220 MOVEMENTS AND DEFORMATIONS 



unloading, changes of temperature, or other local causes, and 

 how far they are the milder phases of great movements or incidental 

 to them, it is difficult to decide. 



The Great Periodic Movements 



1. Mountain-folding. Along certain tracts, the shell of the 

 earth has wrinkled, forming folded mountains. The shell so folded, 

 judged from the nature of the folds, seems to be no more than a 

 few miles thick. The forces that caused the folding took the form 

 of lateral thrusts. The folds themselves were usually lifted, show- 

 ing that there was an upward component to the horizontal thrust, 

 but the horizontal component was the dominant one. Some folds 

 are nearly upright and symmetrical, and some inclined and asym- 

 metrical, as illustrated in Chapter X. Where the folds lean, it is 

 commonly inferred that the active thrust was from the side of the 

 gentler slope, pushing the fold over toward the resisting side; but 

 this is not a safe inference in all cases, for the original attitude of 

 the beds has much to do with the way they yeild. Most systems of 

 folded mountains embrace a series of roughly parallel folds, the 

 whole forming an anticlinorium (Fig. 219). 



Distribution of folded mountains. The location of folded 

 mountains is near the borders of continents in so many cases that 

 the relation is probably significant, but there are folded mountains 



far from coasts, as the 

 Urals, the mountains 

 of Central Europe and 

 of Central Asia. 



Folding move- 

 ments seem to have 



been very common in 

 Fig. 219. Anticlinorium: diagrammatic. (Van - ' 



Rise, U. S. Geol. Surv.) the early ages. The 



Archean rocks(Chapter 



XIII) are almost universally crumpled, and in many places in the 

 most intricate fashion, and the Proterozoic formations are much 

 folded. After the inauguration of the Paleozoic era, folding appears 

 to have taken place chiefly at long intervals, and for any given 

 period to have been concentrated along certain tracts. The Ap- 

 palachian system is an example. 



2. Plateau-forming movements. An important phase of mas- 

 sive movement was the relative settling or raising of great blocks or 



