INTRODUCTION 181 



main, central part of this vast crustal sheet, but even this part alone is 

 considerably greater than any other continent. The peripheral moun- 

 tain ranges of Asia show prominently certain forms — chiefly curved, arc 

 shapes — which are scarcely recognizable or are generally inconspicuous 

 in the peripheral ranges of the other continents. The mountain ranges 

 and plateaus of Asia are greater in every way than those of other conti- 

 nents — greater in areal extent, in height, mass, and breadth of plan. It 

 is a question to be determined whether all these distinguishing attributes 

 are not due mainly to the fact that Asia, considered as a dynamic unit of 

 the earth's crust in the process of diastrophism or mountain making, is 

 larger — greater in extent and mass — than any other similar unit. 



If a crust-flake, like that which we call Asia, should move a given dis- 

 tance as a unit, it would certainly produce greater results in crustal de- 

 formation than would arise from the movement of a flake having only 

 one-tenth of its magnitude, and the difference in the results would be 

 still greater if the larger flake moved several times farther than the 

 smaller one. The products might differ not merely in magnitude, but to 

 some extent in kind also. Australia is a continent of the same type as 

 Asia, but it is relatively feeble in its development; its peripheral ranges 

 are relatively weak. They are mostly submerged, and, so far as known, 

 they show none of the peculiar forms which characterize the great 

 peripheral belt of Asia. 



Figure 1 shows the location of the Tertiary mountain belt. Later re- 

 searches seem to suggest some slight additions in eastern Asia and in the 

 western and southwestern Pacific, but these are welcome changes and are 

 not discordant. 



The mere existence of this vast mountain zone encircling the earth is 

 of itself a remarkable thing, but its significance is prodigiously increased 

 when the distribution and age of these mountains are taken into account 

 and when some of their structural or tectonic relations and character- 

 istics are analyzed. The entire belt is essentially of one age, all made or 

 largely augmented within the limits of one definable and relatively short 

 and recent period of geological time. Moreover, this period of dias- 

 trophism is the last or most recent of the great mountain-making 

 epochs — the nearest to us in time. For this reason partly the mountains 

 of this belt are all relatively young — so new that there has not yet been 

 time for them to be destroyed or very greatly reduced by erosion. This 

 is one of the chief reasons for their great height in many places and for 

 the relative youthfulness of their physiographic expression. It is also a 

 reason for the relatively unmodified state of the tectonic lines which they 

 mark, for these lines have not been disturbed or disarranged by any im- 



