190 A CENTURY OF SCIENCE 



They may be broad geanticlinal arches or bounded by the 

 walls of profound fractures. 



The linear mountain systems made from deep troughs 

 of sediments have come then to be recognized as but one 

 of several classes of mountains. This class, from its 

 clear development in the Appalachians, and the fact that 

 many of the laws of mountain structure pertaining to it 

 were first worked out there, has been called by Powell the 

 Appalachian type (12, 414, 1876). A classification of 

 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 Button in 1876, but especially in 1889, brought 



