DYNAMICAL GEOLOGY. 311 



mountain-systems have unilateral structure, and there has 

 been in North America, Europe, and North Africa a general 

 movement of rock-masses towards the north, in Asia towards 

 the south. 



Suess then enunciated certain principles of mountain- 

 building. The simplest type of a mountain-system is that 

 which begins with the occurrence of a rupture or fault rec- 

 tangular to the direction of contraction, the severed crust-block 

 then moving onward in the direction of the contraction (ex- 

 ample, Erz mountains). The second and most frequent type 

 is that which begins with the disposition of a principal fold 

 striking transversely across the contraction and inclined in the 

 direction of the contraction, a fissure then forming in the fold 

 at the line of maximum tension. The front part of the fold 

 moves in the direction of the contraction and pushes the 

 sedimentary rocks before it into further foldings, the other 

 part of the fold sinks, and volcanic rocks escape at the line 

 of fragmentation and subsidence (example, Apennines and 

 Carpathians). In a third type of mountain-building, several 

 parallel folds arise, occupying a greater surface breadth, and 

 usually ending on the inner side of the innermost fold with 

 a steep crust-fracture (example, folded Jura mountains, 

 Ardennes, Taunus, Appalachians). It depends on the inten- 

 sity and direction of the folding- force, on the nature of the 

 resistance, and on the greater or smaller brittleness of the 

 varieties of rock, whether the secondary folds are preserved 

 or if they are deformed and pass into faults whose planes are 

 inclined inward to the mountains and serve as planes of 

 overthrusting. In extensive regions the contracting force 

 seems to have had the same direction during successive geo- 

 logical epochs. 



Suess agreed with Shaler that the continents represent 

 contractions of the whole earth's crust, whereas the mountain- 

 systems are to be regarded merely as foldings of the more 

 superficial layers of the crust. In addition to the folded 

 mountainous portions of the earth's crust, Suess emphasised 

 the presence of resisting crust-areas which, like Bohemia, are 

 composed of old mountain-masses piled against or across one 

 another like pack-ice, or like the vast Russian block consist of 

 undisturbed horizontal strata. Such unyielding areas of the 

 crust are frequently characterised by considerable gaps in the 

 sedimentary series. Their geographical distribution decides the 



