HOW MOUNTAINS ARE MADE 



409 



We know that the crust of the earth is of rock, because if we 

 dig deep enough, we leave soil behind and reach hard rock 

 (Fig. 274). In deep mines, rock extends everywhere and no 

 soil is seen. As a result 

 of some internal force, 

 masses of rock slip and 

 slide upon each other, and 

 cause changes in the land. 

 These changes may be 

 slight, such as those which 

 are taking place at Stock- 

 holm to-day, or they may 

 be conspicuous like those 

 which caused great changes ^ "^ _ Earthquake crack . 



in the past. Sometimes 



the rocks slide in such a way that fissures or cracks (Fig. 275) 

 occur at the surface similar to cracks in a warping board. 

 Sometimes the rocks are displaced or faulted in such a way 



FIG. 276. Folded beds of limestone. 



that they are compressed against each other and are warped or 

 folded (Fig. 276). Very much as the skin of an apple wrinkles 

 on drying, the earth's crust may wrinkle or fold as a result of 



