ESTIMATES AND CAUSES OF CRUSTAL SHORTENING I 3 



shown 1 in another place that the mashing, flowage, and the 

 shearing motion involved in differential movement between the 

 layers necessarily involves thinning of the limbs of the folds or 

 thickening of the troughs and crests, or both. Even where the 



Fig. i. — Similar monoclinal folds. 



folds are not close, in case the folds are similar, 2 the limbs may 

 not be much more than half as thick as are the troughs and crests 

 (Fig. 4). This distortion becomes more and more important 

 as the folding becomes closer, and in isoclinal and monoclinal 

 folds, in which the strata turn back upon themselves, the amount 

 of thinning of the limbs or thickening of the troughs and crests 

 is very great (Fig. 1). A layer when traced out in such a set 

 of folds alternately thins and thickens, and the section if 

 developed on a plane, would alternately greatly widen and 

 narrow (Fig. 2). The length of the developed layer should be 

 the length of its central part. The excess of material for each 



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

 16th Ann. Rept. U. S. Geol. Survey, Pt. I, 1896, p. 599. 



2 Loc. cit., pp. 599-600. 



