I 8 C. R. VAN HISE 



In this connection it should be remembered, if the theory 

 of a level of no lateral stress be true, that a good reason 

 exists for the lessening folding and distortion of layers with 

 increasing depth. 



Whether or not this theory as ordinarily stated approximates 



Fig. 5- — Parallel upright folds with rounded crests and troughs. 



quantitative correctness, it is certain that the amount of shorten- 

 ing must somewhere decrease with increase of depth ; for infi- 

 nitesimally near the center of the earth the amount of shortening 

 must be infinitesimally small. Since with present knowledge we 

 can only conjecture the law under which folds die out in depth, 

 though we are certain that they must die out, one is not justified 

 in assuming that folds similar to those at the surface continue 

 even for moderate depths. 



If this principle be ignored in estimating shortening, a 

 serious error may be made. The formation being followed may 

 plunge beneath softer formations which show close plications. 

 If it be assumed that similar plications also effect the formation 

 below to be measured, this may lead to a considerable overesti- 

 mate of the amount of crustal shortening (Fig. 6). 



Also the lateral variation in closeness of folding may lead to 

 error. If the layer or formation to be measured is not continu- 

 ously exposed, it may be visible where it chances to be most 

 closely folded and be concealed where more openly folded. If at 



