﻿part 2] 



THE ISLAY ANTICLINE. 



135 



by black slates. In 1913 I visited Degnish for the first time, and 

 came to a like conclusion. 



In 1910 I was sent hj Dr. Home to Jura to collect material for 

 the Geological Survey Memoir then in preparation. In a fortnight 

 I examined the whole of the coast-sections and various inland 

 exposures. 



II. Detailed Descriptions. 



(1) Geology of Colonsay and Islay,, West of 

 the Loch Skerrols Thrust. 



A veiy brief description of the geology of the western part of 

 the Islay Archipelago will suffice. My observations in Colonsay 

 and Oronsay have already been published, in conjunction with 

 Mr. Wright's, in the Survey memoir on those islands, while my 

 knowledge of the western portion of Islay itself is not sufficient to 

 warrant a detailed account at the present juncture. 



Two outcrops of crystalline rocks — a large one in the south of 

 the Bhinns and a small one in the north of Colonsay — have been 

 referred to the Lewisian Complex by Thomson [3, p. 221] 1 and 

 Wright respectively. In contact with the Lewisian lies a mass 

 of varied sediments placed by such good judges as Dr. Peach 

 and (subsecmently) the late Dr. Clough in the Lower Torridonian 

 System. In both localities the junctions between the Lewisian and 

 Torridonian rocks are much sheared, but this is not a sufficient 

 reason to suspect that the original relationship has been materially 

 altered by thrusting. In fact, Dr. Peach and Mr. Wilkinson have 

 detected what appears to be a true basement-conglomerate at 

 various places near the edge of the Torridonian in the Ehinns. 

 The best exposure of this conglomerate is at Dun Mideir. 



From clearly-defined gentle north-easterly pitches which I have 

 observed in traversing the Lower Torridonian sediments lying north 

 of the gneiss-outcrop in the Bhinns of Islay, it seems certain that 

 the gneiss sinks in this direction beneath a covering of Torridonian 

 disposed in a series of sharp but shallow folds (fig. 1, p. 136). I 

 have found north-easterly pitches well marked in the southern part 

 of the sedimentary area, and again between Sanaigmore and Ardnave 

 Point (where shown in PI. XII). Probably a fairly uniform pitch 

 prevails throughout the whole of the peninsula. It is unfortunate 

 that the lines separating psammitic and pelitic sediments in Sheets 

 19 & 27 of the Geological Survey map are not drawn with suffi- 

 cient accuracy to be of assistance in following out the structure ; 

 but I am strongly of opinion that the thickness of the sediments 

 in the Ehinns is very considerable, and that the highest strati- 

 graphical horizon is met with at Ardnave Point. 



Similar gentle north-easterly pitches continue through Oronsay 

 and the southern part of Colonsay, introducing higher groups, 

 probably, than those encountered in the Ehinns. Mr. Wright and 



1 Numbers in brackets refer to the Bibliography, § IV, p. 159. 



