RELATION OF EIGIDITY TO LOAD. 151 



proportional to the depth and the horizontal to rigidity, which is certainly 

 a function of the number of bedding planes and may be assumed to vary 

 inversely as the number. The maximum rigidity is in the middle portion of 

 the column, and on either side are divisions with minimum rigidity. In the 

 upper portion there is an increase to medium rigidity. 



From considerations given in the description of the faults it seems prob- 

 able that the strata were subjected to a certain amount of folding, and prob- 

 ably erosion also, prior to the thrust. With these facts in view, the ideal 

 section forming figure 3, plate 3, may be regarded as representing the atti- 

 tude of the strata when the faulting began. It will be seen that if horizontal 

 compression were applied, the line of weakness, and hence the line of least 

 resistance, which a fracture would certainly follow, is that marked by the 

 broken line P P\ 



But the most obvious result which would be expected from the application 

 of further compression to the mass of strata represented in figure 3 is simply 

 a continuation of the folding. The reason that such was not the result pro- 

 duced must be sought in the relation between rigidity and load. The layer 

 B has a high degree of rigidity, while the load under which it rests is com- 

 paratively small — not enough, certainly, to render it in any degree plastic. 

 Hence the formation of an incipient fold, as at L, would be accompanied by 

 considerable fracturing of the strata on the steep side ; and so a point would 

 be reached early in the process beyond which folding could not proceed, 

 since the slipping upon bedding planes necessitated by the fold would offer 

 greater resistance than fracture across the beds. Further compression would 

 necessarily be taken up by a fault shearing across the weakened rigid bed. 



That such is the correct explanation is clearly shown by Willis,* both 

 experimentally and from a consideration of the stratigraphic relations in 

 faulted and folded areas in the Appalachian region. 



Lateral compression is readily taken up in a mass of fissile shales by close 

 folding, and hence the fracture would not penetrate to a great depth in the 

 lower mass of minimum rigidity. It is on this account probably that, even 

 with the great displacement which these faults show, the rocks underlying 

 the Cambrian are never brought to the surface ; and there is no evidence 

 that the fracture originated much below the lowest beds now exposed. 



As already stated, the rigid mass B presents its weakest points where the 

 compressing force exerts a shear across the beds — i. e., on the sides of the 

 folds If and L. But the point if is in the line of least resistance, since it is 

 nearest to the region of application of the compressing force, and hence the 

 mass of material to be moved is less than if the break were to occur at L. 

 After passing the central rigid mass the line of least resistance follows the 

 upper beds of minimum rigidity C till another fold is reached where it passes 



♦Report on Experiments in Structural Geology; by Bailey Willis (unpublished). 



