108 



STRUCTURAL GEOLOGY. 



strata is represented. The surfaces of walls often become scratched or 

 '' slicJcensided " by the movement. 



107. 



108. 



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It cannot be affirmed in all cases that the downthrow or upthrust 

 exhibited in the beds was the whole movement, but only that it was the 

 final differential result of whatever up or down movement took place. 



Often there are many small faults in a group, as in the annexed figure ; 

 and the group may be of the downthrow or upthrust kind, though usually in 

 such cases, of the former. Frequently one or two blocks in the group of a 

 displacement has undergone a reverse movement ; but this does not change 

 the general character of the faulting. 



109. 



110. 



Faulted by beds. 



Fault with opened fissure filled with fallen 

 masses. Powell, '75. 



Pig. 110 (from Powell) shows a downthrow fault along a vertical 

 fracture ; moreover, the fracture is opened so as to become a wide fissure, 

 and the fissure is filled with masses from the inclosing rocks. For other 

 faults in fissures (veins), see pages 328-330. 



Downthrow faults are often called normal faults ; but only from the fact 

 that they are most common. The smaller faults are usually of this kind, 

 since gravity acts that way. The great faults, thousands of feet in dis- 

 placement, are often upthrust faults. Those in the Appalachian Mountain 

 region of Pennsylvania and Virginia have the upthrust of the enormous 

 extent above stated, 10,000 to 20,000 feet ; and the beds of the eastern side 

 would now have this great height above those on the opposite side were it 

 not that running waters of the sea and land (mostly the latter) had worn all 

 down to a common level. A section of one of these great faults of Virginia, 

 and the worn-off condition of the beds, is shown in Fig. 111. On one side of 



