476 Geikie — Volcanic Rocks of Great Britain. 



yet the high grounds on either side seem to have in great measure 

 escaped, for few of the trap-beds, or of the "necks," marking the 

 points of eruption, have as yet been detected there. Again, the 

 Tertiary basalts of the north-west lie in a long hollow (at least as 

 old as the Lias) scooped out of the metamorphosed Silurian and 

 Laurentian rocks. In these instances it is evident that the numerous 

 volcanic orifices were grouped linear- wise. 



One other part of the subject I would allude to as deserving 

 of inquiry. There seem to me indications that local but well-marked 

 metamorphism and the extravasation of syenitic and granitic rocks 

 have taken place in connection with some of our most recent volcanic 

 phenomena. In Skye, for example, as first pointed out by Maccul- 

 loch, the Lias limestones are much altered and pierced by masses of 

 syenite, which is in some places a true granite. This crystalline 

 rock must have been erupted after the deposition of the middle 

 Oolitic rocks, for it disrupts and sometimes overlies them. It is 

 manifestly connected with the trappean plateaux and dykes of that 

 region. Southwards in Mull, masses of syenite of a like kind 

 ai"e found in the heart of the great Tertiary basalts, and these basalts 

 show there a marked change in texture and aspect, as if they had 

 been more or less metamorphosed. Still further south lies the 

 granite of Arran, which is, at least in part, of later date than the 

 lower Carboniferous rocks, for these are pierced by it. In and 

 around it, as is well known, there is a profusion of trap-dykes 

 like those of Skye and Mull. This association of syenite or granite 

 with hundreds of dykes, or with vast piles of basalt, deserves to be 

 worked out carefully in the field. It will, doubtless be found to 

 furnish additional data towards elucidating the origin of granite, and 

 even perhaps some portion of the still obscure subject of metamor- 

 phism. 



In concluding these somewhat desultory remarks, let me add that 

 I have brought this subject under the notice of the Section with the 

 view of indicating a field of research in British geology where it 

 appears to me that much remains to be discovered, and where the 

 labourers are but few. There was indeed a time, still within the 

 recollection of some of our older members, when the igneous rocks 

 of this country received a much larger share of attention than they 

 do now. After they had ceased to furnish material for the battles of 

 the Vulcanists and Plutonists, they continued to be studied by able 

 observers, more especially in Scotland, where they attain their 

 greatest prominence. Foremost were the names of Macculloch, 

 Jameson, and Boue, who, with their associaies and disciples, worked 

 long and well until they had given to the igneous rocks of this 

 country an European reputation. Since their days, however, this 

 branch of the science seems in this country to have gone sadly out 

 of fashion. De la Beche, Murchison, Sedgwick, Eamsay, and others, 

 have indeed furnished excellent illustrations of the geology of 

 different parts of the country where volcanic rocks abound. But, 

 apart from local or descriptive geology, little has recently been done 

 in the investigation of our volcanic rocks. 



