ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 1 9 



the places of less folding the strata chance to be hidden, the 

 plications of other strata must be selected to fill in the gap. 

 Layers may be selected which exhibit close folding. But even if 

 layers are selected which show the least folding of any in sight, 



Fig. 6. — Simple fold below composite folds. 



there is still a possibility that a considerable overestimate may 

 be made of the closeness of folding and of the amount of crustal 

 shortening. 



For oftentimes, where a formation upon which estimates of 

 shortening are being made, disappears below the surface, this 

 results from its plunging downward as a part of a synclinorium. 

 It is believed that upon the average synclinoria are less closely 

 folded than anticlinoria. Anticlinoria are places of crustal 

 thickening resultant upon close plications, whereas synclinoria 

 are areas ot depression as compared with the anticlinoria, but 

 not really areas of depression as compared with the unfolded 

 districts. If the plications of synclinoria were as composite as 

 anticlinoria, this would involve an equivalent amount of thick- 

 ening of the crust, and consequently equal elevation with the 

 anticlinoria unless a large amount of material, to compensate 

 for the difference in elevation, had flowed from below the 

 synclinornia to below the anticlinoria. Doubtless the flowage of 

 the kind suggested does take place to some extent, but to no 

 such extent as would be involved in this hypothesis 



Willis's experiments most beautifully illustrate the composite 

 character of the folding of anticlinoria as compared with the 

 intervening synclinoria. 1 The above reasoning applies exactly 



•The mechanics of Appalachian structure, by Bailey Willis: 13th Ann. 

 Rept., U. S. Geol. Survey, Pt. II, 1893, Pis. LXXVII, LXXXI, LXXXII, LXXXIV, 

 LXXXV, LXXXVI. 



