MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 201 



those lower in the series. This latter feature is a well-known peculi- 

 arity of the Deckenbau upon the northern border of the Alps 

 (Fig. 34), and it is clearly favored by the sinking crowns of recum- 

 bent anticlines. 



Formation of drag folds and listric surfaces at the front of under- 

 thrust slices. — The new system of stresses, which is inaugurated with 

 the process of underthrusting, may lead to the formation of second- 

 ary folds and eventually to secondary sHdes within the mass beneath 

 which a slice is driven. This driving-in of one slice after another 

 not only produces friction breccia ("mylonite") at the contacts, 



J^-^ 



Fig. 33. — Diagram illustrating underthrusting of slices 



but pushes upward and tends to fold under the frontal portion 

 particularly of each underdriven slice. The active force of com- 

 pression being still directed from in front of the folds and below, 

 the "drag folds" which result ( Figs. 34 and 35) should be under- 

 turned from the front, attenuated in the under turned lower limb, 

 and eventually underthrust in that limb, much as are the folds of 

 larger order of magnitude upon whose fractured remnants they have 

 been superimposed. Instead, however, of being extended for any 

 long distance on bedding planes, these secondary slides should here 

 converge into the subjacent major slide. The shovel-Hke curvature 

 which is so generally characteristic of them has suggested the name 

 "listric surfaces" appHed to them by Suess^ (Fig. 35, Rigihochfluh 

 and My then, and Fig. 37). 



' Listrische Flachen {Das Antlitz der Erie, III, Pt. II, p. 61 2 ; The Face of the Earth, 

 IV, 536). 



