﻿part 2] 



THE ISLAY ANTICLINE. 



137 



I have traced an upward succession culminating at last in two 

 isolated synclines — the one at Kiloran Bay, the other at Scalasaig. 

 The sediments of Oronsay and Colonsay have heen estimated at 

 ahout 5000 feet in thickness. 



Beyond Kiloran a descending sequence is introduced with very 

 high dips, and before long one meets with the outcrop of gneiss 

 already mentioned (fig. 2, p. 136). It is natural to interpret this 

 gneiss as emerging from beneath the neighbouring Torridonian 

 sediments, and continuous underground with the gneiss of Islay. 



When we take into consideration the great thickness of the 

 Lower Torridonian sediments of Colonsay and Islay, we find our- 

 selves bound to postulate an important dislocation along the hollow 

 of Loch Gruinart ; for, immediately east of this hollow, we find the 

 Bowmore Sandstone and Islay Quartzite, divisions unrepresented 

 in even the deepest of the Colonsay synclines. For this dislocation 

 I propose the name of the Loch Grruinart Fault. In the 

 Islay Memoir the existence of such a fault is not recognized, and 

 a description is given of what is supposed to be an unbroken contact 

 between the Bowmore Sandstone and the Lower Torridonian on 

 the shore at Gortan. When I visited this section I was unfamiliar 

 with the rocks, and my opinion is consequently of little value ; but 

 I am inclined to doubt the Bowmore Sandstone of this restricted 

 exposure. If, however, the correlation be correct, then it would 

 appear that the Loch Grruinart Fault, or a branch of the same, 

 runs slightly to the west of the Gortan foreshore. Otherwise the 

 north-easterly pitch, characteristic of the Rhinns, would carry 

 the Bowmore Sandstone outcrop across the Rhinns long before 

 Ardnave Point was reached. 



Without going into details, it seems quite probable that the 

 Loch Gruinart Fault is the south-westerly continuation of the 

 Great Glen Fault. All that is certain is that the Great Glen 

 Fault must pass very close to Colonsay on the one side or the 

 other ; and so it is reasonable to connect it Avith the Loch Gruinart 

 Fault, for both dislocations agree in having a powerful downthrow 

 to the south-east. An additional reason for drawing the Great Glen 

 Fault south-east of Colonsay rather than north-west is illustrated 

 in fig. 3 (p. 138). In the North- West Highlands, wherever 

 undoubted Torridonian rocks are found, they underlie the Moine 

 Thrust. It is probable, therefore, although by no means certain, 

 that the Torridonian rocks of Colonsay and the Rhinns of Islay 

 occupy a similar position. The outcrop of the Moine Thrust was 

 identified by Clough with fair certainty as near Colonsay as the 

 Sound of Iona [C. T. Clough in 14, p. 77] ; and, if it is to clear 

 Colonsay after leaving the Sound, it must bend sharply south- 

 eastwards, as shown in fig. 3. According to this interpretation, 

 the Great Glen Fault can scarcely run between the Sound of 

 Iona and Colonsay, for the effect of this fault, with its great 

 downthrow to the south-east, would be to displace the outcrop 

 of the Moine Thrust south-westwards. On the other hand, 

 it does seem likely that the fault passes south-east of Colonsay, 



