PHYSICAL CHANGES OF THE OUTER SHELL 65 



movements of this sort may be included under the term 

 warping. 



Local crumpling of the shell. Before they were consoli- 

 dated, the stratified rocks were merely layers of sediment 

 which had been deposited in a horizontal or gently inclined 

 attitude. In many places, however, we now find them 

 crumpled and folded (Fig. 45). The folds in any one area 

 are usually parallel to each other, and are arranged in long, 

 narrow bands. Such folds have evidently been produced 

 by compression from the sides, the part between having 

 wrinkled, just as flat-lying sheets of paper will wrinkle if 

 compressed horizontally. 



Both the vertical movements mentioned above and these 

 lateral movements change the surface features of the earth. 

 The former produce plateaus, plains, and broad depressions, 

 while the latter make mountain ridges with troughs between. 1 



Causes of crustal movements. What are the causes of 

 these movements? This question cannot now be answered 

 satisfactorily. The fact that these movements take place is 

 undeniable, but the causes of them are not yet fully under- 

 stood. 2 



EFFECTS OF MOVEMENTS 



Having now in mind the general nature of these slow move- 

 ments within the crust, we are in a position to study the effects 

 which they produce in the rocks. These effects may be 

 grouped as folds on the one hand and fractures on the other. 



Fracturing and folding of rocks compared. Rocks in gen- 

 eral are brittle substances. If quickly bent or squeezed, 

 they will break. If, however, the pressure is applied very 

 slowly, and especially if the layer is kept heavily weighted 



1 Mountains, plateaus, plains, troughs, and basins are formed, not only 

 by body movements, but in a variety of other ways, which are discussed in 

 later Chapters. 



2 The theories relating to crustal movements are discussed at some length 

 in larger textbooks, such as Chamberlin and Salisbury's Geology, Vol. I, 

 2d ed., Chap. IX. 



B. & B. GEOL. 5 



