CHAPTER XIII 

 DISLOCATIONS AND FRACTURES OF STRATA 



Strata are often unable to accommodate themselves by bend- 

 ing or plastic flow to the stresses to which they are subjected : 

 and instead of flexing they are fractured, usually accompanied 

 by more or less dislocation. A simple crack or fracture through 

 the strata, not involving dislocation, is called a fissure. On the 

 two sides of a fissure the beds are the same at corresponding 

 levels, and evidently the crack has been made through continuous 

 strata. 



Faults. — When the strata on one side of a fissure have 

 been lifted lip or dropped down, so that the strata which were 

 once continuous across the plane of fracture are now separated 

 by a greater or less vertical interval, and lie at different levels, the 

 structure is called a fault. A fault is sometimes, but not very 

 often, vertical ; usually it is inclined at a greater or less angle, 

 and the angle made by the intersection of the fault plane with 

 a vertical plane is called the hade or slope of the fault. The 

 fault dip is the angle included between the fault plane and a 

 horizontal plane, and is the complement of the hade. For 

 example, if the fault is vertical, the hade is o° and the dip 90 ; 

 if the fault is horizontal, the hade is 90 and the dip is o°, 

 while a hade of 45 ° gives a dip of the same amount. The side 

 on which the beds lie at a higher level than their continuations 

 across the fracture is called the upthrow side, and the other is 

 the downthrow side. Owing to the obliquity of the fault plane, 

 the beds on one side usually project over those on the other, and 

 hence form the hanging wall; that side which extends underneath 

 the other is called the foot wall. Either the foot or the hanging 

 wall may be the upthrow or downthrow side, according to the 



243 



