262 



PHYSICAL GEOLOGY 



remained stationary or to have moved up. The vertical displace- 

 ment or the vertical distance between the ends of a dislocated 

 stratum is called the throw; the horizontal displacement, the heave. 

 The distance a stratum has moved on the fault surface is indicated 

 by the term slip. The upthrow side is the one in which the beds 

 lie at a higher level than their continuation on the opposite or 



Fig. 254. — Section across Yarrow Colliery, England, illustrating the law of normal 

 faults. The surface is restored. (After De la Beche.) 



downthrow side of the fault. These terms are used without refer- 

 ence to the actual direction of movement. The wall of the down- 

 throw side is called the hanging wall, and the opposite wall, the foot- 

 wall. These last two terms originated with miners, since in mining 

 along a fault the overhanging side was naturally spoken of as the 

 " hanging wall," while the side of the fault upon which they stood was 

 called the " footwall." The fact that the hanging wall in normal 



faults has moved down relative to the 

 footwall is utilized in coal mining. 

 For example, if a seam of coal sud- 

 denly disappears at a fault, the fault 

 plane is followed down or up, depend- 

 ing upon the inclination or hade of 

 the fault surface (Fig. 254). Normal 

 faults sometimes shade into mono- 

 clines (Fig. 248, p. 256). 



Examples of Normal Faults. — 

 The earth's crust is more or less 

 broken by faults; in some cases the 

 faulting has taken place so widely 

 f l*% 255 " T Mosaic ; f the r ° ck fl r and the various systems of faults ex- 



ot the lonopah mining district, Nevada, . ... 



produced by faulting. (After Hobbs.) tend in so many directions that a 



geological map of the region has the 

 appearance of a mosaic (Fig. 255). The Colorado Plateau (Fig. 

 256) has been broken by normal faults, some of which can be followed 

 for hundreds of miles and have a vertical displacement (throw) of 

 several thousand feet. Iii traveling along the Mohawk valley be- 



