is of so low a grade that it is important to have clear 

 evidence of the identity of these rocks and the Lalradian Schists 

 of the mainland. This evidence was obtained long ago by 

 Macculloch [1, vol. ii, p. 159], when he discovered the Portaskaig 

 and Garvellach Conglomerate full of ' granite ' (nordmarkite) and 

 ' limestone ' (dolomite) boulders, and correlated it with the 

 Schiehallion Conglomerate of Perthshire. The significance of 

 Macculloch's comparisons has of late years been heightened, as a 

 result of Dr. Flett's examination under the microscope of the 

 boulders included in these conglomerates [12, p. 75]. 



(3 a) Maol an Fhitliich Quartzite. 



This group consists of fine-grained quartzite. A pocket-lens 

 reveals the clastic structure, and further shows that some of the 

 minute quartz-pebbles are blue in colour. Near its junction 

 with the succeeding phyllites the quartzite is intensely sheared. 

 Mr. Wilkinson regarded the outcrop as a faulted outlier of the 

 main Islay Quartzite ; but this view may be set aside, as the 

 quartzite and the adjacent phyllites are interfoliated and appa- 

 rently also, to some extent, interhedded. 



(3 b) Mull of Oa Phyllites. 



These phyllites are prevalently of a dark-grey tint and rather 

 sandy texture. Colour-striping is common, and is especially well 



