﻿part 2] 



THE ISLAT ANTICLINE. 



151 



Geological Survey Memoir), which occurs towards the base of the 

 Dolomitic Group on the coast, is strikingly developed, and serves 

 as an excellent index. 



In most of the inland tract under consideration the existence of 

 the Islay Anticline is very obvious, because, just as in the coast- 

 section, the dips are practically all normal. About a mile south of 

 Giur-bheinn, however, the western limb of the anticline becomes 

 steeply overturned. 



The Dolomitic Group has a double outcrop in the western limb, 

 as a result of repetition by faulting. In the more easterly of these 

 two outcrops the quartzite intercalation ('Pipe-Rock' of the Survey 

 Memoir) is often traceable, but in the other outcrop I could not be 

 certain of its presence. In their northern portions both outcrops 

 afford quite typical exposures of the Dolomitic Group ; towards 

 the south, especially in the western outcrop, exposures are mainly 

 limited to massive beds of very pale-grey dolomite. 



It is a curious feature in the tectonics of this western limb that 

 the two outcrops of the Dolomitic Group remain equidistant when 

 followed from the district of normal dips into that of steeply- 

 reversed dips. It looks as though the fault, repeating the group, 

 "is of comparatively low inclination in the southern part of the 

 region. East of Loch Cam the outcrop of the fault bends 

 abruptly south-eastwards, and very clearly truncates the Dolomitic 

 Group (with the quartzite-band so often mentioned) and also the 

 whole of the underlying Lower Fine-Grained Quartzite, throwing 

 them against Portaskaig Conglomerate and Islay Limestone. The 

 inclination of the rocks thus brought together is very steep, and, 

 as the fault is running transversely to their strike, its existence 

 is easily demonstrated. From this point north-north-eastwards 

 for 4< miles the fault has been mapped parallel to the strike of 

 the beds, merely so as to account for the observed repetition of the 

 Dolomitic Group. A mile north-north-west of Giur-bheinn it 

 becomes self-evident once more, for quartzite on the west of it 

 is seen aiming directly at dolomitic rocks on the east. 



In the more westerly of the two main outcrops belonging to 

 the western limb of the Islay Anticline, all the groups between the 

 Islay Limestone and the Upper Fine-Grained Quartzite deteriorate 

 greatly south of Loch Cam. It is doubtful how far this is a 

 feature of original sedimentation, and how far due to mechanical 

 thinning connected -with the. Loch Skerrols Thrust. The Upper 

 Quartzite is itself mechanically thinned out, more or less com- 

 pletely, in this neighbourhood and on the south. 



It is unnecessary to point out in detail the evidence that the 

 quartzite outside the cordon of dolomitic outcrops, reaching from 

 the Bonahaven Fault to near Bridgend, must belong to a structu- 

 rally overlying group — sharing, of course, in the steep inversion of 

 the western limb of the Islay Anticline. The mapping may be 

 allowed to speak for itself, but it should be added that the super- 

 position of the outer quartzite is clear in the neighbourhood of 

 the Margadale River (west of Bonahaven) and has been recognized 



