﻿part 2] 



THE I SLAY ANTICLINE. 



149 



it is immensely thick and very white, whiter probably than the 

 Lower Quartzite. 



The authors of the Geological Survey Memoir on Islay regard 

 all the quartzite of the island as belonging to the subdivision 

 under the Dolomitic Group, and, therefore, refer to the Bonahaven 

 Fault as having ' a downthrow to the east.' A glance at the map, 

 however, shows that the fault, all along its inland course, has an 

 important downthrow to the north-west. Thus it appears that the 

 quartzite introduced into the coast-section by the Bonahaven Fault 

 structurally overlies the Dolomitic Group. 



The superposition of the Upper Fine-Grained Quartzite is still 

 more obvious on the northern coast, west of Kudh' a' Mhail, where 

 the Dolomitic Group makes its appearance as an isolated inlier 

 with gentle anticlinal dips. 



The quartzite west of the inlier continues with westerly dips, 

 averaging about 35°, until truncated by a fault, inclining steeply 

 to the east, and excellently exposed for over 100 feet in the 

 cliffs of the raised beaches. Just east of the fault on the fore- 

 shore the quartzite includes a conglomeratic bed with lumps of 

 quartz in a quartzitic matrix. It has been described as carrying 

 granite-pebbles also, but I do not feel confident that such is the 

 case. The authors of the Geological Survey Memoir confused this 

 conglomerate with the conglomerate already described as occurring 

 at the top of the Lower Quartzite ; and, not noticing the fault, 

 imagined that a normal passage existed from it up into the 

 Dolomitic Group, for the latter appears immediately on the west. 



The Dolomitic Group, which occupies the shore west of the 

 fault, undulates at low angles until, passing over an anticline, it 

 dips somewhat steeply westwards at an average angle of about 40°, 

 and once more passes beneath the Upper Fine-Grained Quartzite. 

 The rocks near the junction on the foreshore are broken by a 

 shatter-belt, and this has led to the insertion of a fault on the 

 Geological Survey map. But the apparent superposition of the 

 quartzite over the Dolomitic Group cannot be explained away by a 

 fault, since the actual contact of the two groups may be seen intact 

 where a path goes behind an old sea-stack of the raised beach. 



This is the last appearance of the Dolomitic Group on the coast. 

 The whole of the northern cliffs to the westward are fashioned out 

 of quartzite dipping north-north-westwards at angles of about 

 45°. At Gortantoid Point, east of Loch Gruinart, the Upper Fine- 

 Grained Quartzite passes under Pebbly Quartzite, distinguished by 

 numerous layers of quartz-pebbles, of rather small size and often 

 blue in colour, and by scattered, big, rounded pebbles, also quartz. 

 The pebbly layers and pebbles are merely features in a pre- 

 valently fine-grained quartzite exactly like that of the underlying 

 group. 



Three important results follow from the coastal traverse detailed 

 above : — 



(1) The groups — Portaskaig Conglomerate, Lower Fine-Grained Quartzite, 

 Dolomitic Group, Upper Fine-Grained Quartzite, Pebbly Quartzite — are in 



