THE GENESJS OP THE EARTH. O 



of rocky matter. Thus a very slight step of imagi- 

 nation would carry us back to a time when this slender 

 crust had not yet formed, and the earth rolled through 

 space an incandescent globe, with all its water and 

 other vaporisable matters in a gaseous state, As- 

 tronomical calculation has, however, shown that the 

 earth, in its relation to the other heavenly bodies, 

 obeys the laws of a rigid ball, and not of a fluid 

 globe. Hence it has been inferred that its actual 

 crust must be very thick, perhaps not less than 2,500 

 miles, and that its fluid portion must therefore be of 

 smaller dimensions than has been inferred from the 

 observed increase of temperature. Further, it seems 

 to have been rendered probable, from the density 

 of rocky matter in the solid and liquid states, that 

 a molten globe would solidify at the centre as well 

 as at the surface, and consequently that the earth 

 must not only have a solid crust of great thickness, 

 but also a solid nucleus, and that any liquid portions 

 must be of the nature of a sheet or of detached 

 masses intervening between these. On the other 

 hand, it has recently been maintained that the calcu- 

 lations which are supposed to have established the 

 great thickness of the crust, on the ground that the 

 earth does not change its form in obedience to the 

 attraction of the sun and moon, are based on a mis- 

 conception, and that a molten globe with a thin crust 

 would attain to such a state of equilibrium in this 

 respect as not to be distinguishable from a solid 

 planet. This view has been maintained by the French 



