﻿part 2] 



THE I SLAY ANTICLINE. 



133 



by the Survey authors, but Mr. George Barrow, in conversation, has suggested 

 that an important thrust separates the Khinns from the rest of Islay. 



(3) Collateral evidence is afforded of the existence of the Loch Skerrols 

 Thrust. 



(4) The Maol an Fhithich Quartzite is separated from the Islay Quartzite. 



(5) The comparatively simple anticlinal structure of North Islay, as 

 illustrated in Sections A & B (PI. XII), is traced in detail. In the Siu-vey 

 description Dr. Peach and Mr. Wilkinson recognize the anticlinal structure of 

 that part of the district which is included within the horseshoe outcrop of the 

 Dolomitic Group ; but they correlate the quartzite beyond this outcrop with 

 the quartzite inside, and assume that the Dolomitic Group is everywhere 

 preserved in synclines. The structural relations, although very clear in the 

 field, are not very satisfactorily represented in the official 1-inch map, since 

 certain of the faults are incorrectly drawn and dip-arrows are for the most 

 part omitted. 



(6) The 'Islay Memoir' leaves indefinite the relationship of the Portaskaig 

 and Port nan Gallan Conglomerates, and also that of the North Islay and East 

 Islay Quartzites. The omission is intentional, but does not correspond with 

 any doubt entertained by the writers themselves — as may be judged, for 

 instance, from Dr. Peach's later descriptions of Scarba. I agree with 

 Dr. Peach in considering the rocks of North Islay and East Islay identical 

 in these two instances. 



(7) The quartzite of East Islay is shown to be inter stratified between the 

 Portaskaig Conglomerate and the Port Ellen Phyllites (Section C, PI. XII). 

 In the Geological Survey Memoir this quartzite is described as lying in a 

 syncline, and the conglomerates on its two sides, which I distinguish under 

 the names of Portaskaig and Scarba respectively, are correlated, as are also 

 the Mull of Oa and Port Ellen Phyllites. 



(8) Satisfactory evidence is given showing that the Portaskaig Conglomerate 

 is younger than the Islay Limestone. Many, if not all, the statements bearing 

 upon this point in the ' Islay Memoir ' do not stand examination in the field. 

 It may be added, however, that Dr. Peach's observations in the Garvellachs 

 furnish strong and reliable support to this contention. 



(9) The Jura Slates — a minor group, first separated in my official descrip- 

 tion of Jura — are identified in Islay, where previously they had been regarded 

 as Port Ellen Phyllites exposed along the course of an anticline. The matter 

 is of considerable importance, for on the southern coast of Islay there is very 

 good evidence that these slates are older than the conglomerate and quartzite 

 lying between them and the outcrop of Port Ellen Phyllites farther east. 

 According to my interpretation of the stratigraphy, this points to the Islay 

 Quartzite being older than the Port Ellen Phyllites, whereas in the ' Islay 

 Memoir ' the opposite conclusion is arrived at. 



(10) Taking the evidence afforded by the archipelago as a whole, one is 

 forced to dissent from Dr. Peach's correlation of the Garvallach and Scarba 

 Conglomerates, and also from his view that the Scarba Conglomerate is the 

 oldest part of the Scarba Quartzite. 



(11) The Degnish Limestone is described, in the Geological Survey Memoir 

 that deals with the northern part of the district, as occurring in a syncline 

 with Easdale Slates on both sides. I have failed to find anything to represent 

 these slates on the east side of the limestone. 



(12) Particular care has been taken to distinguish clearly between observa- 

 tion and inference. In reading the Geological Survey memoirs dealing with 

 Islay and the islands north of Jura, one is generally at a loss to know whether 

 the folds so frequently mentioned have been traced in the field, or whether 

 they form part of the theoretical interpretation. 



The above enumeration of addenda and corrigenda will 

 give a very false impression if it conceals the great obligation 

 xinder which Dr. Peach and Mr. Wilkinson have placed all who are 



