﻿150 



ME. E. BATTERSBY BAILEY ON 



[vol. Ixxii, 



ascending structural sequence ; this proves conclusively that the fold, running 

 up Central Islay and determining the horseshoe outcrops of the Islay Lime- 

 stone and Portaskaig Conglomerate, is an anticline. 



(2) In the central or axial belt, where well-defined stratigraphical groups 

 crop out along the seashore, constant repetition by isoclinal folding is non- 

 existent, although there is important repetition by open folding and faulting. 

 One cannot hope to determine structure with like certainty in the waste of 

 uniform, well-bedded, fine-grained quartzite between the westernmost oiitcrop 

 of the Dolomitic Group and the first appearance of the Pebbly Quartzite at 

 Gortantoid Point. I could, however, find no evidence for isoclinal repetition 

 of this fine-grained quartzite (the example, fig. 5, p. 51, of the Geological 

 Survey Memoir is based on an illusory appearance), and I feel confident that 

 the group is some thousands of feet thick on both sides of the Islay Anticline. 



(3) The Upper Fine-Grained Quartzite is stratigraphically distinct from the 

 Lower, as may be seen from its superior position and greater thickness, and 

 more especially from its passing under the Pebbly Quartzite at Gortantoid 

 Point. In order to clinch the matter, it may be stated in advance that the 

 Gortantoid relation is precisely reproduced on the opposite side of the Islay 

 Anticline in Jura. 



Inland exposures in the axial district of the Islay Anti- 

 cline. — On the west side of the inland continuation of the 

 Bonahaven Fault, the Lower Fine-Grained Quartzite clips in a 

 general northward direction away from the Portaskaig Conglo- 

 merate. The junction of the two groups is often hidden,, and the 

 bedding of the quartzite is sometimes rather difficult to make out ; 

 consequently it is fortunate that additional evidence, leaving 

 nothing to the imagination, is afforded by the upward passage 

 of the quartzite at gentle angles under the Dolomitic Group. A 

 traverse along the southern boundary of the latter, from Loch 

 Staoinsha (2 miles south-west of Bonahaven) to be^yond Giur- 

 bheinn, reveals a simple anticlinal arrangement 1 of the beds, 

 affected by a regular northerly pitch, and complicated to 

 some extent by faulting. The fault, which runs through Loch 

 Giur-bheinn (a small loch immediately east of Giur-bheinn), is 

 bordered at the loch by steeply-inclined beds along its western 

 side. Although these beds are on the upthrow side of the fault 

 and dip towards it, they appear to be inverted. Be this as it may, 

 the effect is strictly local, and the dip speedily rights itself. 



It is interesting to note, in passing, a recurrence of conglomeratic 

 conditions towards the top of the Lower Fine-Grained Quartzite in 

 these inland exposures [7, p. 43], just as in the coast-section 

 already described. The bed or beds contain nordmarkite and other 

 pebbles like those of the Portaskaig Conglomerate, but the matrix 

 is sometimes a pebbly quartzite. West of Giur-bheinn considerable 

 outcrops of gritty slate are shown in PL XII as belonging to this 

 conglomeratic position ; it is not certain, however, that they do not 

 belong to the Dolomitic Group. In the neighbourhood of Giur- 

 bheinn the massive quartzite intercalation ('Pipe-Rock' of the 



1 Reference is made to this anticline in the Geological Survey Memoir 

 on Islay (p. 49) as ' by far the most conspicuous example of the system of 

 folding,' and evidence for it is given in some detail. 



