Ixviii PROCEEDINGS OE THE aEOLOtMCAL SOCIETY. [vol. Ixxiii, 



connecting these two classes of effects is found in the consideration 

 that both afford relief to the unequal stresses ^vhich are continually 

 being set up in the earth's crust by causes regional as Avell as 

 cosmic. 



To enquire into ultimate causes is no part of my design. Mellard 

 Keade showed long ago how mountain-building moYements result 

 from prolonged sedimentation in a slowly subsiding geosynclinal 

 basin, but the general problem seems to inyolve also other factors. 

 There is in the greater orogenic displacements an element of uni- 

 lateral progression, which leads us to picture them in imagination 

 as gigantic wayes. In the broad structure of Eurojje we see the 

 results of four successiye earth- wayes of the largest order, all 

 adyancing in a general sense from the south. The actual belts 

 of folding, as laid down on a map, show sweeping curves, with a 

 certain amount of interlocking. At one place only do three of 

 these belts come together within a relatiyeh' narrow space, and it is 

 precisely in this significant situation that the British Isles lie. 



The unique advantages enjoyed by British geologists, to which I 

 have adverted, result then from the fact that three systems of crust- 

 movements, at widely separated ejDOchs, the Lewisian, the Cale- 

 donian, and the Hercynian, have all contributed to the building of 

 our counfay. Moreover, the whole history of Britain can be viewed 

 in relation to these cardinal events, and only when so regarded 

 appears as a coherent sequence. Especially is this true of igneous 

 action, since fluid, or partly fluid, or potentially fluid, rock-material 

 is necessarily more responsive to changes in stress-conditions than 

 the solid crust itself. Accordingly, each great system of crust- 

 movement has been attended by a display of igneous activity, 

 related to it in a manner which clearly bespeaks some underh'ing 

 law of causation. There is in each case evidence of extensive 

 plutonic inti-usion within the disturbed area, either at the crisis of 

 the disturbance, or following it after no long interval by geological 

 reckoning. These copious intrusions of magma, no pai*t of which 

 reached the surface, are not only events of the first magnitude, but 

 seem to differ functionally from the igneous action which chai-ac- 

 terizes other stao'es of the cvcle. This more diffused actixdtv, 

 taking the form of extrusion as well as intrusion, is develojDed 

 during the gathering of crustal stress before the critical epoch, and 

 again especially in the later waning stages. It has many features 

 suggesting that the several episodes which can be distinguished 

 have their jn-oper places in an ordered sequence. When there has 



