﻿154 



MR. E. BATTERSUY BAILEY" OjS t 



[vol. lxxii, 



The Jura Slates are best exposed on the southern coast, where a 

 broad band of grey slate is followed eastwards by a thin belt of 

 intensely black slate. It is interesting to note how close an agree- 

 ment exists between this exposure and those of Jura, save only 

 that the grey slates are relatively more important in the south. 



The Scarba Conglomeratic Group, as in Jura, is represented by 

 pebbly quartzite, generally of extreme coarseness and sometimes 

 definitely conglomeratic. The pebbles are quartz and felspar, along 

 with occasional fragments of quartzite and slate. The rock- 

 fragments are evidently a result of 1 contemporaneous erosion ' — 

 the slate, no doubt, in most cases derived from interbedded seams 

 of slate, generally grey, sometimes black. 



The interbanding of quartzite and slate in parts of the Conglo- 

 meratic Group has been interpreted in the Survey Memoir on Islay 

 as an example of incessant repetition by folding of the conglo- 

 meratic edge of the Islay Quartzite and the neighbouring pelitic 



Fig. 6. — Cliff-section, 200 feet high, on the southern coast of 



Islay: interfolcled Jura Slates and Scarba Conglomeratic 

 Group. 



N..W. S.E, 



rocks of the Port Ellen Phyllites. The slate-fragments have 

 accordingly been taken as conclusive evidence that the Islay 

 Quartzite is of later date than the Port Ellen Phyllites. But, as 

 a matter of fact, the supposed repetitions can nearly always be 

 distinguished one from the other on the score of minor characters, 

 such as colour, composition, or texture ; wherefore it seems certain 

 that they are mostly due to recurrences of type in an originally 

 alternating series of arenaceous and argillaceous deposits. At the 

 same time, with the incoming of softer strata between the quartzite- 

 beds there is a marked increase of buckling, such as is illustrated in 

 fig. 6, above. 



A particulaily interesting section of that portion of the Scarba 

 Conglomeratic Group which comes next to the Jura Slates is 

 furnished by the cliffs of the southern coast of Islay. The black 

 slate of the Jura Slates is here seen folded with the succeeding 

 quartzite of the Conglomeratic Group in the manner indicated in 

 tig. 6. The quartzite, where it approaches the black slate, is 



