Barrel! — Growth of Knowledge of Earth Structure. 169 



mountain systems was proposed by him in which moun- 

 tains are classified into two major divisions, those com- 

 posed of sedimentary strata altered or unaltered, and 

 those composed in whole or in part of extravasated mate- 

 rial. The first class he subdivides into six sub-classes 

 of which the folded Appalachians illustrate one. It 

 appears to the writer that Powell's classification gives 

 disproportionate importance to certain types which he 

 described; but nevertheless, the fact that such a classi- 

 fication was made, indicates the growth of a more com- 

 prehensive knowledge of mountains, — their origin, struc- 

 ture, and history. 



Relations of Crust Movements to Density and Equilibrium. 



A recent important development in the fields of geo- 

 physics and major crust movements consists in the incor- 

 poration into geology of the doctrine of isostasy. The 

 evidence was developed in the middle of the nineteenth 

 century by the geodetic survey of India which indicated 

 that the Himalayas did not exert the gravitative influence 

 that their volume called for. It was clear that the crust 

 beneath that mountain system was less dense than 

 beneath the plains of India and still less dense than the 

 crust beneath the Indian Ocean. This relation between 

 density and elevation indicated some approach to flota- 

 tional equilibrium in the crust, comparable in its nature 

 though not in delicacy of adjustment to the elevation of 

 the surface of an iceberg above the ocean level owing to 

 its depth and its density, less than that of the surround- 

 ing medium. This important geological conception was 

 kept within the confines of astronomy and geodesy, how- 

 ever, until Dutton in 1876, but especially in 1889, brought 

 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 



