AND WATER ON THE EARTH. 323 



of the difference due to the flow of heat from the regions below the 

 continents to those below the oceans, so that the mean difference of 

 our two cones would be less than i6°. A mean difference of i6° 

 would account for about looo meters difference of level at the 

 earth's surface. 



It seems difficult to imagine any probable distribution of tem- 

 perature in the earth that would cause a difference as much as i6° 

 in the mean temperature difference of oceanic and continental cones. 

 And this is only about one-fifth as much as Trabert asks to account 

 for the difference of elevation of 5000 meters ; and when we con- 

 sider that there are considerable tracts of sea bottom and of plateau 

 land that differ in level by twice that amount it seems to exclude a 

 mere diff'erence of temperature as a sufficient cause of the dift'erent 

 levels of the earth's surface. As further confirmation of this con- 

 clusion we notice that the antarctic continent and Greenland are 

 buried under ice which keeps their surface temperature quite as low 

 as the sea bottom, and still they are both land areas. 



Joseph LeConte^° ascribed the ocean basins to greater cooling 

 and contraction on account of greater conductivity for heat of the 

 underlying material. What little information we have on this sub- 

 ject is opposed to the idea. For basaltic rocks, which characterize 

 the oceanic areas, have a smaller conductivity and diffusibility than 

 the granitic rocks, which are mainly continental, or the sedi- 

 mentaries. 



Sollas^^ has suggested the following, on the hypothesis of a 

 cooling earth : When the earth was still very hot, all the water 

 would be in the atmosphere as vapor, and would exert practically a 

 uniform pressure on all parts of the earth. When the temperature 

 fell sufficiently for this water to exist in a liquid form it would 

 occupy the slight depressions which must have existed, increasing 

 the pressure there and reducing the pressure over the high regions 

 As the crust of the earth was then very near its melting point and 

 the pressure due to the water was important, there may have been 

 considerable compression under the oceans and expansion else- 



10 " \ Theory of the Formation of the Great Features of the Earth's Sur- 

 face," Amer. Jour. Sci., 1872, IV., 345-355, 460-472. 



11 B. A. A. S., 1900, pp. 714-716. 



