200 WILLIAM H. HOBBS 



With the advent of failure in an anticHne a new resolution of 

 the active force of compression takes place with components parallel 

 and perpendicular to the slide, movement is facilitated, and the 

 potential energy of the system of stresses falls in consequence. The 

 acute angle which the active force makes with the sHde, and the 

 reduced resistance opposed to shear along that surface, alike favor 

 the underthrusting of the lower portion of the severed limb of the 

 anticline beneath the upper (Fig. 25, A). 



In the case of a well-laminated formation where a "stretch 

 slide" has occurred, the slide, being nearly or quite parallel to the 

 lamination, is extended along the lamination planes on either 

 side within the inferior and superior formations of the series. Since 

 the anticline contains the most steeply inclined of the layers, the 

 dip of the slide is progressively flattened as it passes out from the 

 antichne in either direction. For the case of a "break slide" 

 within massive sediments, the entry into these surfaces of least 

 resistance to sHde may be less easily made but in either case a 

 local swell of the slide surface may result. Both the low dip 

 angles and the undulations of slides are well established by observa- 

 tion in many districts. The entry of the slides into the bedding 

 planes of the formations tends to make of the severed rock forma- 

 tions a series of flatly disposed and widely extended slices capable of 

 individual lateral migrations in mass, which in view of their position 

 relative to the active force should be described as "underthrusting." 



Underthrusting of rock slices separated by slides simulated in 

 experiment. — If we represent the relatively thin slices, which are 

 separated by slides dipping at low angles away from the active 

 force, by a series of overlapping cards resting upon a stretched 

 rubber sheet, and the unfolded neighboring sections on either 

 hand by similar cards laid end to end as essentially continuous 

 and hence relatively rigid sections of strata (Fig. 33), we may illus- 

 trate the underthrusting of the strata if we allow the rubber sheet 

 to contract through releasing the tension upon it. With excessive 

 underthrusting the cards are piled into a low dome elevated by the 

 underdriving of the cards in proportion as the contraction of the 

 rubber sheet has been large. It may happen also that the cards 

 above and at the front dip forward and overlap at the forward end 



