72 WILLIAM H. HOBBS 



Initiation of New Anticlines in Series 



Conditions of Formation of Plunging Crowns and Recumbent Folds 



Attenuation of Under Limb of Anticline after Underturning 



Possible Formation of a Magma Macula beneath an Anticline 



Reduction of Volume of Anticline an Efficient Cause of Elevation of 



Magma 

 Backfolding Anticlines 



Part III 



The Folding Process Studied in the Profile — Formation of Slides 



Imperfect Plasticity of Folding Strata 



Contrasted Cases of Isotropic and Anisotropic Strata — Control of Internal 

 Strains by Lamination 



Shearing Movements within an Anticline for the Contrasted Cases of 

 Isotropic and of Well-laminated Strata 



Earlier Failure in Anticlines of Unlaminated or Poorly Laminated Strata 



Slides and Their Modified Direction after Leaving the Competent Member 



Underthrusting of Rock Slices Separated by Slides Simulated in Experi- 

 ment 



Formation of Drag Folds and Listric Surfaces at the Front of Under- 

 thrust Slices 

 Summary 



PART I 



INTRODUCTION 



The Alps the European type of the Asiatic mountain arc. — • 

 Studies of mountain structure have made use chiefly of profiles, 

 or cross-sections, at right angles to the crests of the ranges. In 

 recent years and to a relatively small extent only, longitudinal 

 sections have also been figured in order to take account of the 

 directions of pitching folds. In the geological map the plan of 

 the region is of course represented, but chiefly with a view to fixing 

 the areal distribution of the formations; and so far as known the 

 discussion of the mechanics of the folding process has been restricted 

 to the two dimensions included in the normal profile. While the 

 reason for this may be in part the difficulty of representing more 

 than two dimensions of space, it is no doubt due largely to a general 

 belief that all significant elements in the problem can be properly 

 set forth in the transverse section. 



