2 2 C. R. VAN HISE 



present point is that if the corrugated parts are alone considered, 

 and the jointed part or the part removed by erosion ignored, 

 that the average crustal shortening may be greatly overestimated. 

 If the considerations presented in the foregoing pages have 

 weight, it is clear that the actual measurements in the field of 

 the amount of crustal shortening involved in folding presents 

 great difficulties, and the question naturally arises as to the best 

 practicable methods of procedure. 



1. So far as practicable, the same formation should be 

 measured throughout a section, and if it is necessary to transfer 

 from one formation to another, the greatest care should be 

 exercised in order to avoid the errors which may result from 

 changing from a formation to another lower or higher, and also 

 to avoid the error which may come in as a result of the lateral 

 change in closeness of plication. 



2. The strongest formations available should be selected for 

 measurement. 



This selection should be made because the stronger forma- 

 tions have less composite curves than the weak ones. As a con- 

 sequence they are less distorted during the folding than the 

 weak formations. These facts may be observed in almost any 

 good section of closely-folded heterogeneous strata. The more 

 composite crenulation, but not the greater thinning and thicken- 

 ing, of the weaker layers may be illustrated by bending a rec- 

 tangular pile composed of bunches of paper alternating with 

 cardboards, the pile being held firmly either mechanically or with 

 the fingers at the edges, so that slipping between the laminae may 

 be hindered at the places held. In this experiment, at the crest 

 or trough, spaces form between the stronger layers, and in these 

 spaces the weaker layers take on secondary crenulations. In 

 natural geological sections the pressure upon the limbs is fre- 

 quently sufficiently great so that the material of the weaker 

 layers flows toward the openings on the anticlines or synclines, 

 and partly or wholly occupies them. In many places some of 

 the weaker layers are quite pinched out upon the limbs. 



The physical cause for the simple folding of strong layers 



