STRUCTURAL GEOLOGY 



521 



great, they probably give place to folds below (Fig. 421) . Other phenom- 

 ena of faulting are illustrated by Figs. 430-435. A fault of thousands, 

 or even hundreds of feet is probably the sum of numerous smaller sUp- 

 pings distributed through long intervals of time. Faulting is probably 

 one of the common causes of earthquakes. 



The significance of faults.^ — Faults afford a valuable indication of 

 the conditions of stress to which a region has been subjected, but some 



Fig. 433. — Figure showing minute faulting. The length of the specimen is 8 inches. 

 The number of faults is nearly 100. (Photo, by Church.) 



iipii ll i l [llilili ll^^ ^^ I , -V-l-r-V^ 



-• • l\V*y •"•*• * •niE^" ^'Tr'^'''''r''''''*'l'"^^ 



Fig. 434. Fig. 435. 



Fig. 434. — Diagram illustrating common phenomena of a faulted region. (Dana.) 

 Fig. 435. — Diagram showing a fault, the plane of which forms an open fissure and 

 has been filled with debris from above. (Powell.) 



caution must be exercised in their interpretation. Normal faults 

 usually indicate an extension of the surface sufficient to permit the 

 fault-blocks to settle down unequally. Reversed faults usually signify 

 a compression of the surface which requires the blocks to overlap one 

 another more than they did before the faulting. In other words, nor- 

 mal faulting usually implies tensional stress, and reversed faulting com- 

 pressional stress. It is not difficult to see, however, that in an intensely 

 compressed and folded region there might be cases of normal faulting 

 1 Reference, Van Hise. Sixteenth Ann. Rept. U. S. Geol. Surv., Pt. I, pp. 672-678. 



