10 THE AGE OF THE EARTH 



according to Lord Kelvin, that the temperature con- 

 tinuously increased from the surface, where it was 

 1,200 C., down to a depth of twenty-five miles, where it 

 was about 1,460 C., or 260 C. above the fusion-point of 

 the matter forming the crust. That the crust at this 

 depth was not molten but solid is to be explained by the 

 very great pressure to which it was subjected just so 

 much pressure, indeed, as was required to counteract the 

 influence of the additional 260 C. Thus if we could have 

 reduced the pressure on the crust we should have caused 

 it to liquefy ; by restoring the pressure it would resolidify. 

 By the time the earth's surface had cooled down to 

 370 C. the depth beneath the surface at which the 

 pressure just kept the crust solid would have sunk some 

 slight distance inwards, but not sufficiently to affect our 

 argument. 



The average pressure of the primitive atmosphere upon 

 the crust can readily be calculated by supposing the water 

 of the existing oceans to be uniformly distributed over the 

 earth's surface, and then by a simple piece of arithmetic 

 determining its depth : this is found to be 1'718 miles, the 

 average depth of the oceans being taken at 2 '393 miles, 

 Thus the average pressure over the earth's surface, imme- 

 diately before the formation of the oceans, was equivalent 

 to that of a column of water T718 miles high on each 

 square inch. Supposing that at its origin the ocean 

 were all " gathered together into one place," and " the 

 dry land appeared," then the pressure over the ocean 

 floor would be increased from 1*718 miles to 2*393 miles, 

 while that over those portions of the crust which now 

 formed the land would be diminished by 1*718 miles. 

 This difference in pressure would tend to exaggerate those 

 faint depressions which had arisen under the primitive 

 anti-cyclonic areas, and if the just solidified material of 

 the earth's crust were set into a state of flow it might 



