422 REVIEWS 



synclinoria of the various normal and abnormal types. Even one toler- 

 ably familiar with the aspects of folds must stop to draw a diagram 

 before he can remember what the definition of a given type is. 1 



A definition of normal and abnormal folds, which may appear 

 simpler, may be based on relative dips and depths of folds. It 

 depends upon a law stated by Heim, 2 which is that from a normal 

 anticline a stratum dips more steeply toward that syncline into which 

 it descends to the greater depth. Thereon we may phrase the defini- 

 tion : 



A normal fold is one in which the steeper dip is toward the deeper 

 syncline of the same order, whereas an abnormal fold is one in which 

 the steeper dip is toward the shallower syncline of the same order. 



This definition has one advantage over that stated by Van Hise. 

 It is related to the initial conditions which determine the positions of 

 folds and their normal or abnormal development, whereas the axial 

 planes are mathematical conceptions only, without actual existence. 

 The relative depths of synclines and the steeper dips are frequently 

 determined in the initial stages of development of folds, and persist 

 as controlling conditions. In any littoral zone the steeper initial dip 

 is usually from the land toward the sea. The resulting steeper dip 

 after deformation holds a corresponding relation. 3 



Van Hise introduces a new use of the common term "mono- 

 clinal," in that he applies it to describe "that structure of a mountain 

 mass in which the axial planes of all folds are inclined in the same 

 direction." Discussing "monoclinal structure" he discriminates 

 between overthrust folds and underthrust folds as follows, page 621 : 



"In monoclinal structure the force, and consequently the move- 

 ment of the strata have usually been supposed to be more largely from 

 one direction than from the other, and the axial planes of the folds 

 have usually been regarded as dipping toward the force." 



After reference to the ideas of H. D. Rogers, Van Hise continues, 

 page 622 : 



1 It has been suggested to me by Mr. L. C. Glenn that a general expression of 

 Van Hise's law of normal folds may be phrased as follows : In a normal fold the 

 axial planes converge centrically with reference to the arc which corresponds to the 

 curvature of the anticlinorium or synclinorium. In an abnormal fold the axial planes 

 converge eccentrically. 



2 Op. cit., pp. 231-233. 



3 Conditions of Appalachian Faulting, Willis and Hayes, Am. lour. Sci., Vol. 

 XLVI, October 1893. 



