246 Br. T. Sterry Hunt — On Volcanic Action. 



which solidification would proceed downwards. There would thus 

 be enclosed, between the inner and outer solid parts, a portion of 

 uncongealed matter, which, according to Hopkins, may be supposed 

 still to retain its liquid condition, and to be the seat of volcanic 

 action, whether existing in isolated reservoirs or subterranean lakes ; 

 or whether, as suggested by Scrope, forming a continuous sheet sur- 

 rounding the solid nucleus, whose existence is thus conciliated with 

 the evident facts of a flexible crust, and of liquid ignited matters 

 beneath. 



Hopkins, in the discussion of this question, insisted upon the fact, 

 established by his experiments, that pressure favors the solidification 

 of matter, 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 effect 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 Eev. 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 Magazine 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 intermediate 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 supposed 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.^ The same conclusion 

 was subsequently adopted by Hanstein. 



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 liquid 

 was yet so deep that the refrigeration from that time to the present 



^ See Scrope on Volcanos, and also his communication to the Geological 

 Magazine for Dec, 1868. 



^ The elevated temperature of the 'interior of the globe would probably oflFer no 

 obstacle to the development of magnetism. In a recent experiment 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 

 of 1300° C, and retained its magnetism while cooling (Comptes Eendus de I'Acad. 

 des Sciences, Feb. 1869). 



