SUMMARY AND CONCLUSIONS. 139 



incapable of complete explanation without invoking some such action, 

 though the ultimate cause to which it is due has not been estab- 

 lished. 



The same action is provided for by Mr. Bailey Willis, who attri- 

 buted the origin of the mountain ranges of Asia to the greater 

 density, and weight, of the crust under the Pacific and Indian Oceans, 

 and to an underground transfer of material, from the oceanic to' the 

 continental regions, in consequence of the pressure set up by this 

 difference of weight. 1 At a later date a similar explanation was 

 adopted by Mr. J. F. Hayford, who puts the action as taking place 

 within narrower horizontal limits. 2 



So far as this undertow is supposed to occur at a depth below 

 that in which the contortion of the rocks, now lying near the surface, 

 took place, it is in effect similar to that of Mr. Fisher's convection 

 currents ; but while these supply a continuous action, ample to 

 provide for all the range of movement required, the alternative 

 process only provides for a limited and insufficient range of move- 

 ment, and in both cases it is questionable whether the pressure 

 requisite to produce compression could be communicated to the 

 upper layers of the crust. If, on the other hand, the compression 

 is supposed to take place within the layer involved in the movement 

 of the undertow, the range of motion might be sufficient, but the 

 pressures developed, especially when supposed to be transmitted, 

 through a long horizontal column of material, appear to be utterly 

 inadequate. As has been pointed out before, we know too little 

 of the conditions actually existing in the interior of the earth to 

 reject this explanation as impossible, but, in view of the many 

 difficulties in the way of acceptance, it cannot be regarded as a 

 satisfactory and sufficient explanation of the facts revealed by 

 observation. 



One more explanation of the origin of mountain ranges, which 

 may be referred to, is the suggestion of Mr. Mellard Reade. 3 He 

 pointed out that if the average temperature of a tract of the earth's 

 crust was raised, it would expand, not merely in a vertical but also 

 in a horizontal direction, and that the cubical expansion of the 

 whole tract would most naturally find relief by yielding along a 



1 Research in China, Vol. II, Chap. VIII, 1907. 



2 Science, new series, XXXIII, 1911, pp. 199-208 ; Journ. OeoL, XX, 1912, G62-578. 

 * The Origin of Mountain Ranges. London, 188G. 



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