258 J. LeConte — Origin of Normal Faults and 



Such displacements are called faults. The amount of vertical 

 displacement is often enormous, especially in the Basin and Pla- 

 teau regions. The vertical displacement on the north side of 

 the Uinta Mountains according to Powell is 20,000 ft, that on 

 the west side of the Wahsatch according to King is 40,000 ft. 

 In the Plateau region according to Button there are faults 

 extending for 200 miles with a vertical displacement of 2000 

 to 12,000 feet. It seems impossible to account for such faults, 

 unless there be a subcrust liquid. 



The displacement of these enormous crust-blocks did not 

 take place all at once, nor by uniform motion, but by a succes- 

 sion of slight slippings, each doubtless attended by an earthquake. 

 This is by far the most common cause of earthquakes. If it 

 were not for erosion, of course every fault would be marked by 

 a great cliff of height equal to the displacement. But in every 

 case the resulting cliff has been greatly lessened, and in many 

 cases entirely destroyed by erosion. If, however, the rate of 

 slipping has been greater than the rate of erosion a cliff will be 

 formed ; and if the time since the displacement was finished be 

 not too great, the cliff will still remain. The faults of the Basin 

 and Plateau region are on an enormous scale and are of compar- 

 atively recent origin, in fact are still growing. For this reason 

 fault-scarps form a very conspicuous feature of this region. 



Law of Faults. — If fissures be vertical so that the crust- 

 blocks are rectangular prisms, then one block may sink bodily 

 lower and another float bodily higher, giving rise to level tables 



Ideal section showing the geueral structure of the Plateau region. 



separated by fault- cliffs (fig. 1). This structure is so common 

 in the Plateau region that it may well be called Plateau-region 

 structure. In such cases the fissures being vertical we can have 

 no distinctive names for the two walls. But in nearly all cases 

 the fractures are more or less inclined, and therefore we have 



Normal Faults. 



Reverse Fault. 



an upper or hanging wall and a lower or foot vjall. Jfow in 

 by far the larger number of faults we find that the foot wall 

 has gone up and the hanging wall dropped down. These there- 



