MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 167 



all such cases, there is a tendency to produce rotation, as appears 

 from the example of the water wave. So far as the form of the 

 resulting fold is concerned, the result would be similar if the active 

 and passive forces were to be reversed; and though conscious of an 

 appearance of presumption in again opposing his own view to such 

 weight of authority, the writer will endeavor to show not only that 

 the principal active force involved in the folding of the Alps must 

 have been directed from the northwest toward the southeast,^ but 

 that the mechanical difficulties which have stood in the way of a 

 more general acceptance of the views of the Swiss geologists, with 

 this modification in large measure disappear. 



Fig. 9. — Diagrams to illustrate the active and passive forces involved in folding: 

 a, position and direction of forces involved in the overturning of a free water wave; 

 h, system of forces assumed by the Swiss geologists to account for the folding of the 

 central Alps; c, the author's modification of this view. 



It is of course to be understood that the active force tending to 

 produce movement may not be solely from a single direction; but 

 of the two opposed directions parallel to the chief compression, the 

 active force as here understood is that one which represents the 

 greater movement. If in Fig. 10 two active forces or thrusts^ 

 which tend to compress a given section of the earth's shell be 

 represented in intensity by the distances a and h, the active force 

 which becomes effective in producing unsymmetrical flexures such 



I Willis has expressed his opposition to the conception of overturning which is 

 apparently the standard doctrine of the day ("The Mechanics of Appalachian Struc- 

 ture," X2,th Ann. Rept. U.S. Geol. Siirv., 1893, Pt. II, p. 233); see also W. H. Hobbs, 

 Earth Features, New York, 191 2, pp. 436-38. 



' This term is not to be confused with that generally applied to the surface of 

 failure in folds, which latter should, we believe, be abandoned for reasons which will 

 be given below. 



