Rev. 0. Fisher — On the Formation of Mountains. 257 



longitudinal elevated region extending from Spain to China, the 

 concomitant Niimmulitic formation scarcely proves that the elevation 

 of that tract was the result of its deposition. True it is, howevei*, 

 that Mr, Medlicott traces a connexion between the " supra-Nummu- 

 litic deposits " and movements of disturbance in the Shillong Plateau, 

 which he considers an example of " the theory of MM. Herschel and 

 Babbage." But that theory, as I understand it, does not include the 

 arch-movement of Capt. Hutton, against which the above objections 

 are chiefly urged. 



Moreover, the effects attributed by Herschel to the weight of new 

 deposits, stand on different grounds, and from the few remarks of 

 Mr. Medlicott, in the passage referred to, it seems to be to these that 

 he alludes.^ 



The above conclusions have been arrived at in accordance with 

 Captain Hutton's assumption that a layer of rock expands through 

 the addition of heat. It will be observed that this introduces the 

 condition that the specific gravity of the disturbed rock is less than it 

 was before. 



If, however, the crushing of the layer arises from contraction of 

 the earth's interior, the reasoning will be somewhat altered. In order 

 to get a clear conception of the circumstances, let us suppose a portion 

 of the crust removed before the contraction occurs. We will suppose 

 the superheated rocks below to be virtually of the same specific 

 gravity as the crust. They would in that case rise level with the 

 surface. Suppose contraction now to take place. Then the edges of 

 the chasm which we have made will approach one another, but the 

 superheated rock will still rise only to the surface-level, so far as 

 anything at present noticed is concerned. We suppose that the width 

 of the chasm we made was I (1 + e), and that it contracts to I. 

 But now we cannot restore all the rock that was removed, for Jc I oi 

 rock will exactly fill the space, and hi {1 -\- e) was what was taken 

 out of it. Hence k I e ot rock must either be left out, or else 

 heaped upon the surface. 



Now, if we were only considering a limited area of the earth's 

 surface, the whole A; Z (1 + e) of rock would sink into the cavity, 

 being, in the case we are now considering, virtually of the same specific 

 gravity as the fluid beneath. It would sink until the surface was 

 level as before with the general surface, the surplus /c Z e of fluid dis- 

 tributing itself throughout the interior, and from its comparatively 

 small amount being, as it were, lost. But since the same amount of 

 shrinking that we have supposed to take place over one area occurs 

 to an equal amount over every equal area, this cannot happen : so that 

 the kle of rock must stand above the datum level and form mountains. 



Moreover, the condition, that no part of the superheated rock can 

 be lost in the interior, necessitates that any portion of it, which is 

 displaced below C D, must be compensated for by an equal portion 

 rising above that line. And this will be expressed by the condition 

 that 



* Geological Sketch of the Shillong Plateau, Geological Survey of India, p. 40. 



VOL. X. — NO. CVIII. 17 



