﻿Ill ME. E. BATTERSBY BAILEY OJS [vol. lxxii, 



doubt in my mind that the Beannan Dubh succession is right 

 way up. But in Beannan Dubh Portaskaig Conglomerate occurs 

 superimposed upon Islay Limestone. Accordingly, I take it that 

 the index on PI. XII gives the rock-groups in their original order. 



Thus it is proved that the core of the Islay Fold is constituted 

 of the older portion of the Islay Schist succession. It does not, 

 however, follow at once that the fold is an anticline, for in the 

 Highlands we must be prepared to meet with folds affecting 

 inverted sequences, so that additional evidence is required before 

 coming to any conclusion in this matter. Meanwhile, it may be 

 noted that the mapping of the Portaskaig Conglomerate further 

 demonstrates the existence of .the Islay Fold, for Mr. Wilkinson 

 has succeeded in tracing the conglomerate intermittently from near 

 Bowmore to Port nan Gallan, east of the Mull of Oa. 1 may add 

 that I have visited all the known exposures, except the one near 

 Bowmore ; between this last and Laggan Bay the solid rocks are 

 completely hidden under superficial deposits. 



In the northern part of the island the Portaskaig Conglomerate 

 reaches a thickness of some hundreds of feet. But even here it 

 varies considerably in character, and in some of the outcrops north 

 of Bridgend crystalline boulders are so scarce that one may walk 

 a quarter of a mile before meeting an example ; the dolomite- 

 fragments are more universally abundant. In the exposures on 

 the western side of the fold the cleavage is very intense, giving a 

 shaly appearance to the rusty weathering conglomerate. 



Followed southwards along the east side of the fold the con- 

 glomerate dwindles, and rarely carries conspicuous crystalline 

 boulders. Only in two of the southern exposures is it found 

 richly charged with the typical nordmarkites. These two occur at 

 Beinn Bhan and Port nan Gallan respectively [7, pi. v]. At the 

 former the unbedded, and at the latter the bedded, types of the 

 conglomerate are exposed. Intervening occurrences of the group 

 are as follows from north to south : — 



(1) Immediately north-east of a farm, Balvickar (2 miles north-west of 

 Port Ellen), are strong beds of pure cream-coloured dolomite, separated by 

 cleaved, reddish-brown, very fine-grained conglomerate or g'rit presenting- the 

 impiu-e, sandy, argillaceous, slightly calcareous character of the matrix of 

 Portaskaig Conglomerate. 



(2) North-east of a farm, Craigabus (2 miles west of Port Ellen), exposures 

 occur showing cleaved, impure, muddy, calcareous, and sometimes slightly 

 gritty rock, occasionally full of fragments of cream-coloured dolomite. A bed 

 of similar dolomite is seen close at hand. 



(3) In a road-metal pit south- south- east of a farm, Coillabus (shown on 

 PI. XII), there is a sandy muddy conglomerate, with quartz-grains and 

 dolomite-pebbles. [It is very like the Portaskaig- Conglomerate elsewhere, 

 and seems to be underlain by dolomite and quartzose schists. 



(4) East of Loch Ardachie (1 mile south-west of Coillabus), the succession 

 just described in (3) is seen to better advantage. A conglomerate with many 

 dolomite pebbles overlies a bed of dolomite which is separated from the 

 Islay Limestone by a band of quartzite. 



The Port nan Gallan outcrop has been figured in the Geological 

 Survey Memoir [7, fig. 3, p. 40] to illustrate the unconformity 



