I02 



NATURE 



[September 23, 1915 



1 



masses occupy the place of strata of pre- 

 Permian age. The great development of ther- 

 mal metamorphism in the Erzgebirge and in 

 Saxony,"^ two classic regions of the dynamometa- 

 morphic school, is now widely recognised, and this 

 activity is also assigned to late Carboniferous times. 

 The work of C. Barrois in Brittany is concerned with 

 absorption-phenomena resulting from intrusions during 

 the same mountain-building epoch. 



Seder holm ^' has suggested that the ground above 

 an area affected by processes 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 

 folding, and subsidences of a regional character may 

 occur. 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 considerable disturb- 

 ance of the zone nearer the surface. 



We usually regard such disturban/ces from the 

 uniformitarian point of view. May not, however, 

 actual mountain-building be the break in a slow pro- 

 cess of "softening," to use Mutton's term? For a 

 long time the isostatic balance suffers only small dis- 

 turbances, 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. O. Ampferer,^* with his customary large- 

 ness of view, has referred superficial evidences of dis- 

 turbance, such as mountain-ranges, to dragging move- 

 ments 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 gravi- 

 tational 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. 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. 



Ampferer and Hammer " have recently considered 

 the overfolded structure of mountains as due to a con- 

 siderable local reduction In volume of the Untergrund. 

 The upper crust presses inwards from opposite sides, 

 and the parts that are thrust downwards become ab- 

 sorbed 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. The Important point for our pre- 

 sent purpose is the restatement of the results of gravi- 

 tation on the flanks of an uprising chain. 



The surprising thing about our folded mountain- 

 chains is the wav in which thov have been eroded 

 parallel to the strike of the overthrust sheets or over- 

 folds. Apart from occasional detached "klips," the 

 distal parts of these masses must have been at one 



16 C. Gabert, Zeitschr. deutsch. g-eol. GesselL, vol. lix. (1907), p. 308 ; 

 R. LeDsiu";, " Oeologie von Deut<:chland " (loio), pt. 2, pp. 107 and 172. 



17 Of>. cii.. Bull. Comm. ereol. Finlande, No. 37. p. 66. 



18 " Das Bevwegunpsbild der Faltengebirgen," Jahrb. k. k. geol. Reichs- 

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



19 O. Ampferer and W. Hammer, " Geologischrr Querschnitt durch die 

 Ostalpen," Jahrh. k. k. Reichstanstalt. vol. Ixi. (1911), p. 700. 



NO. 2395, VOL'. 96] 



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. 



The marine or lacustrine deposits of the age imme- 

 diately 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 Armprican 

 chains : the highest beds of these must have been as 

 yielding and as capable of flow as the Flysch that 

 overlay the growing Alps. In all these cases, familiar 

 to us in Europe, the covering masses must have re- 

 sponded to the crumpling under them, and, when 

 reared to dangerous eminences, rapidly became a prey 

 to denudation and gravitational downsliding. They 

 can scarcely be regarded as protective, and their re- 

 moval would leave the brittle masses below more liable 

 to fracture and to the "calving" process that forms 

 klips. 



In some cases separation seems to have taken place 

 as the moving mass fell forward. The klips of hard 

 material embedded in softer strata are thus a kind of 

 rock-spray, hurled in advance of the breaking earth- 

 wave. 



Termler^" in no wise fears to speak of the progress 

 of a " traineur 6craseur " during mountain-building as 

 " soudain et rapide comme une rupture d'equilibre, 

 le dernier acte, longuement p^par^, mals jou^ d'em- 

 portement, de ce drame grandiose." 



Rupture combined with rapid movement of the 

 rocks need not be the last act of the drama; but, the 

 more we examine the history of folded chains, the 

 more probable it appears as a culminating episode. 

 The original cover of our present ranges has been 

 lost by denudation. Earth-sculpture in "these regions 

 of high altitude and vehement attack has removed 

 much of the evidence that we seek. What remains, 

 however, may lead us to feel that no part of the 

 world in historic times has experienced a mountain- 

 building episode. 



Such relatively catastrophic stages have, Indeed, not 

 been common In the long history of the earth since 

 pre-Cambrian times. It appears that now and again 

 the " orogenic collapse " of some considerable area 

 disturbs the balance in the crust and spreads far 

 through the upper layers like a disease. Or It may 

 be that the thermal cause of the collapse is common 

 to the whole earth at the same time, and becomes 

 manifest in responsive regions far apart. In any case, 

 the weak places give way and the more resisting ones 

 close in. A readjustment is effected, which then 

 endures through long geological time. 



The imminent menace of crustal changes was 

 brought home to us during the terrible period from 

 April 4, 1905, to January 14, 1907, the final twelve 

 months being marked by a veritable earth-storm. 

 Geologically speaking, however, we are near enough 

 to the Tortonian epoch to look forward with some 

 confidence to a quiescent phase. But sorne day, 

 in Its due season, the earth will once more be 

 active. When that time comes, no ingenuity 

 of man will suffice to meet it. Earthquake after 

 earthquake, increasing in intensity, will probably have 

 driven the population to a distance from the threatened 

 zone. Concentration of the folding along a particular 

 earth-line will limit the region of absolute destruction ; 

 but the undulations spreading from It, in response to 

 the heavlngs of the chain, will offer sufiiclent chances 

 of catastrophe. In the case of our youngest mountain- 

 ranges, these undulations remain perpetuated as domes 



20 " Le« Probletnes de la Gioloeie tectonique dans le Mediterranee 

 occidentale," Revue generale dcs Sciences, March 30, 1911. 



