114 ME. E. B. BAILEY ON THE STErCTTJEE OF [vol. Ixxviil, 



The assemblage of interbedded black slate and quartzite just 

 described is referred to the Iltay Nappe for two reasons : — 



(1) Character. — This mixed assemblage, except that it does not appear 

 to contain a definitely conglomeratic horizon, almost exactly reproduces the 

 Scarba development of the Transition Group connecting the Islay Quartzite 

 and Easdale Slates. Only two other groups need be mentioned as possible 

 rivals, namely, the Tayvallich Slates of the Loch-Awe Group (Loch-Awe 

 Nappe), and the Striped Series connecting the Appin Quartzite with the 

 Ballachulish Slates (Ballappel Foundation). Of the Tayvallich Slates it may 

 be said that not only are they more conglomeratic than the Creran rocks, but 

 also they are richer in limestone, while everywhere they are accom- 

 panied by volcanic rocks. Of the Striped Series, that it presents a 

 more definitely continuous interbanding of quartzite and slate ; while its 

 limestones, in my experience, never carry pebbles. 



It will be noticed that the correlation here advanced refers the Garbh-Ard 

 Quartzite to the Islay position. Now, in Islay, Jura, and Scarba, and east- 

 wards to Ben Yrackie in Perthshire, the part of the quartzite bordering the 

 Transition Group is distinctly pebbly, whereas in Garbh Ard it is fijie-grained. 

 Possibly this betokens a local facies, or possibly it indicates a mechanical 

 omission of the pebbly division from the Garbh-Ard outcrop. 



(2) Position. — Island and mainland exposures show that a belt of 

 Easdale Slates of persistent character reaches north-north-eastwards from 

 Luing, through Oban, to the moiith of Loch Etive. The slates are constantly 

 emerging from under a cover of Old Red Sandstone sediments and lavas. 

 Their larger outcrops are shoT^Ti in PL I, while the position of several smaller 

 inliers will be found in a text-figure of the forthcoming Geological Survey 

 Memoir dealing with the 1-inch Map, Sheet 44. 



At Oban, where the Easdale-Slate belt is fully 5 miles wide, the sea hides the 

 probable continuation of both the Transition Group and the Islay Quartzite. 

 The first schists seen beyond the gap (in Mull, Lismore, and neighbouring islets) 

 belong qiiite definitely to the Appin Fold of the Ballappel Foundation. Accord- 

 ingly, itis betweenthe Easdale Slates at the mouth of Loch 

 Etive and the exposure of the Appin Fold in the western 

 part of Ardmucknish that one would naturally expect the 

 Transition Group and Islay Quartzite to reach the main- 

 land. The absence of other members of the Islay succession, as, for 

 instance, the Portaskaig Conglomerate and Islay Limestone, is readily attri- 

 butable to the obvious dislocation which brings the Garbh-Ard Quartzite into 

 conjunction with the Ballachulish Limestone (fig. 6, p. 110). 



The evidence of position forbids the reference of the Ard- 

 mucknish- Creran rocks to the Tayvallich Group of the Loch- 

 Awe Nappe. The Kilbride Inlier, east of Oban (PI. I), almost 

 certainly indicates a northward extension of the Craignish belt of 

 Ardrishaig Phyllites, beneath Old Red Sandstone lavas, on its way 

 to link up with the Pass-of-Brander exposures, where the same 

 phyllitic group extends continuously from Loch Etive to Loch Awe, 

 and so to Loch Fyne and Ardrishaig. Dr. B. N. Peach has de- 

 scribed the Kilbride Inlier as probably in part Easdale Slates, in 

 part Ardrishaig Phyllites (1908 h, ^. 38). I could find nothing in 

 it that did not seem definitely of Ardrishaig tj^pe — but such a 

 difference of opinion is. of course, of quite secondary importance 

 in this particular connexion. 



The evidence of position, if one were restricted to the Loch- 

 Creran district, would, on the other hand, seem to favour a corre- 

 lation of the Ilta}^ Nappe with the Ballachulish Nappe, since little, 



