Papers read be/ore Section C {Geology). 517 



IX. — A Mull Peoblem : The Great Tertiary Breccia.' By E. B. 

 Bailey, B.A., F.G.S.^ 



THERE is in Eastern Mull a great breccia formation, with, in places, 

 several intercalated rhyolite lavas. The breccia consists for the 

 most part of an unbedded assemblage of blunted blocks and fragments 

 of gneiss, granophyre, gabbro, and basalt, often associated with rhyolitic 

 debris, for which latter a truly" volcanic origin can be readily admitted. 

 The problem is whether the non-rhyolitic material of the breccia has 

 been derived through explosion or erosion. If the former alternative 

 is adopted, many of the breccia outcrops must be regarded as marking 

 the sites of volcanic vents, since in several cases the boundaries of the 

 breccia are frankly transgressive. At the present time it is considered 

 that the evidence favours an alternative view, that the breccia is an 

 unconformable formation later than the basalt lavas of Mull, and that 

 its transgressive relations are due to erosion which preceded and, 

 accompanied its formation. The basalt lavas of Mull have been violently 

 folded into a series of anticlines and synclines, and it is in one of these 

 synclines that the main outcrop of breccia in Eastei-n Mull is preserved, 

 with every appearance, moreover, of approximate conformity to the 

 surrounding basalts. Here it is difficult to escape the conclusion that 

 the breccia is a thick layer overlying the basalts and folded with them. 

 Alongside of the syncline is an abrupt anticline, in which are exposed 

 all the rocks commonly recognizable as fragments in the breccia. The 

 anticline has a core of gneiss, flanked locally by upturned Mesozoio 

 sediments, and these by steeply dipping basalt lavas ; intruded, chiefly 

 into the gneiss, are granophyre and gabbro. Patches of breccia, 

 distributed without reference to geological structure, occur in this 

 anticlinal region, and rest upon or against all the rocks mentioned 

 above. Although no positive conclusion can be drawn, it is suggested 

 that the breccia has in large measure resulted from erosion, which 

 operated during the period of upheaval of this and neighbouring anti- 

 clinal ridges. 



X. — The Sequence of Volcanic Eocks in Scotland in relation to 

 THE Atlantic - Pacific Classification of Suess.^ By John S. 

 Flett, M.A., D.Sc, F.G.S. 



THE recognition of two great families of igneous rocks, the Atlantic 

 and the Pacific, and their relation to certain types of earth- 

 movement, which we owe to Harker, constitutes one of the greatest 

 advances in rational petrology. 



In Scotland we may take the Carboniferous volcanic rocks as typical 

 Atlantic, while the volcanic rocks of Lower Old Red Sandstone age are 

 characteristic of the Pacific group. We may add to the Atlantic two 

 other series, the Permian or late-Carboniferous volcanic rocks of Ayrshire 

 and East Eife and the nepheline-basalts (presumably Tertiary) of 

 Caithness, with their associated camptonites and monchequites. 



The Tertiary volcanic rocks of the Hebrides are Atlantic, and are 

 associated with movements of Atlantic types. There is much reason 

 in ascribing also to this period the north-west dykes, so abundant in 

 Scotland, which contain not a few nepheline-b earing rocks. 



The remaining- volcanic rocks of Scotland are of distinct type. They 

 comprise the Tayvallich lavas (perhaps pre-Cambrian), the Upper 

 Cambrian volcanic rocks of the Highland border, and the Silurian (and 



^ Abstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



- "With the permission of the Director of the Geological Survey. 



