the Earth's Contraction from Cooling. 215 



could hardly have been due simply to radial contraction from 

 cooling; for this would make the cooling over the tropical part 

 of the ocean in this small part of geological time sufficient to 

 produce there a sixth of the oceanic depression. Is it not 

 proof that even then the plastic layer had enough of extent 

 beneath the tropical part of the oceanic crust to permit of such 

 a sinking under the irresistible lateral pressure at work ? How- 

 ever this may be, it stands as a fact to be explained. 



In view of the conclusions here reached with regard to the 

 earth's interior, I present the following statements : — 



1. That this restriction of the interior liquidity of the earth 

 to an undercrust layer does not require in itself any modifica- 

 tion of the views I presented more than twenty-five years since 

 on the results of the earth's contraction, since there is still a 

 flexible crust and mobile rock beneath it. 



2. The condition of the earth's interior here recognized is, 

 as many readers will have observed, that suggested long ago by- 

 Professor W. Hopkins, the author who first offered a mathema- 

 tical argument in favour of the earth's either having a very thick 

 crust or being solid throughout*. In a paper " On Theories 

 of Elevation and Earthquakes" in 1847 f, Professor Hopkins 

 argues that the central mass of the earth became solid in conse- 

 quence of the pressure whenever the temperature within reached 

 a limit that permitted of it, that crusting at the surface from 

 cooling commenced afterwards, and that between the regions 

 of interior and exterior solidification there long remained a 

 viscous layer, which in the progress of time was gradually 

 contracted by the union of the solid nucleus to the thickening 

 shell. 



3. The possibility of solidification at centre from pressure in 

 the face of a temperature too high for consolidation from cool- 

 ing has not been experimentally demonstrated ; yet a number 

 of facts favour the principle. It has been urged that since the 

 solidification of rocks is attended by contraction (that is, by in- 

 crease of density), and since pressure tends to produce this 



* Trans. Roy. Soc. 1839, 1840, 1842. 



t Report Brit. Assoc, for 1847, p. 33. The theory of elevation advo- 

 cated in this paper attributes elevation, not to lateral pressure from con- 

 traction, but to evolved vapours underneath the elevated region. The 

 array of facts which have been presented respecting the positions of moun- 

 tain-ranges, their relations to the great areas of depression, their successive 

 formation on sea-borders in parallel ranges, and the natural evolution of 

 the whole from the universal action of the one great cause (contraction), 

 has appeared to me to afford the most complete demonstration that the 

 vapour theory is not necessary, at least as regards mountain-ranges. The 

 fact also that mountains so raised could not hold themselves up has 

 seemed to be an insuperable difficulty to the success of the method. 



