KNOWLEDGE OF EARTH STRUCTURE 189 



American Institute of Mining Engineers, the thirteenth 

 annual report of the United States Geological Survey, 

 and the Journal (46, 257, 1893). 



Finally should be noted the contributions of the Lake 

 Superior school of geology, in which the work of Van 

 Hise stands preeminent. Under the economic stimulus 

 given by the discovery and development of enormously 

 rich bodies of iron ore, hidden under Pleistocene drift 

 and involved in the complex structures of vanished moun- 

 tain systems of ancient date, structural geology and met- 

 amorphism have become exact sciences to be drawn upon 

 in the search for mineral wealth and yielding also rich 

 returns in a fuller knowledge of early periods of earth 

 history. 



Crust Movements as Revealed by Physiography. 



During the last quarter of the nineteenth century 

 another division of geology, dominantly American, was 

 taking form and growth, the science of land forms, 

 physiography. The history of that development is 

 treated by Gregory in the preceding chapter but some of 

 its bearings upon theory, in so far as they affect the sub- 

 ject of mountain origin, are necessarily given here. 



Powell, Dutton, and Gilbert in their explorations of the 

 "West saw the stupendous work of denudation which had 

 been carried to completion again and again during the 

 progress of geologic time. The mountain relief conse- 

 quently may be much younger than the folding of the 

 rocks, and may be largely or even wholly due to recurrent 

 plateau movement, a doctrine to which Dana had pre- 

 viously arrived. But the introduction of the idea of the 

 peneplain opened up a new field for exploration in the 

 nature and date of crust movements. Davis by this means 

 began to study the later chapters of Appalachian history, 

 the most important early paper being published in 1891. 18 

 Since then Davis, Willis, and many others have found 

 that, girdling the world, a large part of the mountainous 

 relief is due to vertical elevatory forces acting over 

 regions of previous folding and overthrust. In addition, 

 great plateau areas of unfolded rocks have been bodily 

 lifted one to two miles, or more, above their earlier levels. 



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