496 R. T. CHAMBERLIN AND F. P. SHEPARD 



forming to this shrinking surface, necessarily was folded or faulted. 

 In many cases the axes of these folds were curved; in some places 

 they formed arcs as intricate as the mountain ranges in the East 

 Indies and southeast Asia. They suggest that arcuate folding may 

 be the characteristic way in which a globe is deformed by internal 

 shrinkage. 



It would seem probable, therefore, from these various experi- 

 ments, that mountain arcs are produced in several ways. A natural 

 tendency of a globe undergoing deformation to yield along curved 

 rather than straight lines is apparently a basal factor in the develop- 

 ment of such ranges. This factor would operate, either by produc- 

 ing curving geosynclines through a long chain of ancestral conditions, 

 or in some cases more immediately by developing curving folds 

 directly. Such arcs would have their curvature already outlined 

 in the early stages of the deformation. In addition, if the active 

 force is from one side, or more from one side than the other, a 

 progressive bending of the trend lines may occur as folding pro- 

 gresses, due to the holding back of the extremities while, with the 

 maximum thrust in the center, that portion moves forward in a hori- 

 zontal shearing movement, forming an arc which is concave toward 

 the active pressure. 



OVERFOLDS AND UNDERFOLDS 



In Suess' explanation of the arcuate mountain zones of Asia 

 and the festooned islands off the Asiatic coast, the active thrusting 

 was supposed to come from the inner side of the arcs. Hobbs, how- 

 ever, approaching the problem from a different point of view, made 

 the cardinal principle of his explanation active pressure from the 

 outside of each arc. His criticism of Suess was based on his belief 

 that there must be an actual stretching of the strata inside the arc 

 to make possible the outward migration of materials to form the 

 arc. Such an explanation, he believes, must require a noteworthy 

 thinning of the strata in the upper limbs of the overturned folds.^ 

 If, on the other hand, the active force be directed from without, he 

 believes that the under limbs of the folds (in this case underfolds) 



'W. H. Hobbs, Earth Evolution and its Facial Expression, Macmillan and Co., 

 1921, pp. 124-26. 



