'ffs applied to the Origin of Mountains. 319 



sinking continues, there is a transfer of material, squeezed out from 

 under the depressed area, and forced below the bordering land region. 



As a great thickness of sediment accumulates on the margins 

 of the main area of depression (which is itself far from land), 

 owing to the establishment of a temperature gradient throughout 

 this sediment, the contraction of this border region becomes con- 

 tinually less and less than that of the main area of depression ; 

 and at last the region of sedimentation ceases to sink and begins 

 to rise owing to the introduction of material, squeezed out from 

 under the more rapidly contracting area. 



To sum up, the sequence of events will be briefly as follows : — 



First, when sediment is accumulating, the neighbouring land 

 area is rising, owing to actual transference, from under the sinking 

 area to beneath the neighbouring land region. 



Secondly, this continues until, owing to the thickness of sediment 

 accumulated, the area of deposition contracts at a much slower rate 

 than the main area of depression, and, in consequence, is forced 

 up by the introduction of material squeezed out from beneath tliat 

 area. Thus the tendency is, as I stated above, to narrow the main 

 area of depression. 



The last point in Mr. Eeade's paper, to which I wish to call 

 attention, has reference to the effects of denudation. 



As far as I am able to understand the very brief statement con- 

 tained in his paper, Mr. Reade argues that, as foot after foot of rock 

 is removed by denudation, the temperature of any point beneath 

 is gradually lowered, so that contraction sets in, which doubtless 

 tends to draw the rocks in such an area of denudation away from 

 those surrounding it. 



The actual results which Mr. Reade considers would follow are 

 best given in his own words : — 



" The weight of the crust itself squeezes up all vacuities, and the 

 area adjusts itself to its decrease of volume by normal faulting and 

 keying up in a wedge-like manner and by compressive extension." 



Now to bring about normal faulting the rocks must not only split 

 apart, which is the necessary result of contraction, but this must be 

 accompanied by vertical displacement, and I can see no possible 

 cause contained in the theory which accounts for the vertical sinking 

 of one part of the area of denudation relative to another. 



Again, as regards what Mr. Reade calls compressive extension, 

 I think it is a sufficient answer to point out that contraction is 

 greatest nearest the surface ; for it is always there that the reduction 

 of temperature must originate. In other words, the first layers to 

 contract are the outer ones, or those under the least overlying 

 weight ; and further, in contracting, each layer must tend to assist 

 the contraction of one lower. To bring to a close a paper perhaps 

 already inexcusably long T will briefly recapitulate my own views 

 on the results to be expected in such a contracting area of denudation. 



In so far as contraction is general over the area, the tendency will 

 be for the whole area to attempt to bend inwards, and the result 

 of such a tendency is to check the introduction of material which, 



