MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 



199 



formation at an early stage and near the point of inflection of the 

 lower limb; whereas anticlines in tough laminated strata persist 

 until strongly underturned and then fail only after excessive stretch- 

 ing of the under limb and at an angle with the base which would 

 indicate that the adjustments of individual particles were nearly 

 or quite parallel to the laminae. These contrasted examples illus- 

 trate respectively Willis' types of "break thrusts" and ''stretch 

 thrusts"^ (Fig. 32). 



The second arch within a series of anticlines may in its turn 

 fail in a manner similar to the first, and others of the series in 

 succession ; thus yielding a 

 series of slices which in 

 early stages at least dip 

 away from the active force 

 of compression and are 

 described as "imbricated 

 structure" {Schuppenstruk- 

 tur, Fig. 2S,B). 



Slides and their modi- 

 fied direction after leaving 

 the competent member.- — 

 The names in most general 

 use, or at least in longest 

 use, for the surface of fail- 

 ure within an anticline are 

 "thrust"and "overthrust." 

 The former is in every way undesirable because so likely to be 

 confused with the same term in general use in mechanics for 

 an active force; while the latter term is further objectionable 

 in that it assumes that the active force responsible for the surface 

 of failure operated from above and behind the anticline. Suess's 

 term "sole" (Sohle), now employed by many, is without these 

 objections, but the word "slide" recently suggested by Bailey^ is 

 perhaps even better. 



• Willis, op. cit., p., 223. 



^ E. B. Bailey, "Recumbent Folds in the Schists of the Scottish Highlands," 

 Quart. Jour. Geol. Soc, LXVI (1910), 593. 



Fig. 32. — Diagrams to illustrate failure in 

 anticlines; a, in an isotropic member such as 

 limestone — break slide, or "break thrust"; b, 

 in a tough laminated member such as schist — 

 stretch slide, or "stretch thrust." 



