328 MAN AND THE GLACIAL PERIOD. 



to produce a contraction of the earth's volume and a 

 shortening of its diameter. Heat is constantly being ab- 

 stracted from the earth by conduction and radiation, but 

 perhaps to a greater extent through ceaseless volcanic 

 eruptions which at times are of enormous extent. It re- 

 quires but a moment's thought to see that contraction of 

 the volume of the earth's interior means that the hard- 

 ened exterior crust must adjust itself by wrinkles and 

 folds. For a long period this adjustment might show 

 itself principally in gentle swells, lifting portions of the 

 continents to a higher level, accompanied by correspond- 

 ing subsidence in other places. This gradually accumu- 

 lating strain would at length be relieved along some line of 

 special weakness in the crust by that folding process which 

 has pushed up the great mountain systems of the world. 



Careful study of the principal mountain systems shows 

 that all the highest of them are of late geological origin. 

 Indeed, the latter part of the Tertiary period has been 

 the great mountain-building epoch in the earth's history. 

 The principal part of the elevation of the Andes and the 

 Eocky Mountains has taken place since the middle of the 

 Tertiary period. In Europe there is indubitable evidence 

 that the Pyrenees have been elevated eleven thousand feet 

 during the same period, and that the western Alps have 

 been elevated thirteen thousand feet in the same time. 

 The Carpathians, the western Caucasus, and the Hima- 

 layas likewise bear explicit evidence to the fact that a 

 very considerable portion of their elevation, amounting to 

 many thousand feet, has been effected since the middle of 

 the Tertiary period, while a considerable portion of this 

 elevation of the chiefest mountain systems of the world 

 has occurred in what would be called post-Tertiary time — 

 that is, has been coincident with a portion of the Glacial 

 period. 



The Glacial period, however, we suppose to have been 

 brought about, not by the specific plications in the earth's 



