xgoS.] THE PHYSICS OF THE EARTH. 229 



sion of the Tibetan mountain area from east to west may be about 2,000 

 miles, while its average breadth somewhat exceeds 500 miles ; about 100 miles 

 on either side constitute the sloping faces, the central tableland having a 

 width of about 200 miles on the west and probably 500 miles at its eastern 

 border." 



General Strachey thus shows that the Himalayan mountains and 



Tibetan Plateaus are directly and intimately connected as merely 



different parts of one great continuous moveinent of the earth's 



crust. 



After describing many features of the Himalayas, General 



Strachey continues : 



" The general conclusion that may be drawn from the facts of structure 

 thus briefly indicated is that the elevation of the Himalaya to its present 

 great height is of comparatively recent occurrence. An area of land must 

 have existed where the main line of snouy peaks now stands, which has 

 not been submerged since the Palaeozoic period, and which then had its 

 northern boundary somewhere along what has been termed the Indian water- 

 shed. Evidence of a similar ancient sea on the south also exists, but in 

 less definite shape ; and whether it was united with the northern sea or not 

 is still a matter of conjecture, though the distinctive character of the fossils 

 rather indicates that there was no direct union. The possible connection of 

 this ancient Himalayan land area with the pre-Tertiary land of the peninsula 

 of India is also only a matter for speculation. 



" There is further reason to infer that the existence of the great line 

 of peaks is rather due to some previous line of elevation on the ancient 

 land, which has continued to retain its relative superiority while the whole 

 areas have been raised, rather than to any special line of energy of upheaval 

 of recent date ; and that the fundamental features of its former configura- 

 tion of surface in mountain and valley have been preserved throughout. 

 There is evidence for the conclusion that the chief rivers of the pre- 

 Tertiary land issued from the mountains where the present main streams are 

 found, and this embryo Himalaj-a may have been of such moderate height 

 as to have permitted the passage across it of the Siwalik mammals, the re- 

 mains of which appear both on the border of the Indian plain and in Tibet. 

 It is after the middle Tertiary epoch that the principal elevation of these 

 mountains must have taken place, and about the same time also took place 

 the movements which raised the tablelands of Afghanistan and Persia, and 

 gave southern Asia its existing outlines. 



" The best answer that can be given to an inquiry as to how changes 

 of level could have arisen, such as those which are observed in the Hima- 

 laya, is that they should be regarded as due rather to secondary actions 

 consequent on the general contraction of the cooling terrestrial sphere than 

 to direct elevating forces, for which no known origin can be assigned. The 

 contraction of the cooling but now solid crust of the earth must have set 

 up great horizontal strains, partly of tension and partly of compression 



