F.U'LTS 



283 



plains to a distance of at least seven miles. 1 Overthrusts of com- 

 parable displacement have been detected in Scotland 2 and 



Some faults branch, and in some cases the faulting is along a 

 srnVs of parallel planes near one another, instead of being along a 

 single plane. Such a fault is 

 distributive (Fig. 274). Faults 

 are found to die out when 

 traced horizontally, in some 

 cases by passing into mono- 

 clinal folds (Fig. 275), and in 

 some cases without connection 

 with folding. In depth they 

 probably die out in various 

 ways (Fig. 276). A fault of 

 thousands, or even hundreds of feet is probably the sum of numer- 

 ous slight slippings distributed through long intervals of time. The 

 faulting along one plane may be the cause of many earthquakes. 



Fig. 274. A branching fault. 

 U. S. Gcol. Surv.) 



(Powell, 



Fig. 275. Diagram of a fault pass- 

 intr into a monoclinal fold. 



Fig. 276. The fault above 

 grades into a fold below. Thick- 

 ening and thinning of layers next 

 the fault-plane is evident. Based 

 on experiments of Willis. (i3th 

 Ann. Kept., U. S. Geol. Sun-.) 



The rock on either side of a fault-plane may be smoothed as 

 the result of the friction of movement. Such smoothed surfaces are 

 dickensidcs. 



The significance of gravity and thrust faults. 3 Faults afford 

 an indication of the conditions of stress and tension to which a 

 has been subjected, but some caution must be exercised in 



1 Willis, Bull. Geol. Soc. of Am., Vol. XIII, pp. 331-336, and McConnell, 

 Canada (ir.l. and Nat. Hist. Surv., 1886, Pt. II. 



( ieikie, Text-book of Geology. 



* Van Hise, Sixteenth Ann. Kept., U. S. Geol. Surv.. Pt. I, pp. 672-678. 



