494 Fisher — Elevation of Mountain Chains. 



of the opposite sides of any cubical element of this shell wiir be 

 equal to the weight of a column of rock of the same sectional area 

 and density, and of the length of half the earth's radius. This 

 would be sufficient to crush any strata, and is, I believe, the force 

 to which the elevation of mountains is due. 



But why should the outer crust have ever lost the support 

 of the inner portions of the earth ? Probably from the effects of 

 contraction in cooling. Granites contract in volume in passing 

 from a fluid to a crystalline state, ^ and the columnar structure 

 proves the like for Basalts. It may be objected that this is not 

 true of all bodies, for some, like water, expand in solidifying. 

 But if it be true of those of which the outer layers of our globe 

 chiefly consist, it is sufficient for my argument. Suppose then that 

 the internal strata of the earth have lost some heat since the outer 

 crust became consolidated, and we have a cause adequate to the ele- 

 vation of mountain chains. In this view there is now nothing 

 original (although I excogitated it for myself in 1841) except that, 

 as far as I know, the approximate amount of the horizontal com- 

 pressing force has not been calculated before. 



It does not appear to me that this view requires us to suppose the 

 interior of the earth to have been fluid before the mountains arose, 

 but only highly heated. There are strong reasons, as Professor Sir 

 W. Thomson and Mr. Hopkins have shown, against supposing the 

 earth generally fluid internally. Sir W. Thomson, in his valuable 

 paper " On the Secular Cooling of the Earth," ^ expresses an opinion 

 that "at depths greater than 100 miles, the whole mass, or all except 

 a nucleus cool from the beginning, is (whether liquid or solid) pro- 

 bably at, or very nearly at, the proper melting temperature for the 

 pressure at each depth." 



But since it is rendered almost certain by other considerations that 

 it is solid, we arrive at the conclusion that that solidity must be due 

 to pressure. Eemove part of the pressure and the matter must pass 

 into a fluid state. 



Here, then, it appears to me, we find an explanation of volcanic 

 phenomena. Volcanoes, for the most part, follow mountain chains, 

 and earthquake phenomena still more constantly do so. 



Archdeacon Pratt has shown that the density of the Earth's crust 

 beneath mountain chains is less than elsewhere,^ and it seems natural 

 that it should be so, if they be corrugations of the surface formed by 

 lateral pressure ; because the matter of which the mountain is composed 

 will be partl}^ supported by the pressure which elevated it. Hence 

 there is a cause in diminished vertical pressure why the interior layers 

 beneath mountains should pass into a state of fusion, and the water 

 contained in them assuming, as it ascends, a gaseous state, will, under 

 favourable circumstances of facility of exit, cause that ebullition of 

 lava, in which a volcanic eruption essentially consists. 



It is the general opinion at the present time, that trains of vol- 



^ Jukes' Manual, p. 96. 



2 Trans. Roy. Soc. of Edinburgh (and Phil. Mag. Series 4, Vol. xxv.) § 16. 



' Fig. of the Earth. 3d. Edn., Art. 116. 



