132 DYNAMICAL GEOLOGY. 



this the temperature is invariable. The upper limit of 

 the region of invariable temperature is called the stratum 

 of invariable temperature. Its depth varies with latitude, 

 being nearest the surface at the equator, and lying deeper 

 as we go poleward. 



As already said, below this stratum the temperature 

 is invariable, but it increases as we go deeper. This 

 important fact has been proved by observations in mines 

 and artesian wells. It is true everywhere, but the rate 

 of increase varies, being in some places more rapid (1 

 in thirty feet), in some less rapid (1 in ninety feet). 

 The average may be taken, for convenience, at 1 for 

 every fifty-three feet, or 100 for every mile of depth. 



The Interior Condition of the Earth. Now it is 

 easy to see that at this rate the melting temperature for 

 most rocks, say 3,000, would be reached at a depth of 

 about thirty miles. Hence, many persons have rashly 

 concluded that the earth is essentially an incandescent, 

 liquid mass, covered with a comparatively thin shell of 

 thirty miles. This would correspond, in a ball of two 

 feet diameter, to a shell of less than one tenth inch thick. 

 On this view, volcanoes are supposed to be openings into 

 this general interior liquid. 



A little reflection, however, suffices to show that this 

 condition of the interior is improbable. It is almost cer- 

 tain, in the first place, that the rate of increase is not 

 uniform, but decreases, and therefore that the temperature 

 of 3,000 would be found only at a much greater depth 

 than thirty miles. In the second place, 3,000 is the 

 fusing point under atmospheric pressure ; but under the 

 enormous pressure of thirty to fifty miles of rock, the fus- 

 ing point would probably be much higher. Taking these 

 two things into account, it seems certain that, if there be 

 a universal interior liquid at all, the solid shell is much 

 thicker than is usually supposed, and even probable that 

 there is no universal interior liquid at all and that vol- 



