412 TRANSACTIONS OF SECTION C. 



west of Caslav in Bohemia and in the Eisengebirge are now attributed by 

 Hinterlechner and von John '- to the metamorphism of Ordovician strata by 

 younger granite, which intruded in post-Devonian and probably in Carboniferous 

 times. Much of the gneiss and granite of the Black Forest and the Vosges is 

 now, moreover, removed from the Archaean, and is shown to be associated with 

 the Armorican movements/'' These vast intrusive masses occupy the place of 

 strata of pre-Permian age. The great development of thermal metamorphism in 

 the Erzgebirge and in Saxony," two classic regions of the dynamometamorphic 

 school, is now widely recognised, and this activity is also assigned to late Car- 

 boniferous times. The work of 0. Barrois in Brittany is concerned with 

 absorption-phenomena resulting from intrusions during the same mountain- 

 building epoch. 



Sederholm"^ has suggested that the ground above an area affected by pro- 

 cesses of mountain-building cracks and becomes faulted, while the more plastic 

 zone below flows under pressure into folds. But the blocks of the ' brittle ' layer, 

 as Lawson has it, may be seriously displaced by movements in the zone of fold- 

 ing, and subsidences of a regional character may occur. The example of the 

 Minaun Cliffs in Achill shows that the plastic zone may become locally thickened 

 by softening and overfolding. The pressure that has driven an excess of matter 

 to the region of overfolding has squeezed it from beneath an adjacent region. 

 Crumpling and overfolding are accompanied by a shearing away of the matter 

 in one zone from that of another which overlies it; this must result in consider- 

 able disturbance of the zone nearer the surface. 



We usually regard such disturbances from the uniformitarian point of view. 

 Earthquakes are often bad enough, but they are treated as breaks in a slow 

 process of folding that is always going on beneath our feet. May not, however, 

 actual mountain-building be the break in a slow process of 'softening,' to use 

 Mutton's term ? For a long time the isostatic balance suffers only small disturb- 

 ances, restoring itself automatically on a gently-yielding underworld. Then 

 something gives way ; something — a large mass of supporting rock — suffers a 

 change of state. The balance is destroyed abruptly, and mountain-building 

 and rapid subsidences have their day. 0. Ampferer,^'' with his customary large- 

 ness of view, has referred superficial evidences of disturbance, such as mountain- 

 ranges, to dragging movements of a mobile Untergrund. He urges that physical 

 and chemical changes within the earth may produce considerable local changes of 

 volume. Vertical movements lead to upfolding, and this leads to gravitational 

 sliding. The zone of folding that we have been considering as normal near the 

 Untergrund thus becomes transferred to the surface of the earth. 



I am not now concerned with the causes of folding, beyond the fact that at 

 a certain critical stage the material involved may move at a rapid rate. When 

 R. A. Daly speaks of an ' orogenic collapse,''"' he implies something of a 

 different order in time from the slow processes of sinking and accumulation of 

 sedmient that have gone before. Changes of state, physical and chemical, occur 

 with some abruptness. In the case of rocks, the softening or melting of even 

 one constituent may allow of flow, and, as we have observed, this flow in a lower 

 layer may soon become manifested in surface-changes. 



Ampf erer and Hammer " have recently considered the question of collapse 

 in an opposite sense to that of Daly, who pictures it as the herald of an upward 

 movement. These authors, on the other hand, regard the overfolded structure of 



'^ Verhandl. k. k. Reichsanstalt, 1910, p. 337, and Jahrh. ibid., vol. lix. 

 (1909), p. 127. 



" P. Kessler, ' Die Entstehung von Schwarzwald und Vogesen,' Jahresberic/ite 

 Oberrhein. geol. Vereines, vol. iv. (1914), p. 31. 



" C. Gabert, Zeitschr. deutscli. geol. Gesell., vol. lix. (1907) p 308- 

 R. Lepsius, ' Geologie von Deutschland ' (1910), Pt. 2, pp. 107 and 172 



"Op. cit., Bull. Comm. geol. Finlande, No. 37, p. 66. 



" 'Das Bewegungsbild der Faltengebirgen,' Jahrb. k. k. geol. Reichsanstalt, 

 vol. Ivi. (1906), p. 607. 



" Igneous Rocks and their Origin (1914), p. 188. 

 r> ^\ ^' .^PiP^fi'er and W. Hammer, ' Geologischer Querschnitt durch die 

 Ostalpen, Jahrb. k. k. Reichsanstalt, vol. Ixi. (1911), p. 700. 



