Ixviii PROCEEDINGS OE THE (tEOLOCMCAL SOCIETY. [vol. Ixxiii, 



connecting these two classes of effects is found in the consideration 

 that both afford rehef to the unequal stresses which are continually 

 being set up in tlie earth's crast bv causes regional as well as 

 cosmic. 



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

 Eeade showed long ago how mountain -building movements result 

 from prolonged sedimentation in a slowly subsiding geosynclinal 

 basin, but the general problem seems to involve 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 waves. In the broad structure of Em-ope we see the 

 results of four successive earth- waves of the largest order, all 

 advancing 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 relatively 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 epochs, the Lewisian, the Cale- 

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

 our country. 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 pai-th' 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 Avhich clearly bespeaks some underlying 

 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 charac- 

 terizes other stages of the cycle. This more diffused activity, 

 taking the form of exti'usion as Avell as intrusion, is developed 

 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 ])roper jjlaces in an ordered sequence. When there has 



