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242 EECAPITULATION OF CHAPS. XXXIL AND XXXIII. [Ch. XXXIII. 



of a central nucleus slowly cooling from a state of fusion by 

 heat, has had to be given up, now that granite is found to be 

 of all ages, and now that we know the metamorphic rocks to 

 be altered sedimentary deposits implying the denudation of a 



previously solidified crust. 



12. (Chap. XXIII.) The powerful agency of steam or 

 aqueous vapour in volcanic eruptions leads us to compare its 

 power of propelling lava to the surface with that which it 

 exerts in driving up water in the pipe of an Icelandic Geyser. 

 Various gases, 



also rendered liquid by pressure at great 



may 



and convulsing the rocks during earthquakes. 

 13. The number of active volcanos on se^ 

 islands is probably connected with the agency of water in 



volcanic operations. The latest chemical observations on 

 the products of recent eruptions, favour the doctrine that 

 large bodies of salt water gain access to the volcanic foci. 



14. The flexibility of certain parts of the earth's crust, as 

 deduced from observations on earthquakes, may imply the 

 continuous existence of vast reservoirs of melted matter 



mi 



very subordinate place in the earth's crust. 



15. The existence of electrical currents in the earth's crust, 

 and the changes in direction which they may undergo after 

 great geological revolutions in the position of mountain chains 

 and of land and sea, the connection also of solar and terres- 

 trial magnetism, and of this last with electricity and chemical 

 action, may help us to conceive such a cycle of change as may 

 restore to the planet the heat supposed to be lost by radiation 



into space. 



16. The permanent elevation and subsidence of land now 

 observed, and which has been going on throughout past 

 geological ages, may be connectad with the expansion and 

 contraction of parts of the solid ^-^rust, some of which have 

 been cooling from time to time, while others have been 

 gaining fresh accessions of heat. 



17. In the preservation of the average proportion of land 

 and sea, the igneous agents exert a conservative power, re- 

 storing the unevenness of the surface, which the levelhng 





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