i8o 



WILLIAM H. HOBBS 



that after the antichne has begun to rise, a couple arm separates 

 the two opposing forces and that the point of application of the 

 active force of compression is below that of the passive force of 

 resistance. So soon, however, as underturning has set in, a new 

 couple enters which involves not the compressive forces in generally- 

 horizontal positions, but the vertical external forces; namely, the 

 load and the passive resistance of the mass to depression (Fig. 20). 

 Within the underturned portion of the antichne this passive force 



Fig. 20. — Diagrams to show the new couple which enters with the underturning 

 of anticlines. 



may be concentrated near the base of the under limb, while the load 

 upon the underturned section of the arch is centered in front, so 

 that the action of the couple tends to rotate the crown of the arch, 

 not forward as before, but downward. Such sinking of the crown 



is resisted by any support of 

 the superior strata which have 

 been underfolded beneath the 

 crown. If these are of suffi- 

 ciently strong material, the 

 crown is not bent; but in 

 the event of their being 

 weak, the crown is sunk as 

 in Fig. 21. Such "plunging crowns" {tete plongeante, sinkende 

 Gewolbe) are the normal feature in the northern zone of the 

 Alps, where the weak Flysch (Eocene) overhes the competent 

 Helvetian limestones.^ 



Whether the crown be sunk or not, as the antichne becomes 

 increasingly underturned, it is forced down as a whole by the action 

 of this couple and so becomes a "recumbent fold" (Fig. 13, stage 9). 



' For another excellent example of a plunging anticline crown see E. B. Bailey and 

 M. Macgregor, "The Glen Orchy Anticline," Quar. Jour. Geol. Soc, LXVIII (1912), 

 164-78, PI. 10, and especially Fig. 3. 



Fig. 21. — Plunging or sinking crown of an 

 underturned anticline due to weak strata 

 above the competent member. 



