PHYSICAL CHANGES OF THE OUTER SHELL 73 



faults or overthrusts are known to have been caused by the 

 excessive overturning of a fold (Fig. 56) ; but in other cases 



FIG. 54. Diagram of normal faults in a segment of the earth's crust. 



the rocks have merely been sliced through by a diagonal 

 fracture (Fig. 57). 



Reversed fault planes are usually not steeply inclined. 

 Some, indeed, are almost horizon- 

 tal. In the mountains of North 

 Carolina great masses of the older 

 rocks have been thrust along 

 gently inclined fractures for dis- 

 tances as great as 15 miles. 



Earthquakes. - The disloca- 

 tions which result in faults, es- 

 pecially normal faults, are often 

 felt at the surface as earthquake 

 shocks. Minute slippings in the rocks give rise to mere tre- 

 mors, which, although of common occurrence, are often 

 imperceptible to our senses. Their existence is detected by 



FIG. 55. Block diagram of a 

 normal fault. 



FIG. 56. An overturned fold passing into a reversed fault. 

 (After Heim.) 



means of a delicate instrument known as the seismograph. 

 Greater ruptures of the crust generate more violent shocks 

 which often dislodge huge masses of loose rock from moun- 



