6 THE STORY OP THE EARTH AND MAN. 



physicist, Delaunay, and for some time it made geo 

 legists suppose that, after all, the earth's crust may 

 be very thin. Sir William Thomson, however, and 

 Archdeacon Pratt, have ably maintained the previous 

 opinion, based on Hopkins' calculations; and it is 

 now believed that we may rest upon this as repre- 

 senting the most probable condition of the interior 

 of the earth at present. Another fact bearing on 

 this point is the form of the earth, which is now 

 actually a spheroid of rotation; that is, of such a 

 shape as would result from the action of gravity and 

 centrifugal force in the motion of a huge liquid drop 

 rotating in the manner in which the earth rotates. 

 Of course it may be said that the earth may have 

 been made in that shape to fit it for its rotation ; but 

 science prefers to suppose that the form is the result 

 of the forces acting on it. This consideration would 

 of course corroborate the deductions from that just 

 mentioned. Again, if we examine a map showing the 

 distribution of volcanoes upon the earth, and trace 

 these along the volcanic belt of Western America and 

 Eastern Asia, and in the Pacific Islands, and in the 

 isolated volcanic regions in other parts of the world ; 

 and if we add to these the multitude of volcanoes now 

 extinct, we shall be convinced that the sources of 

 internal heat, of which these are the vents, must be 

 present almost everywhere under the earth's crust. 

 Lastly, if we consider the elevations and depressions 

 which large portions of the crust of the earth have 

 undergone in geological time, and the actual crump- 



