MOUNT AIX GROWTH 239 



and from thermal expansion probably serves chiefly to disturb equi- 

 librium, setting in motion the larger forces due to contraction. The 

 materials of the orogen become more plastic with heat — perhaps portions 

 actually liquefy — and the deeper part of the zone becomes honeycombed 

 with magma. Under lateral pressure from the rigid plates the plastic 

 mass tends to rise and the overlying sediments are folded. The entire 

 orogenetic zone acts as a passive mass between the jaws of a vice, and 

 as great folds rise on either side of the geosyncline they are underridden 

 hy the rigid plates, and so turned back over the forelands. Other folds 

 form inside of and parallel to the first, and if compression is intense and 

 long continued the entire body of sediments will be crumpled. In a 

 wide geosyncline the forces will ordinarily be spent in folding up the 

 border ranges, and the sediments of the middle zone will escape intense 

 crumpling. This interior (Zwischengebirge) zone occupies the deepest 

 and most plastic portion of the orogen. There isostatic compensation 

 will be almost completely maintained, even during periods of active 

 mountain growth, and very little actual elevation may occur. The rigid 

 plates in part support the overfolded border ranges, and thus in some 

 degree superior elevation may represent an overload. Yielding to the 

 overburden may take the form of sagging or major faulting, either at 

 some stage of mountain growth or at a later date, and considerable 

 portions of a foreland may be carried down in the failure. 



Kober's ideas of mountain growth as illustrated in the Alps are best 

 •expressed in his own words, as follows :' - * 



"The Alps arise on the site of the Paleozoic mountains. During the 

 Mesozoic these were in large part leveled down and sank more and more 

 into the geosyncline. But not all parts alike. The pressure from contraction 

 allows some parts to sink deeper than others, and thus there are formed more 

 •or less parallel sea basins on the floor of the geosyncline lying in the area of 

 •contraction. Major sjnclines and geanticlines originate, alternating from 

 north to south. The great anticlines become embryonic decken; these change 

 in the course of mountain-making through the Mesozoic to trunk decken, 

 from which secondary decken (Teildecken) later branch off. The decken 

 move slowly and steadily on the sea-floor against the foreland, gradually rise 

 above sealevel, develop into islands, rows of islands, island chains, and finally 

 into connected mountain features (Argand and Staub), It is a 'mechanics 

 of great folding,' as we may say with Abendanon, a moving of ground folds 

 <Penck), which increases in magnitude, develops into geanticlines, between 

 which great synclines lie deeply depressed. The geanticlines are finally over- 

 turned ; their own weight drags them down into the synclines ; these are 

 concealed (overthrust). A period of quiet follows. Then the next geanti- 

 cline is thrust forward. The process is repeated. Then the previously formed 

 geanticline revives, is thrust over the one lying before it ; both move together 



Der Bau der Erde, pp. 92-93. 



