32 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



deeply seated portions would not enter into consideration. It 

 must also be remarked that any hypothesis which regards the 

 elevation of mountain ranges as a result of compression, seems 

 necessarily to involve some form of isostasy by ilotation, in order 

 to account for compensation, for if the whole of the thickening 

 took place in an upward direction the mountains would be an un- 

 compensated excrescence of additional matter, but if the thicken- 

 ing took place both upwards and downwards, and the outer crust 

 consisted of less dense matter than that underlying it, there would 

 be a defect of attraction which, at a sufficient distance, would 

 neutralise the attraction of the mountain mass to a greater or less 

 degree, according to the ratio of the excess and defect of matter. 

 For complete compensation the two would have to be equal in mass, 

 a condition which would imply complete isostasy and a support 

 of the mountains by flotation. 



Much the same effect, and the same considerations will apply 

 to any form of hypothesis which attributes the elevation of the 

 surface to an intrusion of fluid matter below it. Here again, if 

 the whole effect was the raising of the crust between the upper 

 surface and the intrusive mass, the range would be a mere excres- 

 cence of the surface and its attraction would be unmodified by com- 

 pensation, unless we could assume that the intrusion was devoid 

 of density, which is inconceivable, or that the displacement of the 

 upper surface was accompanied by a downward displacement of 

 the lower surface, leading to the replacement, under the upraised 

 tract, of denser material by lighter. Any hypothesis of this kind, 

 therefore, falls into the same great category as the supposition 

 that the elevation of the range is due to a thickening of the crust 

 by compression, in that it would imply an actual transfer of matter 

 from a region outside, resulting in an increase of the mass of the 

 outer crust underneath the upraised tract ; and on any hypothesis 

 involving this, it seems impossible to account for the accepted 

 fact of compensation, without admitting that the upward pro- 

 tuberance of the upper surface is accompanied by a downward 

 protuberance of the under surface of the crust, the "root" of Mr. 

 Fisher's investigation, with the consequences of a displacement 

 of the denser material under the crust by the lighter material of 

 the crust itself, and an isostasy and support by flotation. 



An hypothesis of this kind opens up a further possibility of a 

 considerable departure from locally complete isostasy and a dis- 



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