20 GEOLOGICAL FORMATIONS. 



ally the opinion of geologists, and is confirmed by the follow- 

 ing, among other considerations : 



1. The earth is not a perfect globe, but an oblate spheroid, 

 flattened at the poles the polar diameter being about twenty- 

 six miles shorter than the equatorial. This is the form which 

 it would necessarily have assumed from the centrifugal force 

 caused by diumal revolution, supposing it to have been orig- 

 inally in a fluid state. 



2. There is good evidence that our planet is still a vast ball 

 of liquid fire, surrounded by a thin crust, which, in thickness, 

 bears no greater proportion to the general mass of the earth, 

 than the egg-shell bears to the general mass of the egg. From 

 careful observations which have been made during many 

 years, upon the temperature of deep mines and the waters of 

 artesian wells, in various parts of the 'world, it is found that, 

 after descending beyond the reach of solar influence, the tem- 

 perature invariably increases, in all places, at the average rate 

 of about one degree Fahrenheit for every forty-five or fifty 

 feet of descent. And this rule uniformly holds good to the 

 greatest depths to which the earth has been penetrated. 



Now, assuming fifty degrees as the average temperature at 

 the surface of the earth, and taking the mean ratio of increase 

 at one degree for every fifty feet of descent, we should, at this 

 rate, at a depth of a little more than sixty-five miles, reach a 

 temperature of seven thousand degrees, which would be suffi- 

 cient to melt all known rocks. Supposing this state of igneous 

 fusion to extend from the comparatively thin crust of the earth 

 on all sides, to the center, we have still a mass of molten lava 

 of more than seven thousand miles in diameter. If we suppose 

 this mass to become sometimes agitated in its higher portions 

 by internal gasses, or by the percolation of water through 

 fissures in the superincumbent strata, we have a sufficient ex- 



