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



The geometrical equation (1) will then become as before, 

 kle = :S (a) - H {b). (2) 



And if there be no depressions below the upper datum leyel, we 

 haye, as in the former case, 



kle=l!(a). (A) 



It may be asked why the weight of the rock hl(l + e) should not 

 force back the walls of the chasm until it was wide enough to 

 admit of its reintroduction. The reason is because the lateral 

 pressure caused by the shrinking of the interior is so enormous that 

 that could not happen. Hence, in truth, the rock replaced would be 

 supported somewhat in the manner of a piston. 



If this reasoning be correct, as I believe it to be, a very remark- 

 able result follows from it. The pressure of the superheated rock 

 upwards upon the given area, whose base is Z (1 -f e) x 1, equals 

 the weight of the rock above it, and upon a unit of area will be 

 k (1+e) p g Bit the lower datum level, where g is the force of gravity. 



Now the pressure Ic pg would be sufficient to bring the super- 

 heated rock to the upper datum level. Hence there is an excess of 

 pressure equal to Jc pg e, which would serve to raise it to an addi- 

 tional height Jc e above the upper datum level. 



This will account for the steady welling out of lava from a vent, 

 or for its standing at a nearly uniform level in a crater, as in 

 Hawaii. By our former investigation, which we see by (A) to be 

 applicable to the present case, the average height of the mountains 

 formed by the contraction would be 



h = 2k me. 



Hence the lava would rise to the height -— , or to the surface which 



2 m 



would be produced if the mountains were all levelled down. 



This is much less than the height to which mountains might be 

 thrown up, and leads one to ask whether there may not be here an 

 explanation of the fact mentioned by Mr. Scrope, that although the 

 peaks of the South American Volcanoes (which are the most lofty in 

 the world) are frequently seen to emit vapour and ashes, we rarely 

 hear of their giving issue to any great lava currents in recent times. 

 The lavas of these mountains, he tells us, generally appear to have 

 burst forth from lateral fissures on the flank of the lofty snow- 

 covered volcanic peaks which crown the range. A distinction must 

 be made between ebullition and a steady welling oxit of lava. 



It must be remembered in these estimates, that no mountains in 

 nature can reach the full height above the datum level which the 

 formulae give, partly because the curved contour gives a less altitude 

 than the assumed triangular one, and partly on account of the full 

 effect of elevation not being produced owing to friction and other 

 causes, and also on account of subsequent denudation. 



To revert to the connexion which Mr. Medlicott perceives between 

 the supra-Nummulitic deposits of India and movements of elevation. 



On the view which I advocate of mountain chains being produced 

 by the shrinking of the earth's interior, the local pressure, caused by 

 a fresh deposit disturbing the equilibrium in a certain region, in 



