ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 17 



The fact that the distortion is less in folds with sudden 

 turns than in those with rounded turns, is believed to be a cause 

 why this deformation so frequently occurs in closely-folded 

 rocks. The angular deformation requires less work, and there- 



FlG. 4. — Similar upright folds with rounded crests and troughs. 



fore less energy than the other form. As the stresses slowly 

 accumulate until the elastic limits of the rocks are surpassed, the 

 deformation which will occur in any given case in order to relieve 

 the stresses is that which requires the least expenditure of energy 

 under the existing conditions. This is the old principle that a 

 mass under stress gives way along the lines of least resistance. 

 2. In estimating the amount of superficial shortening 

 involved in folding, it is necessary to consider what and where 

 strata shall be selected for estimation. I have shown that if 

 there is no thinning or thickening of the layers (Fig. 5) folds 

 rapidly die out above and below any given folded layer. I 

 have also shown that similar folds are only possible by profound 

 distortion of the layers (Figs. 1 and 4), unless the turns are very 

 abrupt. Agreeing with theory, actual geological sections are a 

 compromise between parallel folds and similar folds, the folds 

 rapidly varying in closeness in different parts of a mountain mass, 

 vertically and laterally. 1 



1 Principles of North American pre-Cambrian geology, by C. R. Van Hise : 

 16th Ann. Rept. U. S. Geol. Surv., Ft. I, 1896, pp. 599-601. 



