lxXXViii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May lpll, 



spread outwards from the famous overthrust region of the North- 

 West Highlands of Scotland, and the more central region marked 

 by the recumbent folds recently described by Mr. Bailey. How 

 much of the metamorphism is attributable directly to the move- 

 ment, to the association of intrusive rocks with the movement, to 

 the original condition of the rocks, or to the thermal conditions 

 under which the movement was carried on, is at present an 

 unsettled question. The intensity of the movement diminishes 

 when traced through the Southern Uplands into the Lake District 

 and again into North AValcs ; even here it is accompanied by 

 thrusting and cleavage. The latter affects Silurian as well as 

 Ordovician and Cambrian rocks, but it is not known to affect 

 anything newer. Mr. Harker has shown that in Carnarvonshire 

 the location of the cleavage can be connected with the presence of 

 hard, old, masses of resistant rock ; it is remarkable that the 

 Longmynd has not produced like results in its neighbourhood. It 

 may be argued that the pressures had here considerably diminished. 

 A similar absence of cleavage is evident in the Nuneaton Cambrian 

 rocks, despite the presence of the Charnian mass. 



The Pennine movement has been restricted in its effects to the 

 production of folds and faults without cleavage or overthrust ; but 

 the Armorican movement proper at its maximum intensity has 

 been enough to produce the gneisses and schists of Britauny and of 

 Devon and Cornwall. All later movements have resulted in 

 nothing more serious than a certain amount of folding and faulting, 

 crushing, thinning out, and occasional overthrust. 



The connexion of vulcanicity with earth-movement has long 

 been recognised, and is being more fully illuminated by the active 

 petrographical research that is now going on. Each period of 

 orogenic movement in Britain has been connected with one or 

 more of the phases of extrusive or intrusive action, volcanoes, 

 plutonic intrusion, or minor intrusion. So far as the composition 

 of the rocks is concerned, as Mr. Harker has pointed out, the 

 Pacific type dominates over the Atlantic type. The igneous action 

 tends to occur on the margin of the great Atlantic continent, to 

 break through the older rocks where the cover is thin or absent, 

 and to be associated with regions in which contemporary or 

 posthumous folding is asserting or about to assert itself. Thus the 

 dominance of the Pacific type of rocks may possibly be connected 

 with the general dominance through British geological history 

 of geographical features of 'Pacific' rather than 'Atlantic' type. 

 The exceptions will probably be explicable when we possess a 



