TEREANES. 



109 



the fault F are coal-beds, on the other, one of the lower limestones of 

 the geological series, which, by upthrust action, has been put on a level 

 with the coal-formation. By the same forced movements, downward dis- 

 placements or faults are sometimes made, and these have been distinguished 



111. 



Section of the Paleozoic formations of the Appalachians, in southern Virginia, between Walkers Mountain 

 and the Peak Hills (near Peak Creek Valley) : F, fault; a. Lower Silurian limestone; b, Upper Silurian; 

 C.Devonian; d, Subcarboniferous with coal-beds. Lesley. 



from the gravity-made downthrow faults by using the term downthrust fault 

 (E. A. Smith). 



The following figures show that after erosion the same surface features 

 may result from a downthrow (Fig. 112) along a vertical fracture and from 



112. 



113. 



\- ■■• 













= 



-:\:;;:::::::;^\;:;/; 



114. 



115. 



an upthrust (Fig. 113) along an oblique fracture; the dip of the fracture- 

 plane is here about 25°. 



ISTot unfrequently a flexure changes, in one direction or the other, into a 

 fault, showing that the force causing the break first produced, as is natural, 

 a bend. Many examples of such flexure-faults have been described by Major 

 Powell, and later by others, from the plateaus 

 of Colorado, where the absence of vegetation 

 and soil affords unusual opportunities for ob- 

 servation on the positions and inside con- 

 dition of strata. A bend (Fig. 114, from 

 Powell) represents, ideally, the upper layer 

 of a region of a low anticline in the eastern 

 part of the Uinta Mountains. The bend in 

 the part to the right shows that a fracture 

 is begun ; and in Fig. 115, which represents 

 the same line of faulting, the actual displacement amounts to thousands of 

 feet. 



A succession of monoclines along faults produces, in the region of the 

 Colorado plateaus, the features shown in Fig. 116, from Powell ; and Fig. 

 117, from the same region, illustrates a section across a large fault having 

 two branches. 



E 



Flexure-fault. Powell, '76. 



