304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



beneath the earth's crust, the application of the force of growing 

 crystals, in association with molten areas, explain certain facts in 

 orogeny better perhaps than any of the other theories. 



CONCLUSIONS 



The foregoing brief discussion of some of the better-known theories 

 of the origin of folded mountains shows that the question of origin 

 is far from being settled. Some of the newer and more startling 

 theories, such as the continental drift theory, have been carried to 

 absurd extremes by some of the more enthusiastic and less well bal- 

 anced advocates. On the other hand, a number of ultraconservative 

 scientists, believing in absolutely fixed continents, can see no value 

 in the drift theories. These reactions are according to the laws of 

 human nature. I think it can be said in truth that none of the 

 newer theories is completely in the right and that none is completely 

 wrong. Here again we are traveling the same old scientific path. 



No one, at the present, questions the truth of great horizontal and 

 vertical movements in the earth's crust, in the formation of folded 

 mountains, but only the causes back of such movements. Most geol- 

 ogists who believe in fixed continents and ocean basins think also of 

 the horizontal movements largely as the result of differential settling 

 of continental and subocean masses in a radially shrinking earth ; or 

 they think of them as due to actual enlargement of the crust by 

 wedge action of igneous injections or by the pressure exerted by 

 great numbers of crystals growing, in horizontal orientation, in the 

 zone of metamorphism. 



As a result of lateral compression from one or many causes, the 

 rigid crust of the suboceanic section is gouged into the continental 

 section, beneath its more rigid superstructure. This underthrusting 

 by the ocean segments results in an overthrusting by the continents. 

 Since the Pacific Basin is larger and deeper than the other ocean 

 basins, it has a greater and more unbalanced underthrust than other 

 ocean segments, and we have, therefore, the greatest foredeeps in the 

 Pacific. 



The geosynclines provide the location for the maximum buckling. 

 Their elevation provides sediments for new geosynclines. 



The theories of drifting and of sliding continents give us addi- 

 tional causes for horizontal compression. They also give reasons for 

 directionally variable forces of tension and compression. 



Arcuate mountains are best explained by either the gravitational 

 sliding of continents or by Bailey Willis' theory of asthenoliths. 



The Joly hypothesis of a periodically molten subcrust removes one 

 of the chief objections to the theory of continental migration, namely, 

 the nonmobility of the earth. It also makes possible the explanation 

 of a number of facts previously unsatisfactorily explained. 



