SOME EXPERIMENTS IN FOLDING 



495 



tendency was not very marked. In general the trend of the fold 

 did not conform to the shape of the rigid arc. 



This would suggest that the curving mountain chains flanking 

 the more rigid Tibetan mass do not, in the main, owe their concen- 

 tric trend directly to the outline of the resisting mass. More likely 

 in most cases it would seem that the belts of heavy sedimentation in 

 geosynclinal tracts bordering the periphery of an elevated land mass, 

 which also should have a slight curvature, may be the real factors 

 which have determined the curving chains subsequently developed. 

 A more or less circular or rounded land mass should naturally yield 

 heavy sedimentation along its borders. Because of the relative 

 weakness of these zones of sediments, and because of other factors 



Fig. 3. — Result of deforming model between pressure board and rigid plaster mass. 

 Folds contoured. Dash line indicates crest of principal anticline. 



which cannot be discussed here, folding into mountain ranges sub- 

 sequently appears in these marginal belts. Then, in the long geo- 

 logic history the destruction of one range furnishes the material close 

 at hand in more or less concentric belts, for the elevation of the next 

 chain in some subsequent orogenic outburst. These successive 

 generations of mountain chains follow a sort of cycle. The details 

 of trend in a given range thus may be foreshadowed in part long in 

 advance, and constitute an inheritance from a long sequence of 

 ancestral conditions. 



The problem of arcuate folding was studied from another angle. 

 Inflated toy balloons were coated with paraffin by immersion in the 

 hot liquid. After the parafiin had cooled somewhat, but was still 

 quite plastic, the air in the balloon was allowed to escape slowly, 

 causing the surface of the balloon to shrink. The paraffin, con- 



