THE CRUST OF THE GLOBE. 



35 



at 100 miles beneath the surface of the earth, even the 

 least fusible rocks and minerals may be in a melted state. 



8. Nature of the ckust of the globe. — The total 

 thickness of that portion of the rocks and strata accessible 

 to human observation, reckoning from the highest mountain 

 peaks to the greatest natural or artificial depths, is vari- 

 ously estimated at from ten to fifty miles. As the earth is 

 nearly eight thousand miles in diameter, the entire series of 

 strata hitherto explored is, therefore, but very insignificant 

 compared with the magnitude of the globe; bearing about 

 the same relative proportion as the thickness of this paper 

 to an artificial sphere a foot in diameter; the inequalities 

 and crevices in the varnish of such a ball would, in fact, 

 be equal in proportionate size to the highest mountains and 

 deepest valleys. 



The depth of the crust of the earth, and the greatest ine- 

 qualities of its surface, as compared with its mass, are ex- 



Lign\ 2. — Diagram to illustrate the proportionate thickness of the 

 earth's crust. 



pressed in the annexed diagram.* The line from e to k, 

 represents a depth of oOO miles ; to the point i, 100 miles ; 

 to the line Z, /, forty-five miles above the surface, the supposed 



* To preserve a k s far as possible the language and spirit of the 

 original lectures, the references to the diagrams and- specimens are 

 retained. 



d2 



