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to the surface. This is easily accounted for by the variable na- 

 ture of the chemical actions which are going on in the rocks. 

 Parts of the rocks in deep mines, which are found intensely hot 

 at one time, are at another time found at a very low temperature 5 

 and such is the nature of the heat observed in all mines. 



That the intensity of action should be greater in depth gene- 

 rally than near the surface, is a reasonable supposition 5 but we 

 have no proof of any intense decomposition or igneous effects 

 taking place at those depths without the oxygen of the air or 

 water being connected with such points. And if so, the decom- 

 position of either element would be sufficient to account for any 

 intense action taking place without having recourse to the hypo- 

 thesis of central heat. All that we require to account for the 

 observed geological phenomena is a current circulating within 

 the crust of the globe, acting chemically and mechanically on 

 those elements which form the external part. 



It is the opinion of some very profound geologists, that the 

 globe was originally in a state of igneous fusion, and that as 

 this heated mass began gradually to cool, an exterior crust was 

 formed, first very thin, and afterwards gradually increasing, 

 until it attained its present thickness, which they calculate as 

 amounting to sixty miles. During this process of gradual re- 

 frigeration, some portions of the crust cooled more rapidly than 

 others, and the pressure on the interior igneous mass being un- 

 equal, the heated matter, or lava, burst through the thinner parts 

 and caused high peaked mountains. The same cause they allege 

 produces volcanoes. 



According to this theory, we live upon a thin crust enclosing 

 matter in a state of intense heat, which in particular districts 

 agitates the earth in its pressure to escape, thus causing earth- 

 quakes, or occasionally bursting forth and producing volcanoes. 



The arguments adduced for such a doctrine are the following : 



First. That the form of the earth is just that which an igneous 

 liquid mass would assume, if thrown into an orbit with a motion 

 similar to that of the earth ; as if an aqueous liquid under similar 

 circumstances could not produce the same kind of figure. 



Second. That it is found that heat increases with the depth in 

 mines ; thus inferring that there must be fire to produce heat ; 

 whereas it is proved by experiments that chemical action is the 

 immediate cause of heat. «, 



