418 REACTION OF THE INTERIOR OF THE EARTH 



In inquiries respecting the geographical distribution 

 of volcanoes, and their greater frequency on islands and 

 coasts (i. e. margins of elevation of continents), the 

 probable great inequality of the already attained thick- 

 ness of the earth's crust has also been brought into 

 discussion. One is inclined to assume the surface of 

 the interior molten mass to be nearer the points where 

 volcanoes have broken forth. But inasmuch as we 

 can imagine many intermediate degrees of consistency 

 in the solidifying materials, it is difficult to form 

 any clear conception of such a supposed surface of 

 the molten mass, if we are to look for the principal 

 cause of all overthrowals, fissurings, elevations, and 

 basin-shaped subsidences, in an alteration of capacity in 

 the external already solidified shelL If it were admissi- 

 ble to determine the so-called thickness of the solid 

 crust of the globe ( 573 ) from the data we have obtained 

 in Artesian wells and the temperature of fusion of 

 granite, by an arithmetical series, i. e. by assuming 

 throughout equal geothermic gradations of depth, we 

 should find it not quite twenty-one geographical miles, 

 or 3-^-9 th of the polar diameter ( 574 ) ; but the effects of 

 pressure and of conduction of heat in different kinds of 

 rock give us reason to suppose that the increment of 

 heat in descending will become less rapid as the depth 

 increases. 



Notwithstanding the very small number of points at 

 which the molten interior of our planet is at the present 

 time in active communication with the atmosphere, it is 

 yet a question not without importance if we ask, what 

 is the kind and the measure of influence exercised by 

 volcanic gaseous exhalations on the chemical compo- 





