as applied to the Origin of Mountains. 317 



with the very unknown volume which has, in reality, been employed 

 in constructing all the mountains since that period. 



I now wish to examine Mr. Reade's theory, and to compare it 

 with that put forward in this paper. 



In the first place, Mr. Eeade imagines a great depth of sediment 

 to be laid down over a large area, and, to fix ideas, suggests an 

 average depth of 4 miles of sediment deposited over an area 2000 

 miles by 1000 miles. 



No doubt it follows, with perfect accuracy, that, as the layers 

 accumulate with great slowness, the temperature gradient at any 

 time during deposition will simultaneously attain its average value, 

 and be roughly, say, 1° F. in 50 feet downwards ; so that, as any 

 laj'er becomes buried deeper and deeper beneath successive deposits, 

 its temperature will continually rise. 



For the same reasons, the rocks which constitute the floor upon 

 which sediment was first deposited will suifer a continual increase 

 of temperature. After a certain accumulation of sediment, the expan- 

 sion produced by this rise of temperature will cause the underlying 

 rocks to curve upwards, so that the curvature of any layer, which 

 was at first approximately that of the surface of the earth, becomes 

 increased and allows of expansion, much in the same way that a 

 bar of iron, bent into an arc and fixed at each end, will, if heated 

 in the middle, bend into a steeper curve. 



I have taken great care to state exactly what I understand 

 to be Mr. Reade's views, in order that my criticism may be 

 comprehended better. I may, perhaps, just point out that this 

 extension has, of course, the effect of a vertical lift upon the over- 

 lying mass, and that it is to this vertical motion that Mr. Reade 

 attributes the inception of mountain ranges. 



Now, it is a fair deduction from the very slow rate at which 

 sediment accumulates, that the full eifect of extension proper to any 

 given depth of sediment will have been brought about at the time 

 at which that depth is established. Such a process would necessarily 

 go on until the central portion of the region was raised to the sea- 

 level and the deposition of sediment in consequence ceased, and 

 no further rise could take place, at least so far as due solely to 

 expansion of underlying rocks. 



The mechanism by which the fold thus formed is further com- 

 pressed and raised by lateral pressure, I fail to understand ; especially 

 in the light of Mr. Reade's insistence that expansion is internal and 

 shading off to zero at the boundaries of an area of deposition, so 

 that any new area of sedimentation which may be established on 

 the sides of the old fold would produce a new central fold, and not 

 exert lateral compression on the earlier one. It is also hard to see 

 how such an area of deposition could exist outside the first fold, 

 except as derived by marine denudation of that fold itself. 



Again, the conception that any fold is formed must necessarily 

 imply that the rocks which form it are drawn away from the under- 

 lying mass ; for it is upon the amount of lateral expansion, and not 

 upon the comparatively small vertical expansion, that the theory 



