ISOSTASY AS A RESULT OF EARTH SHRINKAGE 



FRANCIS PARKER SHEPARD 

 University of Illinois, Urbana, Illinois 



In spite of the apparently indisputable evidence that the earth's 

 crust is in almost perfect isostatic adjustment, geologists have been 

 slow to accept the theory of isostasy. The subject received wide 

 interest at the 192 1 meeting of the Geological Society of America. 

 At this meeting it developed that the opposition to the theory was 

 directed in part against the common interpretations of some 

 geological phenomena made by the isostasists and also against the 

 lack of interpretations of other geological phenomena which have a 

 bearing on isostasy and appear to be inconsistent with it. Therefore 

 the question arises as to the possibility of explaining these various 

 phenomena in such a way as to satisfy both geological and isostatic 

 requirements. 



Those isostasists who claim that mountain ranges have been 

 uplifted by the vertically acting forces of isostasy rather than by the 

 horizontal forces produced by earth shrinkage, have still to explain 

 much in regard to folding and thrust faulting. Bowie believes that 

 these are due to the irregularity of the application of the vertical 

 forces.^ Irregularities of uplift might produce tilting of surfaces, 

 block faulting, similar folding, and other phenomena which do not 

 indicate a distinct shortening of the crust, but where the cross-section 

 of a range shows that points on the sides of the range have actually 

 moved toward the center so much that there has been shortening of 

 40 or 50 per cent of the original section, then not vertical forces, but 

 horizontal forces must have been the cause of the structure. It is 

 easy to duplicate experimentally the effects of lateral shortening 

 and to reproduce on a small scale any type of fold found in nature, 

 but it is dynamically impossible to produce overturned folds and 

 thrust faults of large proportions by contrivances which apply 



^ W. Bowie, Geol. Soc. of America, Vol. XXXIII (1922), p. 280. 



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