60 THE PROBABLE SEAT OF VOLCANIC ACTION. [VI. 



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 reser- 

 voirs or subterranean lakes ; or whether, as suggested by Scrope, 

 forming a continuous sheet surrounding 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 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 

 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 Rev. 0. 

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

 by 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 (p. 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, sepa- 

 rated from the solid envelope by a fluid medium, and revolving, 



* See Scrope On Volcanoes, and his communication to the Geological Mag- 

 azine for December, 1868. 



