SECTION C— GEOLOGY. 



THE TERTIARY PLUTONIC CENTRES 



OF BRITAIN. 



BY 



HERBERT H. THOMAS, ScD., F.R.S., 



PRESIDENT OF THE SECTION. 



In presenting to you the subject of the Tertiary Plutonic centres of 

 Britain, I do so with some diffidence, for of late years so much has been 

 written concerning them and so much work has been done on related 

 subjects that, of necessity, a great deal of what I am going to bring to 

 your notice will not be new to you. I cannot pretend that I am armed with 

 an array of fresh facts and observations, nor do I propose to follow the 

 fashion and propound some new theory relating to petrogenesis. All I 

 desire to do, and feel capable of doing, is to stress the importance of certain 

 features displayed by the igneous rocks of these centres, feeling that 

 they are not merely matters of local interest but are such as must influence 

 fundamentally any conception of igneous intrusion and the explanation 

 of the variability of rock-types and rock-composition. 



The great Brito-Icelandic Province of Tertiary igneous activity, as you 

 are well aware, stretches over a known area of some hundreds of thousand 

 square miles, and so far as its major development is concerned, reaches 

 from north-east Ireland, through the Inner Hebrides to the Faroe Islands, 

 Iceland and beyond. Although broken up by the sea into more or less 

 isolated areas that represent but a fraction of their original extent, these 

 areas are sufficiently large and of sufficient relief to offer unparalleled 

 opportunities for detailed study, both as regards the lateral and vertical 

 distribution, and mutual relations of their component rock-masses. 



In the early days the lava-field received the greater share of attention, 

 and it was not till the later decades of the last century that the importance 

 of the major intrusive bodies was either suspected or realised. 



It is not necessary to delve into the early history of petrographical 

 work connected with this region, but sufficient to pay tribute to the 

 astute observations of such pioneers as Pennant, Ami Boue, Jameson and 

 Macculloch, and to acknowledge the great debt that we owe to Judd and 

 Archibald Geikie who, although often holding divergent views, together 

 laid the sure foundations on which the whole fabric of subsequent work 

 has been erected. 



The definite establishment of the Tertiary age of the igneous rocks as 

 a whole, the conception of a great petrographical province, and the 

 proof that the lavas of the great field were of the plateau-type due to 

 fissure-eruptions, constituted the first real advance in our knowledge. 



What I may refer to, however, as the intensive study of the Tertiary 



