292 ME. E. B. BAILEY ON THE [June 1913, 



that, in the neighbourhood of Dalmally, there is undoubtedly an 

 extensive development of black slate or schist, and subordinate 

 limestone interposed between the Ardrishaig Phyllites and a pebbly 

 quartzite, of which last he speaks as the Loch Awe Grit. I have 

 purposely excluded these northern rocks from the scope of the 

 present paper, since I consider their relation to the Loch Awe Group 

 a very much more difficult question to decide than is the sequence 

 of the rocks actually included within the Loch Awe Syncline. 



And now for the evidence upon which the present subdivision of 

 the Loch Awe Group is based. It may be summarized under various 

 headings : — 



(1) Mr. Hill's contention that the Loch Avich Slates and Grits 

 form part of the Loch Awe Group is justified by the nature of the 

 grit or quartzite intercalations which accompany the slates : many 

 of these agree precisely in character with the rocks of the main 

 Crinan Quartzite. 



(2) At the same time, the Loch Avich Slates and Grits furnish a 

 definite stratigraphical horizon, as maintained by Dr. Peach. This 

 much is apparent from the fact that their outcrop occupies a 

 symmetrical position in the centre of the Loch Awe Syncline, and 

 is, in its own central part, characterized by a complete absence of 

 epidiorite. 



(3) There is fairly conclusive direct evidence that the Loch Avich 

 Slates and Grits structurally overlie all the other rocks of the Loch 

 Awe Syncline. At the northern end of their outcrop, on the east 

 side of Loch Awe, there is a very definite boundary between the 

 slates and grits which form the central exposures and the surrounding 

 epidiorites. In a north-easterly direction the sedimentary outcrop 

 narrows and disappears, in a manner that almost certainly indicates 

 the existence of a fold. It was obviously important to ascertain 

 whether the disappearance of the sediments is the accompaniment 

 of a north-easterly or of a south-westerly pitch : whether in fact 

 the fold, if such there be, is anticlinal or synclinal. On visiting 

 the exposure, it was found that grits are prominent where the 

 sedimentary outcrop is wide, that is, towards the loch. They con- 

 sist of strong pebbly quartzo-felspathic rocks with obscure bedding, 

 so that their structure is not clear. Appearances suggest, however, 

 that they occupy a syncline having a triplex termination. After 

 the grit ceases, a considerable thickness of green cleaved mudstone 

 with widely-spaced, faintly-marked bedding occupies the whole 

 breadth of the narrowing sedimentary outcrop, and persistently 

 shows south-easterly pitches, in accordance with the view that the 

 outcrop is of synclinal nature. 



(4) The Tayvallich Black Slate and Limestone division has an 

 assemblage of characters which distinguishes it from the Shira 

 Limestone. Among these characters the importance of black slate 



