12 THE AGE OF THE EARTH 



state to that of glass ; in the case of diabase * the 

 difference in volume of the rock in the two states at 

 ordinary temperatures is 13 per cent. If the relief of 

 pressure over the site of continents were accompanied by 

 volume changes at all approaching this, the additional 

 elevation of seventeen twenty-sevenths required to raise 

 the land to the sea-level would be accounted for.f How 

 far down beneath the surface the unloading of the 



* C. Barus so names the material on which he experimented ; 

 apparently the rock is a fresh dolerite without olivine. 



f Professor Fitzgerald has been kind enough to express part of the 

 preceding explanation in a more precise manner for me. He writes : 

 " It would require a very nice adjustment of temperatures and 

 pressures to work out in the simple way you state it ; but what is 

 really involved is that in a certain state diabase (and everything that 

 changes state with a considerable change of volume) has an enormous 

 isothermal compressibility. Although this is very enormous in the 

 case of bodies which melt suddenly, like ice, it would also involve 

 very great compressibilities in the case of bodies even which melted 

 gradually, if they did so at all quickly, i.e., within a small range of 

 temperature. What you postulate, then, is that at a certain depth 

 diabase is soft enough to be squeezed from under the oceans, and that, 

 being near its melting-point, the small relief of pressure is accompanied 

 by an enormous increase in volume which helped to raise the con- 

 tinents. Now that I have written the thing out in my own way it 

 seems very likely. It is, anyway, a suggestion quite worthy of serious 

 consideration, and a process that in some places must almost certainly 

 have been in operation, and maybe is still operative. Looking at it 

 again, I hardly think it is quite likely that there is or could be much 

 squeezing sideways of liquid or other viscous material from under one 

 place to another, because the elastic yielding of the inside of the earth 

 would be much quicker than any flow of this kind. This would only 

 modify your theory, because the diabase that expands so much on the 

 relief of pressure might be that already under the land, and raising 

 up this latter, partly by being pushed up itself by the elastic relief of 

 the inside of the earth, and partly by its own enormous expansibility 

 near its melting-point. The action would be quite slow, because it 

 would cool itself so much by its expansion that it would have to be 

 warmed up from below, or by tidal earth-squeezing, or by chemical 

 action, before it could expand isothermally." 



