﻿part 2] 



THE I SLAY ANTICLINE. 



139 



bedding is very faintly marked. They are much shattered, as well 

 as considerably sheared. Near Laggan their weathering tints are 

 paler than usual, and some of the group might be styled ' fine 

 felspathic quartzite.' At Blackrock pebbly beds occur, and are re- 

 presented by slides 6231-6236 in the Geological Survey collection. 



As mentioned above, there is often a difficulty in making out the 

 dip of the Bowmore Sandstone. Appearances certainly suggest a 

 very general inclination towards the south-east at angles varying, 

 according to Mr. Wilkinson, between 10° and 40°. 



Several competent judges, including Dr. Peach, have been 

 impressed by the resemblance of the Bowmore Sandstone to the 

 Middle- and Upper Torridonian. The recognition of the Loch 

 Gruinart Fault does not greatly weaken the correlation which has 

 been based upon this similarity. If, indeed, the Bowmore Sand- 

 stone belongs to the southern continuation of the Torridonian, 

 then it is extremely likely that the Loch Skerrols Thrust is the 

 same as the Moine Thrust of the North- West Highlands. 



It is well to bring this section to a close on a note of warning. 

 There is something very attractive in the view propounded above 

 that the Lewisian and Torridonian rocks of Colonsay and Western 

 Islay belong to the disturbed foreland up on to which the Moine 

 Nappe has ridden. And also in the further hypothesis that the 

 Moine and Loch Skerrols Thrusts are identical, in which case one 

 must suppose that the Moine Thrust has transgressed from its 

 position in the North- West .Highlands under Moine Schists — 

 extending into Mull — until in Islay it lies directly beneath 

 Dalradian Schists of the Central Highlands. There is, however, 

 no abrupt change of metamorphism on crossing the Loch Skerrols 

 Thrust, for the so-called 'schists' of Islay are included in an area of 

 extremely low metamorphism which embraces much of Argyllshire. 

 On this account, Dr. Peach is encouraged to recognize in the Torri- 

 donian of Colonsay and West Islay the southward continuation 

 of the Moine Schists in an unmetamorphosed condition, and to 

 refer them to the Moine Nappe rather than to the underlying 

 foreland. This interpretation is, of course, only a part of 

 Dr. Peach's well-known though but partly-published theory, in 

 which he maintains the Torridonian age of the Moine Schists as a 

 whole. 



(2) The Loch Skerrols Thrust. 



The Bowmore Sandstone passes eastwards beneath the Islay 

 Quartzite. The junction can be followed with approximate 

 accuracy as far south as Bowmore. Beyond this it is completely 

 covered beneath superficial deposits — in fact, it is quite doubtful 

 whether quartzite persists along the eastern margin of the 

 Bowmore Sandstone. 



The recognition of the Loch Skerrols Thrust by the officers of 

 the Geological Survey was due in the first place to the marked 

 deformation and mylonitization of the Islay Quartzite and Bowniore 

 Sandstone at their mutual contact. The mechanical evidences are 



