Summary and conclusions. 131 



bution of resistance to compression, needed to bring this condition 

 about, would be somewhat peculiar, but by no means impossible, 

 yet it must remain merely a suggestion, in the absence of any means 

 of testing it. Another possibility is that, in addition to the thicken- 

 ing of the crust by compression, its density is actually reduced in 

 some way or other, and here Dr. Fermor's suggestion of the pass- 

 age of rocks, belonging to the same norm, from a mode of greater 

 density to one of lesser, affords a feasible explanation, but, like 

 the previous one, it must remain a mere suggestion. 



Neither of these suppositions involves the implication of any 

 fresh material, from outside the portion of the crust covered by 

 the hills, in the process of mountain formation, but the excess of 

 support under the main range might equally be accounted for 

 by an invasion, of the tract under the hills, by material from out- 

 side, whether by the injection of acid intrusions, or by a differen- 

 tial movement of the lower and upper parts of the crust, such as 

 could be described indifferently, according to the point of view, 

 as an over-thrust of the upper portion towards the south, or an 

 under-thrust of the lower towards the north. 



It is in the last-named direction that the easiest relief occurs from 

 the difficulties arising from a limitation of the cause to the area 

 actuallv covered by the range. The attribution of part of the down- 

 ward prominence to an invasion of material from outside the limits 

 of the range, would enable the neutral zone to be brought down to a 

 level which would remove any difficulty in explaining the production 

 of complicated folding of the rocks, but it is important to note that 

 any process of this sort can only be subsidiary to the effect of com- 

 pression, and that we cannot, on the hypothesis under consideration, 

 attribute the whole, or even the major part, of the elevation of the 

 range, to the invasion of material from outside. The case can be put 

 simply enough : using Mr. Fisher's constants, the total thickness of the 

 crust under the range would have to be just about twice as much 

 as the normal thickness of the undisturbed crust, but these cons- 

 tants, as has been pointed out, 1 represent what may be regarded 

 as a minimum value for the thickening, which may amount to 

 three times the normal thickness ; the hypothesis, therefore, 

 demands a compression of from one-half to two-thirds of the original 

 horizontal extent of the crust. The actual amount of compres- 

 sion, indicated by geological structure, cannot be estimated with 



1 See p. 48. 



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