The Interior of the Earth 



us regarding the nature of these internal 

 media. 



Thus the study of sudden convulsions of the 

 ground confirms the admitted hypothesis con- 

 cerning the probable thickness of the solid crust. 

 But the ground is also subject to other motions, 

 which would certainly have escaped our notice, 

 if long and careful observation had not estab- 

 lished their existence beyond a doubt ; these are 

 secular variations, elevations, subsidences and 

 lateral displacements, which are continually 

 modifying the general configuration of our 

 globe. 



The sea supplies in its unchanging level a 

 surface of reference which enables us to calculate 

 these displacements. The total volume of water 

 in the ocean has, in fact, not varied sensibly 

 since the commencement of historic time, as 

 otherwise the coast line would be found to have 

 risen or sunk as a whole. Now this is not borne 

 out by our observations, and we cannot admit 

 the view of the ancients that the earth was once 

 a small island, and has gradually grown by the 

 deposit thrown up by the waves. The move- 

 ments of the ground have invariably been of 



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