SOME EXPERIMENTS IN FOLDING 491 



folding, and is far better adapted to experiments on faulting. On 

 the other hand, a layer of pure vaseline between stronger layers is 

 extremely incompetent. The competency of experimental materials 

 can thus readily be regulated by varying the relative proportions 

 of the softening and hardening constituents. In some cases a small 

 amount of plaster of Paris was added to develop the most competent 

 layers. In preparing the succession of strata each layer was molded 

 separately in a wooden frame having a paper bottom. With the 

 materials used the layers were sufficiently plastic to weld together in 

 a firm block of strata which was then placed in the compression box. 



Criticism has sometimes been leveled at compression-box experi- 

 ments, but the opinion may perhaps be ventured that observation 

 of the stages of deformation of three-dimensional blocks gives one 

 a much clearer conception of the processes of folding than can be 

 obtained by a study of field sections. In addition there were many 

 interesting and unexpected developments which arose independently 

 of the main purpose of the investigation. 



Mead has suggested that pressure-box experiments do not make 

 accurate reproductions of folding as it occurs in nature, because 

 they do not take into account the shrinkage of the earth beneath 

 the mountain zone.^ In Mead's very neat and effective experiments 

 a tightened rubber band with a paraffin coating was allowed to 

 shrink and thus deform the paraffin. This method would seem to 

 rest on the assumption that the sub-crustal portion beneath the 

 mountain range is shortening by compacting as much as is the moun- 

 tain range by folding, but one may perhaps urge the alternative 

 hypothesis that the shortening by compacting of the sub-crustal 

 zone is more widely distributed under plain and mountain range, 

 while in the crustal portion the shortening tends to be concentrated 

 in the mountain ranges. On this hypothesis the compressive move- 

 ments which produce the folding are not only the product of the 

 compacting beneath the ranges, but also to a greater degree the 

 product of the accumulated movement of the non-mountainous 

 parts of the crust toward the growing mountains. The very com- 

 mon location of mountain ranges parallel or concentric with conti- 



^ Warren J. Mead, "Notes on the Mechanics of Geologic Structures," Jour, of 

 Geol., Vol. XXVIII (1920), p. 507. 



