﻿part 2] 



THE IS LAY ANTICLINE. 



155 



markedly coarse in texture, blackish, and charged with black-slate 

 fragments. It is, therefore, exceedingly likely that the Scarba 

 Conglomeratic Group is later than the Jura Slates. This deduc- 

 tion has already been drawn from the exposure, and is stated in 

 the Geological Survey Memoir on Islay (p. 36). Unfortunately, 

 however, the authors of that memoir regarded the Jura Slates as 

 merely Port Ellen Phyllites, brought to the surface along an anti- 

 cline, and accordingly they thought that the section showed the 

 Islay Quartzite to be later than the Port Ellen Phyllites. On 

 the interpretation of the stratigraphy advanced in the present 

 paper, exactly the reverse conclusion is attained. 



Scarba, Lunga, etc. — The following succession of groups has 

 been recognized from west to east in the Islay Quartzite as deve- 

 loped in Scarba. Dr. Peach, to whom we owe the classification 

 and also the recognition of worm-casts in certain of the groups, is 

 of opinion that the succession is probably a stratigraphical one — 

 not reduplicated by folding, — and that the Conglomeratic Group is 

 the oldest group of the series. I agree with Dr. Peach, except 

 that, in the light of the evidence from Islay, I think that the Scarba 

 Conglomerates are probably the latest members of the series : — 



Scarba Conglomeratic Group. 



? Jura Slates (not recognized as a group by Dr. Peach, and for the 



greater part cut out by the Scarba Fault). 

 Pebbly Quartzite with numerous Flags (sometimes crowded with 



worm-casts). 



Massive Quartzite, sometimes pebbly, with quartz and felspar. 

 Fine-Grained Quartzite, with concretions resembling the infilled 



mouths of worm-pipes. 

 Quartzite with Flags (sometimes crowded with worm-casts). 



It seems not improbable that the western Quartzite with Flags 

 of Scarba may belong to the upper part of the Dolomitic Group of 

 Islay. The eastern Quartzite with Flags is, of course, the con- 

 tinuation of the Jura Flag Group ; while the Massive Quartzite on 

 the west of it belongs to the belt of vitreous quartzites which 

 occupies the western coast of Jura for about 7 miles. 



The eastern Quartzite with Flags is bounded on the east in 

 Scarba by an important fault, recognizable also in the islands on 

 the north, where it increases considerably in downthrow. In those 

 portions of the northern islands that lie west of the Scarba Fault 

 (leaving out of consideration the Garvellachs and Dubh-fheith, 

 which I have described as constituted wholly of Portaskaig Con- 

 glomerate), Dr. Peach believes that he can recognize the Massive 

 Quartzite of Scarba and also (in Lunga) the eastern Flag Group. 



It seems that the Jura Slates are wholly faulted out in Scarba, 

 unless represented for a short distance by a strip of black slate, 

 noted by Dr. Peach on his field-maps along the course of the 

 Scarba Fault. In Lunga it is not impossible that the group has 

 been locally removed by ' contemporaneous erosion,' since a fairly 

 important outcrop of what appears to be the eastern Flag Group of 



