354 GEOLOGY 



Minor Movements 



Gentle movements seem to have affected nearly every portion 

 of the surface of the lithosphere at nearly all stages of its history, 

 They have often had much to do with the particular places and forms 

 of deposits. Slow relative sinkings of sea-borders have permitted 

 deposition to go on in shallow water for long periods without 

 building the bottom up into land, and slow relative swellings of 

 land tracts have permitted the agents of erosion to get sediments 

 for such deposits. Such movements have also shifted the borders 

 of lands and seas, and with them, the areas of erosion and depo- 

 sition. These movements may have amounted to a few inches, 

 a few feet, or a few fathoms per century. They are sometimes 

 reciprocal, one area being bowed up while another near by is bowed 

 down. How far these are merely local or regional, due to loading, 

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

 how far they are the milder phases of the great body movements 

 of the earth or the incidental creep arising from these, it is difficult 

 to decide. 



The Great Periodic Movements 



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

 earth has been folded into wrinkles, thus forming folded mountains. 

 The shell so folded, judged from the nature of the folds, seems to 

 be quite superficial, reaching only a few miles in depth. Some- 

 times the strata were very closely folded, and even intensely 

 crumpled. The forces that caused the folding took the form of 

 lateral thrusts. The folds themselves were usually lifted, showing 

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

 the latter was the dominant factor. The folds were crumpled 

 outward rather than inward, which implies that there was crowding 

 below, rather than shrinkage. The folds are sometimes nearly 

 upright and symmetrical, and sometimes inclined and asymmet- 

 rical, as illustrated in Chapter XL When the folds Jean, it is 

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

 gentler slope, and pushed the fold over toward the resisting side, 

 but this is not always a safe inference, for the original attitude of 

 the beds has much to do with the way they yield. Systems of 



