of the Structure of the Basin Region. 



259 



fore are called normal faults (figs. 2 and 3). In many cases 

 however, especially in strongly folded and crumpled strata 

 whether in existing mountains or in the places of extinct moun- 

 tains, we find faults in which the hanging wall has slidden 

 upward and forward over the foot wall. These therefore are 

 called reverse faults (fig. 4). 



Theory of Faults. — The explanation of the reverse faults 

 seems obvious enough. They occur, as we have already said, 

 mostly in strongly folded regions. Such folds can only be 

 produced by lateral pressure. The pressure when extreme 

 often produces overfolds. If such overfolds break, the dip of 

 the fissure will be toward the direction from which the pressure 

 came and the hanging wall he pushed forward and upward over 

 the f ootwall by the sheer force of the lateral thrust (figs. 5, 6 



Diagrams showing made of formation of reverse faults. 

 (After DeMargerie and Heim.) 



and 7). Extreme examples of this are found in the Scottish 

 highlands in which the plane of dis23lacement is nearly or quite 

 horizontal. These are called by Geikie thrust-planes (fig. 8). 



But the explanation of normal faults which are by far the 

 most common is not so obvious. I will give very briefly what 

 seems to me the simplest explanation — an explanation which I 

 have used in my class lectures for many years. 



Suppose then the earth-crust in any place to be not crowded 

 together by lateral pressure, as in the formation of mountains of 

 the Appalachian type, but uplifted into an arch by intumes- 

 cence of the subcrust liquid. Such local intumescence of the 

 subcrust liquid may be the result {a) of elastic force of steam 

 incorporated in the magma in more than usual quantity by 

 the access of water from above, or (b) of hydrostatic pressure 

 transferred from a subsiding area in some other perhaps distant 

 place. Such an arch being put upon a stretch would be broken 

 by long fissures more or less parallel to one another and to the 

 axis of uplift into oblong prismatic crust-blocks many miles in 

 extent. After the outpouring of liquid lava or the escape of 

 elastic vapors had relieved the tension, these crust-blocks 

 would again be re-adjusted by gravity. If the blocks are rect- 

 angular prisms, some may float bodily higher and some sink 

 bodily lower, giving rise to level tables separated by fault cliffs 

 as in the Plateau region already explained. But if the fissures 



