83 



were warmer, it may be equally said that formerly these regions 

 were much colder, for we find signs of extensive ice action down 

 as low as latitude 40'=' north, and clear proofs that, at any rate in 

 the Pliocene and upper Miocene strata, an arctic degree of cold 

 once existed. In fact that the surface of the globe, at least 

 between the pole and latitude 40° north, has not gradually and 

 steadily become cooler, but that it has alternately been hot and 

 cold. 



Now bearing in mind that cold is a negative quality merely, 

 that it is an absence of heat ; what does this prove ? Clearly that 

 tlie heat or the want of it was due to causes, exterior to the 

 earth, and not from within. For if the heat were from within, 

 it follows that, if the earth were gradually cooling down, it would 

 cool most rapidly where the radiation was greatest, and where it 

 was less supplied with fresh heat from without, as at the poles, 

 and if it had once cooled down in those parts to a great degree, 

 although the cooling might be afterwards checked by an increase 

 of solar heat from without, yet that there would exist a great 

 degree of diiference between the intrinsic heat of the earth below 

 its surface, in the trojjics which have never 1)cen subject to the 

 extremes of surface cold, and a latitude of, say 45°, which has 

 been so subject to arctic cold. Now this is just what we do not 

 find to be the case, for at a distance from the surface of the earth 

 to which the sun's rays do not penetrate, we find that the earth's 

 heat is the same in both latitudes, viz., 50°, and the difference of 

 depth to reach that heat is nearly the same in both latitudes or 

 within a depth of about 20 feet — that is to say, that at about 100 

 feet deep at the equator, or 80 feet in latitude 45^* the tempera- 

 ture is constant at 50"^. 



Again, how is it that at the bottom of the deep seas, where 

 you may ajiproach 7 miles nearer to the earth's centre, we find 

 the temperature is under 40°, and does not show any signs of 

 being heated from below — in fact is exactly the temperature it 

 should be, assuming that it is only heated from without, by the 



