AGE OF EXISTING MOUNTAINS 385 



The main belt of each group is relatively stable and is Tertiary or 

 possibly even Pleistocene in part, whereas the age of the extreme outer 

 groups is Pleistocene at least for the Pacific region. This statement 

 refers only to the present general form and the general altitude of the 

 island belts considered. The history prior to the formation of the present 

 mountain forms is considered in another chapter. These greater arcs^ 

 together with their subsidiary ridges, may be separated by ocean deeps 

 or by deep intermontane valleys (land troughs) or they may occur merelj 

 as associations of high and less high plateaus arranged in parallel zones. 



These complete units are progressively stable in proportion to tJieir 

 proximity to the continental nucleus from the great ocean deeps. 



Examples are the Society Islands with the Paumotus, Tonga with its 

 parallel zones of volcanoes, coral islands, and ocean trenches ; Fiji with 

 Lau, New Caledonia with the Loyalties, Australia with its continental 

 shelf and outer islets. 



The works of Barrell, Berry, Bowman, Brouwer, W. M. Davis, Diller, 

 Button, Gilbert, Ellsworth Hunting-ton, Lawson, Le Conte, Lindgren, 

 Molengraaff, Powell, Tangier Smith, Warren D. Smith, T. W. Vaughan,. 

 the Indian and the Alpine geologists, and others, all supply evidence a 

 study of which indicates that the main mountain blocks existing todays 

 are not older than Pliocene, and that they are associated with pressure 

 ridges in certain instances of Pleistocene age. Especially convincing in. 

 this particular are the works of the authors mentioned above for the 

 Pacific, the Indian, the European, and the West Indian regions discussed 

 by them. 



DiSTEIBUTION AND ARRANGEMENT OF PAST MOUNTAIN SYSTEMS OF 



THE Earth 



In a. general way it may be said that existing mountains are similar 

 and similarly situated to earlier mountain ranges. On the other hand,, 

 there are decided deviations from this general law. 



In the first place, the grandest examples of mountain chains of the 

 present day occur within or near the margins of the Pacific and the 

 Tethyan regions.® The existing mountain chains decrease in grandeur 

 of appearance in confocal zones traced progressively from the ocean 

 toward the continental nuclei in directions at right angles to the general 

 movement of earth-wave pulsation. 



® In this connection the Pacific is considered as inclusive of Mexico, the Antillean 

 zones, and their continuation through Venezuela and Colombia. The Tethyan area is 

 considered as equivalent to the region named Tethys by Suess, together with its con- 

 tinuation to the southeast. 



