WARPING IN CENTRAL ASIA. 113 



The limiting ranges hold apparently homologous relations to the 

 plateaus; but they differ greatly in history and structure. The Altin-tagh, 

 occupying part of the northern Tethys, was folded primarily in the Permo- 

 Mesozoic revolution, and its essential structures thus date from a some- 

 what remote time. The Himalayas did not suffer compression till the 

 beginning of the period of diastrophism that marks the late Tertiary and 

 Quaternary. The Altin-tagh seems to rise from the Tarim basin as a con- 

 tinuation of the eroded surface which plunges beneath the Gobi deposits 

 of the desert; it resembles apparently the warped surface by which the 

 Shan-si mountain region sinks beneath the plains of eastern China; and 

 though possibly in some sections broken by normal faults, it does not differ 

 from other mountain slopes of Asia. The Himalaya range, on the other 

 hand, is unlike them. It is separated from the plains at its southern base 

 by thrusts of great magnitude, which dip beneath the upraised mass. The 

 effect is as if the range were pushed southward were overthrust; it may 

 equally well be expressed by the statement that the lowlands are pushed 

 northward are underthrust. Since the overthrusting or underthrusting, 

 whichever is the dominant fact, involves movement on an inclined plane, 

 the mechanical condition is that which would result from driving a wedge 

 under the range. Either the range must be raised or the wedge must be 

 depressed, or both movements may occur. In discussing the analogous 

 case of the Lewis range of the Rocky Mountains in Montana, I have shown 

 that an elevation of 3,400 feet is probably attributable to a displacement 

 of 7 miles on the flat thrust which underlies the Algonkian strata.* The 

 thrusts beneath the Himalayas are apparently even more important factors 

 in the relative elevation of the mountain mass. Among Asiatic mountains 

 the Himalayas thus present a unique case of mechanical relations. 



The elevation of the Tibetan plateau (Isle Tibet) is apparently an effect 

 of the underthrusting, to which we may attribute some considerable part 

 of the altitude of the Himalayas. I conceive that the plateau is the surface 

 of a deep-seated, strongly compressed sub-Tibetan mass. 



* Stratigraphy and Structure, Lewis and Livingston Ranges, Montana, Bull. G. S. A., vol. xni, p. 345. 



