198 WILLIAM H. HOBBS 



upward, thus indicating that there has been tension upon the convex 

 side and compression upon the concave side of the stratum. These 

 relations are reversed in the syncHnal portion. The points of 

 inflection of the double curves into which the guide circle diameters 

 have been transformed define a surface within the stratum which 

 has been neither expanded nor contracted and is known as the 

 "zero surface." This surface is in the true anticUne section below, 

 and in the syncHne section above the median surface of the stratum.' 



Earlier failure in anticlines of unlaminated or poorly laminated 

 strata. — At the point of inflection in the steeper antichne limb, 

 deformation is clearly at a maximum, and failure, if it is to occur, 

 will be located here. The enlarged diagrams a and b of Fig. 31, 

 which set forth the original and the deformed conditions of the 

 guide circles at this critical point, show that there are two exactly 

 similar common diameters to the circle and ellipse and that the 

 observed deformations might have taken place through migrations 

 of individual particles either in one or in the other of two ways, 

 dependent upon which offered the minimum of resistance. Obvi- 

 ously the sHght cohesion to be overcome by the shear of the paper 

 laminae over each other has determined that in the case of our experi- 

 YnQnt — a well-laminated (anisotropic) stratum — adjustments shall 

 take place parallel to the laminae, for which reason failure is Httle 

 likely to result until a late stage of the antichne has been evolved. 

 Had our stratum been, on the other hand, without lamination planes 

 (isotropic), adjustments would have taken place in the sense of 

 diagram b, and failure would have occurred parallel to the other 

 common diameter in the figure as an apphcation of the principle of 

 greater weakness on the section of minimum area. 



As an apphcation of these considerations we find that unlami- 

 nated strong members Uke limestone fail in the process of anticline 



I When well-laminated rock formations lie beneath a competent member of heavy 

 massive rock like limestone, the strong tendency to shear along lamination surfaces 

 may result in a complicated puckering of the laminated inferior formation even though 

 the arch in the competent formation remains comparatively simple. This phenomenon 

 so often observed in folded mountain regions and reproduced by Willis in his experi- 

 ments {The Mechanics of Appalachian Structure, PI. 90) involves an attenuation of the 

 puckered laminae. It is also to be observed that the puckerings or plications are 

 concentrated in the crown of the anticUne where tension in the stratum protected 

 may supply the space necessary for duplication by plication. 



