6l6 STUDIES FOR STUDENTS 



the predominant force, and the fault is a reverse one (Fig. 15). 

 As the hade becomes steep, gravity has greater and greater rela- 

 tive power, and if the hade is very steep, gravity may be able to 

 overcome the tangential thrust, even if the latter is several times 

 as great as the former. So, also, if the hade is flat, tangential 

 thrust even much weaker than gravity may overcome it and pro- 



FiG. 14. — Normal or gravity fault. 



duce a reverse fault. This is one reason why, as a rule, normal 

 or gravity faults have steep hades, while reverse or tangential 

 faults have flat hades. 



There is, however, another reason. Since tension faults form 

 in the normal planes, they are usually steeply inclined or nearly 

 vertical. But the very idea of tension faults implies that there 

 is no thrust. Hence, gravity has its full effect, and the overhang- 

 ing side goes down with reference to the other side. It does not 

 follow, therefore, that all gravity faults are tension faults, 

 although this may be the case. Compression faults form in the 

 shearing planes, and they are therefore likely to be much inclined 

 to the vertical. In order that rupture shall occur the thrust must 

 be great, and hence compression faults are usually, and perhaps 

 always, thrust faults. 



Perhaps it would be well to classify faults as gravity faults 

 and thrust faults rather than normal and reverse faults, and thus 

 give them names which refer them to the predominant causes. 

 For the present this classification is preferable to the classifica- 



