1 6 C. R. VAN HISE 



sure in positions which average greater than 45 °, may be very 

 important. In reference to the other factors, the greater the pres- 

 sure, the greater the tendency for thickening and shortening at 

 angles less than 45 °, and the greater the tendency for thinning 



Fig. 3. — Similar upright folds with angular crests and troughs. 



and lengthening at angles greater than 45 °. The greater the 

 shearing between the layers, the greater the thinning. The 

 greater the rigidity of any given laver, the less the thinning. 



The foregoing statements are understood to apply to strata 

 which are so deeply buried that the deformation of the layers 

 results from true interior distortion of them. In the upper zone 

 of fracture these statements need to be modified, as subsequently 

 explained.' 



With present data only these qualitative statements can be 

 made. But, it seems probable that in the earlier stages of the 

 development of folds, the average thrust thickens instead of 

 thins the layers. However, where the folds are very close, and 

 especially in isoclinal and overturned folds, it can hardly be 

 doubted that upon the average there is considerable thinning 

 and consequent important elongation of the layers. For folds 

 of a given average closeness the average amount of distortion is 

 not so great where the strata bend back upon themselves with 

 sudden turns, as where the bends occur gradually (compare 

 Figs. 3 and 4), although the distortion at the angles may be 

 greater than at the corresponding places upon the gentle turns. 

 In nature the deformation is ordinarily between the two extremes 

 fiofured. 



