DEFORM A TION OF ROCKS 6 1 5 



before the upper two series were deposited, and that the two sets 

 of joints found in the lower series, one of them also affecting the 

 middle series, and both absent in the upper series, were present 

 before the latter series was deposited. That is, the lower series 

 was subjected to three orogenic movements, the middle series to 

 two, and the upper series to one. Considering that two or more 

 sets of joints may be developed by a single orogenic movement, 

 it would seem that such a conclusion should be supported by 

 other criteria. 



FAULTS. 

 ORIGIN OF FAULTS. 



Faults differ from other rock fractures, in that there is import- 

 tant dislocation along the fractures and often also they are far 

 more extensive. Like joints, faults may be classified into tension 

 faults and co7npression faults, the first forming in the normal planes 

 and the second in the shearing planes. Faults are, however, usu- 

 ally defined as normal and reverse. A normal fault is one in 

 which the overhanging side descends in reference to the other, 

 while in the reverse fault the overhanging side ascends in refer- 

 ence to the other. Another term applied to reverse faults is 

 thrust faults, implying that tangential thurst is the controlling 

 factor. As equivalent to normal fault may be placed the gravity 

 fault, implying that gravity is the predominant force. 



In the case of the normal fault the overhanging side has a 

 smaller base than the other. Consequently by force of gravity it 

 descends, as compared with the other side. In all cases both of 

 normal and reverse faults, gravity is a never-ceasing force. At 

 first explained by Le Conte,^ the principle of the inclined plane 

 thus applies to these two forces, the hade of the fault giving the 

 inclination of the plane. Where the hade is greater than 45° 

 if the forces of gravity and tangential thrust are equal the fault 

 is normal, because gravity controls the movement (Fig. 14). If, 

 on the other hand, the hade is less than 45°, tangential thrust is 



' On the origin of normal faults, and of the structure of the Basin region. Joseph 

 Le Conte. Am. Jour. Sci. (3), Vol. XXXVIII, 1889, pp. 257-263. 



