Appalachian Faulting. 263 



region and has been most fully described by Heim from exam- 

 ples observed in the Alps. 



This explanation of the origin of stepfolds rests upon con- 

 ditions of sedimentation along shore and may suffice for those 

 cases in which the observed facts of stratigraphy indicate the 

 antecedent development of a syncline of deposition. But 

 geologists familiar with the number and relations of Appala- 

 chian folds and thrust- faults may fairly object to the assump- 

 tion of a shore line and an original syncline for each one. A 

 supplementary explanation is required and it must account for 

 the extraordinary parallelism generally existing among the 

 axes. This explanation may be found in the results of the 

 development of competent anticlines. 



Competent structure. — When a force tangential to the earth's 

 surface affects a stratified mass, it is transmitted by the strata 

 in the direction of their lateral extent and by each stratum 

 according to its rigidity. To illustrate, if pressure be applied 

 to the side of a pile of tissue paper in which are leaves of 

 card board of the same size, the predominant effective pressure 

 apparent on the other side is that transmitted by the card- 

 boards. The stratum which thus most effectively transmits 

 pressure may be called the competent stratum ; and it is plain 

 that the term "competent" is significant of relative rigidity, 

 not of an absolute quality. In the Appalachian province the 

 competent stratum was the great limestone. 



If the pressure transmitted by a competent stratum arrives 

 at a bend due to initial dip, the force is resolved into two com- 

 ponents ; the one acts in the direction of the stratification, the 

 other acts in a line normal to the stratum, or radially. If the 

 curve be convex upward it will develop into an anticline of 

 deformation when the radial component of the force is strong 

 enough to bend the strata and carry up the superincumbent 

 load. The strength of this component is determined by the 

 strength of the original force and by the initial curvature of 

 the stratum. No anticline can develop unless the radial com- 

 ponent of the force for a given original curvature is equal to 

 the task, nor for a given force unless the original curvature is 

 sufficient to deflect a competent component. If these condi- 

 tions be fulfilled the competent stratum will rise in anticlinal 

 form and will carry the overlying load on its crown. The 

 zone beneath the crown is then relieved of load, which is 

 transferred to the limbs.* 



If the stratified series be made up of many beds of nearly 

 equal competency alternating with relatively incompetent beds 

 such as thin interbedded limestones and shales, then each com- 



*H. Schardt, T?ull. de la Societe Vaudoise des Sci. Nat., vol. xx, 1884. pp. 

 143-146. B. Willis, Mechanics of App. Structure, Xlllth Ann. Rep. U. S. G. S. 



