KNOWLEDGE OF EARTH STRUCTURE 191 



it into. the geologic field. A test of isostasy was made for 

 the United States by Putnam and Gilbert in 1895 and 

 much more elaborate investigations have since been made 

 by Hayford and Bowie. These investigations demon- 

 strate the importance and reality of broad warping 

 forces acting vertically and related to the regional varia- 

 tions of density in the crust. 



There are consequently two major and unrelated 

 classes of forces involved in the making of mountain 

 structures, the irresistible horizontal compressive 

 forces, arising apparently from condensation deep within 

 the earth, and vertical forces originating in the outer 

 envelopes and tending toward a hydrostatic equilibrium. 

 In this latter field of investigation, America, since the 

 initial paper by Button, has taken the lead. 



Conclusion on Contributions of America to Theories of 



Orogeny. 



The sciences arose in Europe, but those which treated 

 of the earth were still in their infancy when transplanted 

 to America. The first comprehensive ideas on the nature 

 of mountain structures arose in Great Britain and 

 France. These ideas served as a guide and stimulus to 

 observation in the recognition of deformations in the 

 strata of the Appalachian system. Since 1840, however, 

 America has ceased to be a pupil in this field of research 

 but has joined as an equal with the two older countries. 

 New ideas have been contributed, new and striking illus- 

 trations cited, first by the scientists of one nation, next by 

 those of another. The composite mass of knowledge has 

 grown as a common possession. Nevertheless, a review 

 of the progress since 1840 as measured by the contribu- 

 tion of new ideas shows on the whole America at least 

 equal to its intellectual rivals, and at certain times 

 actually the leader. This is true of the science of geol- 

 ogy as a whole and also of the subdivision of orogeny. 



Thus far no mention has been made of German geolo- 

 gists, with the exception of Suess, an Austrian. German 

 geology is voluminous and the names of many well-known 

 geologists could be cited. But this article has sought 

 to trace the origin and growth of fundamental ideas. 



