﻿part 2] 



THE I SLAY ANTICLINE. 



147 



the Central Islay Fold. The following suhclivisions are recognized 

 in the Quartzite Series, in descending order : — 



Pebbly Quartzite. 



Upper Fine- Grained Quartzite. 



Dolomitic Group (' Pucoid Beds ' of the Geological Survey Memoir), 

 with often a massive quartzite intercalation (' Pipe Rock ' of the 

 Memoir) near the base. 



Lower Pine-Grained Quartzite. 



In regard to the two lower subdivisions and their relations to 

 the Portaskaig Conglomerate, the above is in agreement with 

 the succession as stated by Mr. Wilkinson. But, whereas he 

 believes that the Dolomitic Group is the highest stratigraphical 

 division preserved in the island, I propose to show that it is over- 

 lain by much the greater portion of the Islay Quartzite. 



The structure and succession of the district can be determined 

 equally well along the coast or inland. The coast-section furnishes 

 a good introduction, and will be considered first. 



Coastal traverse from PortasTcaig to Loch Gruinart. — 

 The Lower Pine - Grained Quartzite succeeds the Portas*kaig 

 Conglomerate with an even, moderate northerly dip in two clear 

 sections (repeated by normal faulting) on the shore north of 

 Portaskaig. The quartzite here is very fine-grained, pure though 

 slightly felspathic, vitreous, well-bedded, and ripple-marked. It 

 must be some hundreds of feet thick. In the northern exposure 

 thin conglomeratic seams occur towards the top, one bed con- 

 taining pebbles of nordmarkite, quartzite, and shale. 



I have seen specimens from the Islay Quartzite which could 

 be placed right way up with confidence, becatise they carried 

 narrow-crested broad-troughed ripple-marks. During my earlier 

 visits I had not realized the value of ripple-marks in determining 

 the original order of superposition of a little -altered series of 

 sediments. On my last visit my time at Portaskaig was unfor- 

 tunately too short to enable me to investigate the point satis- 

 factorily. There is no doubt, however, that this line of research 

 is a very promising one in Islay and elsewhere. 



The succeeding Dolomitic Group consists, in the Portaskaig 

 sections, of the following subdivisions, in descending order : — 



Thick zone, very largely made up of biiff -weathering, banded, impure 

 dolomitic beds, with a very small proportion of grey or blue limy seams ; 

 also dark and silky-grey, impure quartzose slates ; and, towards the top, 

 important beds of pure, cream-coloured and white dolomite. 



Minor band of well-bedded, massive, fine-grained quartzite, about 100 feet 

 thick. 



Grey silky phyllites, sandy-grey flags, banded, flaggy, fine-grained quartzite 

 (one pebbly layer has been noticed), alternating with calcareous or dolomitic 

 flags and fine-grained quartzite. Almost every bed ripple-marked. 



Some of the flags are sun-cracked in the manner that is so 

 common among the Orcadian Flags of Old Red Sandstone age. 

 This feature was remarked upon by Thomson, as also the presence of 



