June] HUNT — ON VOLCANIC ACTION, 167 



solidification of matters which, like rocks, pass in melting to a less 

 dense condition, and hence concludes that the pressure existing at 

 great depths must have induced solidification of the molten mass 

 at a temperature at which, under a less pressure, it would have 

 remained liquid. Mr. Scrope has followed this up by the 

 ingenious suggestion that the great pressure upon parts of the 

 solid igneous mass may become relaxed from the efiect of local 

 movements of the earth's crust, causing portions of the solidified 

 matter to pass immediately into the liquid state, thus giving rise 

 to eruptive rocks in regions where all before was solid.* 



Similar views have been put forward in a note by Rev. 0. 

 Fisher, and in an essay on the formation of mountain chains, by 

 Mr. N. S. Shaler, in the proceedings of the Boston Society of 

 Natural History, both of which appear in the Geological Maga- 

 zine for November last. As summed up by Mr. Shaler, the 

 second hypothesis supposes that the earth " consists of an 

 immense solid nucleus, a hardened outer crust, and an inter- 

 mediate region of comparatively slight depth, in an imperfect 

 state of igneous fusion." In this connection it is curious to 

 remark that, as pointed out by Mr. J. Clifton Ward, in the same 

 Magazine for December (page 581), Halley was led, from the 

 study of terrestrial magnetism, to a similar hypothesis. He sup- 

 posed the existence of two magnetic poles situated in the earth's 

 outer crust, and two others in an interior mass, separated from 

 the solid envelope by a fluid medium, and revolving, by a very 

 small degree, slower than the outer crust. f The same con- 

 clusion was subsequently adopted by Hansteen. 



The formation of a solid layer at the surface of the viscid and 

 nearly congealed mass of the cooling globe, as supposed by the 

 advocates of the second hypothesis, is readily admissible. That 

 this process should commence when the remaining envelope of 



* See Scrope on Volcauos, and his communication to the Geological 

 Magazine for Dec, 1868. 



t The elevated temperatm-e of the interiorof the globe would probably 

 offer no obstacle to the development of magnetism. In a recent experi- 

 ment of M. Treve, communicated by M. Faye to the French Academy 

 of Sciences, it was found that molten cast iron when poured into a 

 mould, surrounded by a helix which was traversed by an electric 

 current, became a strong magnet when liquid at a temperature ot 

 1300o C., and retained its magnetism while cooling (Comptes Eeudas 

 de I'Acad. des Sciences, Feb., 1869.) 



