PART III, 



PHYSICAL STRUCTURE OF THE EARTH, 



CHAPTER 1. 



Structure of tlie Earth. 



" Ye mighty ones who sway the souls that go 

 Arnid the marvels of the world below! 

 Ye, silent shades, who sit and hear around ! 

 Chaos! and streams that burn beneath the ground! 

 All, all forgive, if by your converse stirred, 

 My lips shall utter what my ears have heard; 

 If I shall speak of things of doubtful birth, 

 Deep sunk in darkness, as deep sunk in earth." 



Virgil, 



WE have before shown that our globe is a planetary orb of a 

 few thousand miles in diameter, and of a spheroidal shape, the 

 difference between the polar, and equatorial diameters being 

 twenty-six miles. The mean density of the earth, is about five 

 times that of water, the interior being double that of the solid su- 

 perficial crust, hence if the interior of the earth be cavernous, 

 its crust must be composed of very dense materials. The crust, 

 or outer covering of the earth, significantly called " Erdrinde," 

 or Earth-rind, by the Germans, is that part to which our investi- 

 gations are naturally directed. The greatest thickness of this su- 

 perficial crust, which man has been able to explore, estimated 

 from the highest mountain peaks, to the greatest natural or arti- 

 ficial depths, does not exceed ten miles ; this, in comparison with 

 the diameter, 8000 miles, is a distance, utterly insignificant, bear- 



