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WILLIAM H. HO BBS 



as are the rule in all much compressed mountain districts is the 

 difference between the two forces, c. Had the active forces a and 

 b been just equal and the beds under compression offered uniform 

 resistance, the folds produced should be symmetrical, and in the end 

 have constituted a series of vertical isoclinal flexures, which are as 

 rare in nature as they would upon this assumption be expected to 

 be (Fig. ii).^ The lack of a fore-and-aft symmetry in mountain 

 arcs clearly indicates that the radially directed thrusts are not in 

 equilibrium, but that one notably overbalances the other. 



Fig. io. — Diagram to illustrate the re- 

 sultant of two parallel but opposed active 

 forces, or that effective in producing un- 

 symmetrical flexures. 



Fig. II. — Vertical isoclinal folds 



Incompetence of folding layers to transmit compressive stresses to 

 long distances. — -At the outset it is important to emphasize that the 

 capacity of bodies to fold shows that they cannot transmit com- 

 pressive stresses to long distances, due to the dissipation of energy 

 in producing internal strains within the folding mass. With 

 constant diminution of intensity, therefore, the active force is 

 transmitted to certain moderate distances only from its place of 

 application. Although perhaps self-evident, this fact has been 

 demonstrated in experiments by Cadell, who has expressed its 

 application to folding strata in the following sentence: "Horizontal 

 pressure applied at one point is not propagated far forward into a 

 mass of strata."^ The distance to which the active force is carried 



' This discussion obviously rests upon the assumption that the folding of any given 

 district is due to forces from without the area itself, though independent of any special 

 theory of planetary contraction. Any assumption which requires that the active force 

 causing compression originate within the district itself requires a wholly different 

 analysis. Thus the view of Mellard Reade, which conceives the cause of folding to be 

 expansion by heat of areas of sediments due to depression and consequent rise (relative 

 to the beds) of isogeotherms, would require that the active forces proceed outward 

 toward relative rigid formations, and zones of folding should develop simultaneously 

 on opposite margins of the more plastic interior area. 



'Henry M. Cadell, "Experimental Researches in Mountain Building," Trans. 

 Roy. Soc. Edinburgh, XXV (1890), 356. 



