﻿part 2] 



THE ISLAT ANTICLINE. 



153 



where its most obvious feature is an absence of the flags so charac- 

 teristic of the much thicker group on the west. The quartzite is 

 massive and not very pebbly, and has no conglomeratic tendencies. 

 In the Survey Memoir I correlated this band with the Conglo- 

 meratic Group of Scarba ; but, on further consideration, I feel 

 convinced that the conglomeratic pebbly quartzite south-east of 

 the Jura Slates is the true representative of the Scarba Conglo- 

 merates. 



The Jura Slates, in their typical development, include a western 

 portion of grey slate or phyllite and an eastern portion of black 

 slate. Except in certain exposures half way up the coast, the grey 

 slates are quite subordinate, and in the extreme north they fail 

 altogether. In the north the black slates are accompanied by thin 

 beds of dark-grey or black limestone, some of them pebbly. 



The Scarba Conglomeratic Group, according to the correlation 

 now adopted, dips off the Jura Slates, and consists of pebbly 

 quartzite of an unusually coarse texture, and often of a dark-grey 

 or black hue. These coarse pebbly quartzites everywhere carry 

 intercalations of black slate, which, so far as one can judge, 

 increase in importance northwards. 



The pebbles are generally quartz and felspar, ranging up to the 

 size of a pigeon's egg. At a few points, indicated by dots in 

 PI. XII, fragments of grit and slate occur, imparting to the 

 rocks a definitely conglomeratic facies. From the appearance of 

 the matrix, and from the quite abnormal size of the associated 

 quartz- and felspar-pebbles, I have no hesitation in regarding these 

 rock-fragments as products of erosion : they have not resulted 

 from crushing connected with the folding of the schists. 



East I slay. — The following succession has been determined in 

 East Islay : — 



Scarba Conglomeratic Group. 



Jura Slates, black above, grey below. 



Quartzite, prevalently pebbly above, non-pebbly below. 



It has already been pointed out that the Portaskaig Conglo- 

 merate can be recognized at intervals from Beinn Bhan to Port 

 nan Gallan, near the Mull of Oa. A little to the east of its course 

 one might expect to meet with the Dolomitic Group of North 

 Islay. Unfortunately, however, only a single rather doubtful out- 

 crop has so far been found. It occurs on Beinn Bhan, where it 

 was recognized by the officers of the Geological Survey. The 

 persistent non-appearance of the Dolomitic Group elsewhere may, 

 perhaps, be due to an untraced continuation of the Beinn Bhan 

 Fault ; it is more likely, however, a result of a deterioration of the 

 Dolomitic Group in a southerly direction. 



The great mass of quartzite lying north-west of the Jura Slates 

 in East Islay is susceptible of a rough division into a lower group 

 with few pebbly bands, and an upper group which is very generally 

 pebbly. All the quartzite east of the Beinn Bhan Fault belongs 

 to the pebbly group. 



