ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 23 



and the composite folding of the weaker layers is that already 

 assigned for another kind of deformation on p. 17, namely, 

 under given conditions, the deformation occurs which demands 

 the least expenditure of energy. To deform the strong layers 

 in a composite way requires a large amount of energy. To 

 deform the weak layers in a composite way requires much less 

 energy. The simple deformation of the strong layers and the 

 composite deformation of the weak layers demands less energy 

 than would be required to deform all the layers in a similar 

 manner, so that the deformation would average the same as in 

 the case of the unequal deformation of the strong and weak 

 layers. 



Under different circumstances the strong layers vary greatly 

 in the simplicity of their deformation. In case the load is not too 

 great, as explained by Willis, the strong layers are bent into 

 large, simple folds. If, upon the other hand, the load is too great 

 for the strata to support, the strong layers are folded in a com- 

 posite manner. Both of these cases fall under the principle that 

 the deformation occurs which requires a minimum expenditure 

 of energy, for in the case of the lighter load, it requires less 

 energy to elevate the load on the anticline, or to somewhat depress 

 it on the syncline, than it does to greatly distort the strong 

 formations. But in the case of the great load it requires less 

 expenditure of energy to distort the strong layer a sufficient 

 amount to make it develop composite folds than it does to 

 elevate the load. But as above stated, even in the case of 

 composite folding of the stronger layers, the weaker layers 

 adjacent to them show still more composite deformation. 



The statement made that the strong formations should be 

 selected for tracing above the surface and for measurement is 

 therefore justified. 



3. From the places where the strong formations plunge 

 below the surface to the places where they reappear, only the 

 most gentle curves should be assumed (Fig. 6). 



4. From the places where the formation which is being 

 measured is lost because removed by erosion, only the most 



