﻿part 2] 



THE ISLAY ANTICLINE. 



141 



seen in the shore-section of the south-eastern corner of Laggan 

 Bay. Grey or greenish phyllites are exposed on the foreshore 

 here, and are constantly interlaminated with very dark seams 

 approaching black. 



Cream-coloured sandy dolomites are common in some sections, 

 and may be examined in the cliffs a mile north of the Mull of Oa. 

 The lower 1 bands of the grey Islay Limestone undoubtedly make 

 their first appearance intercalated in the upper portion of the Mull 

 of Oa Phyllites. There are also occasional outcrops of white lime- 

 stone and associated fine-grained quartzite, which appear to be 

 referable to a like position. 



Rough dark-grey, or blackish, slates are quarried at Esknish, 

 and belong to a horizon somewhat below the main mass of Islay 

 Limestone. 



The group is much more sheared in the southern part of its 

 outcrop than in the northern, where bedding surfaces are some- 

 times found glistening with clastic micas. 



(3 c) Islay Limestone. 



The limestone-beds, here classed together, make their first 

 appearance in the upper portion of the Mull of Oa Phyllites, and 

 attain a well-defined maximum at the summit of the group. They 

 are somewhat sandy in composition, and generally dark grey, blue, 

 or black in colour. Oolitic structure is well developed in one or 

 more bands, which have been noted by Mr. Wilkinson at widely- 

 separated points along the limestone-outcrop. Thomson examined 

 the oolites microscopically in the hope of finding fossils, but 

 without success [3, p. 216]. 



The uppermost bed of the Islay Limestone, near Loch Lossit, is 

 a rather pale-grey rock, which, on testing, proves to be dolomite. I 

 think that dolomite also occurs, north-east of Bridgend, as cream- 

 coloured bands interstratified with the ordinary grey type of Islay 

 Limestone. I did not test them, but they are very similar in 

 appearance to dolomites that occur on higher horizons in the 

 island. Such exceptions, however, do not vitiate the general rule 

 clearly stated by Mr. Wilkinson, that the Islay Limestone is a 

 true limestone; whereas the calcareous beds of the Portaskaig 

 Conglomerate and the Dolomitic Group of the Islay Quartzite 

 are just as definitely dolomite. 



Locally — as, for instance, west of Esknish — it appears that 

 certain bands of the limestone become conglomeratic. Fragments 

 of oolitic limestone are crowded together, sometimes cemented in 

 an oolitic matrix. It seems that these beds, if more than one 

 exist, occur near the top of the limestone group. Mr. Wilkinson 

 regards them as belonging to the Portaskaig Conglomerate, and 

 describes their outcrops as outliers ; but, as careful search fails to 



1 The terms lower and upper used in this connexion are justified by 

 structural considerations developed in the sequel. 



Q.J.G. S. No. 286. M 



