44 SECTIONAL ADDRESSES. 



igneous rocks of Britain, made possible by the progress of petrographical 

 methods and ideas, commenced when Dr. Harker undertook in 1895 the 

 detailed mapping of the Island of Skye — work that led up to the pro- 

 duction of the most complete account of any extensive and complicated 

 region of igneous rocks which up till then had been presented to the 

 scientific public. Later he extended his researches into Rum and the 

 Small Isles of Inverness, where problems of a similar nature confronted 

 him, and which he elucidated with the same skill and perseverance. Since 

 that time the Geological Survey undertook and completed the investiga- 

 tion of what is probably the most complex, but at the same time most 

 illuminating, plutonic centre in the Tertiary province, namely, the Island 

 of Mull. Here many of the discoveries of Dr. Harker in Skye and else- 

 where received ample confirmation, but, as might be expected from a 

 region of greater size and complexity, many features that were obscure or 

 unrepresented in the previously described region were made clear and 

 established as matters of general importance. 



A complete account of this region, prepared largely by Mr. E. B. 

 Bailey, has now appeared and is probably familiar to most of you. No 

 memoir, however good, can do Mull justice, and no student, even with the 

 aid of map and memoir, can fully appreciate all that Mull has to teach 

 without a study of the actual exposures. The companion centre of 

 Ardnamurchan, although of smaller extent, is equally important because, 

 in addition to reproducing many of the salient features of Mull, it supple- 

 ments as well as confirms many of the deductions framed on work done 

 in the larger area. Mr. Richey was mainly responsible for the survey of 

 Ardnamurchan, and although references to the progress of the work have 

 been made from time to time, the complete account of this centre has 

 yet to appear. I am glad to be able to say, however, that the publication 

 of the memoir on this interesting area will not long be delayed. 



In other districts, the petrography of the Island of Arran, with its 

 two centres, has been made the subject of revision by Dr. Tyrrell of 

 Glasgow, who is now preparing an official memoir and explanation of the 

 map. The Mourne Mountains in Ireland have received attention at the 

 hands of Mr. Richey, and his account of the granite masses has been 

 communicated to the Geological Society. Still farther afield, Dr. Hawkes, 

 following on the work of Thoroddsen, has undertaken an investigation of 

 the plutonic centres of south-eastern Iceland, the results of which we 

 await with interest, while others have studied the remnants of the great 

 Tertiary lava-field in even higher latitudes. 



When Dr. Harker commenced his work in Skye the magnitude of the 

 lava-field was fully appreciated, and in addition it was well known that 

 breaking through this field were strictly localised intrusive masses of 

 considerable extent. Till then, however, none of these plutonic areas 

 had been studied in detail, the form and mutual relations of their com- 

 ponent rock-masses were but imperfectly understood, and only the 

 dominant rock-types had received serious attention. Now, practically 

 every one of the Tertiary centres has come under review in recent years. 

 Each has yielded its quota of new facts, and we have reached a stage at 

 which generalization is both possible and legitimate. 



The plateau lavas which rest directly upon a platform of denuded 



