14 C. R. VAN HISE 



curve on the convex side of such a central line would equal the 

 deficiency on the concave side, and consequently such a 

 developed layer truly represents its mass and average length. 

 If it be assumed that the orisfinal horizontal stratum had a 



d' 



c' 

 Fig. 2. — Development of a part of a layer of Fig. i. 



thickness equal to the thicker parts of the folded stratum, it 

 would follow that the developed stratum is much longer than it 

 was originally. However, probably in no case is this assump- 

 tion wholly correct, and in many cases it is far from the truth. 

 The weaker and stronger layers require consideration separately. 



At most places the evidence seems to show that during 

 deformation the anticlines and synclines of the weaker layers 

 have been thickened. However, since during the deformation 

 there is shearing motion under pressure all along the limbs, it 

 can hardly be doubted that in many cases the thinning of the 

 layers is a more important phenomenon so far as their length is 

 concerned, than thickening on the anticlines and synclines. 



In the stronger layers, often no evidence of thickening is 

 anywhere seen. Upon the contrary in many cases, as will be 

 shown below, the layers are actually elongated by tensile fracture 

 upon their convex sides and therefore cannot have been thick- 

 ened. Radial open fractures upon anticlines and synclines, due 

 to tension, are beautifully exhibited in the southern Appalachians 

 (Fig. 8). The openings in many cases have been filled with 

 quartz. These joints are evidences of tensile forces. Where 

 stretching of anticlines and synclines occurs in the zone of flow- 

 age, this is undoubtedly due to the great friction between the 

 layers, and to their positions on the convex sides of the neutral 

 planes. The limbs of the stronger layers, like those of the 



