PRESIDENTIAL ADDRESS. 413 



mountains as due to a considerable local reduction in volume of the Untergrund. 

 The upper crust presses inwards from opposite sides, and the parts that are 

 thrust downwards become absorbed and carried away with the retreating region 

 of the Untergrund. The surviving parts fall over on either side, producing, as 

 the whole continues to close in, folds that are not so very different from the now 

 familiar nappes de recouvrement which these authors hesitate to accept. It is 

 not clear why the postulated reduction in volume in the substratum should 

 occur along a certain line, so as to give rise to axial folding at the surface. 

 The folded mass, broken up by overthrusts, as sketched by Ampferer and 

 Hammer, suggests in its general outlines a Jerusalem artichoke rather than a 

 breaking wave. 



The important point for our present purpose is, however, the restatement of 

 the results of gravitation on the flanks of an uprising chain. The chain is 

 said " to result from ' overthrustings with occasional involutions, accompanied by 

 the rolling over and pushing forward of blocks' (' Ueberschiebungen mit 

 gelegentlichen Einrollungen, Walz- und Schubschollen '). So long as these move- 

 ments occur in the depths, they may be retarded considerably by friction; but, 

 when they produce mountain-bulging at the surface, freedom is given to the 

 ' waltzing ' masses, and gravitation comes into play on the unsupported strata 

 that flank the dome or anticline.*" 



Ampferer and Hammer'" point out incidentally that much of the covering of 

 Flysch and Molasse strata was worn away from the Alps before the final folding 

 which gave the chain its eminence in early Pliocene times. They argue that the 

 uplifted masses were thus less imposing than those pictured by G. Steinmann or 

 C. Schmidt in their diagrams of the Dechen or recumbent folds. But the view 

 advanced by Ampferer and Hammer allows the principal folding to take place 

 in contact with the upper air. The cover is usually supposed to act as a re- 

 strainer, and the long duration of the folding movements is held to have allowed 

 of contemporaneous denudation. Cases, for instance, are known to us where 

 rivers have maintained their level while crust-blocks rose beneath them." The 

 surprising thing, however, about our folded mountain-chains is the way in which 

 they have been eroded parallel to the strike of the overthrust sheets or overfolds. 

 Apart from occasional detached 'klips,' the distal parts of these masses must 

 have been at one time continuous with those proximal to the root-region. The 

 forward movement could not have occurred if denudation had negatived the 

 effects of folding on the surface. A. Tornquist " has suggested that Jurassic 

 limestones were pushed in among unconsolidated mudbanks while the Eocene 

 sea still lay across the Alpine area.' Anything comparable with this during the 

 final folding could not fail to produce a largely felt disturbance at the surface. 

 It is impossible to believe that the ground does not part asunder in Sederholni's 

 zone of Assuring during the exceptional movements that rear a folded mountain- 

 chain. Where such a zone has remained with its parts in contact, as in 

 Fennoscandia, mountain-building has not really taken place. Intense folding 

 may have occurred in the plastic zone below ; but no one of the lines of super- 

 ficial weakness has been continued as a plane of fracture into the depths. 'When 

 this continuation occurs, the material of the folded zone may be forced up to 

 the surface. The general deep-seated crumpling then involves the beds over it 

 and becomes concentrated as an axial chain. 



The marine or lacustrine deposits of the age immediately preceding that of 

 uplift obviously cannot be consolidated at the epoch of upheaval. Gotlandian 

 sands and muds must have overlain the heaving masses that rose as Caledonian 

 land. The swamps of the Coal Measures were contorted in the Armorican 

 chains ; the highest beds of these must have been as yielding and as capable of 



" 7 hid., p. 701. 



*" Compare 0. Ampferer, 'Das Bewegungsbild von Faltengebirgen,' Jalirb. 

 k. h. Beichsnnstalt, vol. Ivi. (1906), p. 601. 



" Op. cit. (ref. 38), p. 708. 



" This is urged even for the Himalayas. Medlicott, Blanford, and Oldham, 

 Geology of India, ed. 2 (1903), p. 463. 



■" ' Noch einmal die Allgau-Vorarlberger Flyschzone und der submarine 

 Einschub ihrer Klippenzone,' Verhandl. k. k. Reichsanstalt, 1908, p. 330. 



