MOUNTAIN BUILDING 33 



than probably any other branch of their subject, not 

 excluding even the age of the earth. It was at first 

 imagined that during the flow of time the interior of the 

 earth lost so much heat, and suffered so much contraction 

 in consequence, that the exterior, in adapting itself to the 

 shrunken body, was compelled to fit it like a wrinkled 

 garment. This theory, indeed, enjoyed a happy existence 

 till it fell into the hands of mathematicians, when it fared 

 very badly, and now lies in a pitiable condition neglected 

 of its friends.* 



For it seemed proved to demonstration that the con- 

 traction consequent on cooling was wholly, even ridicu- 

 lously, inadequate to explain the wrinkling. But when 

 we summon up courage to inquire into the data on which 

 the mathematical arguments are based, we find that they 

 include several assumptions the truth of which is by no 

 means self-evident. Thus it has been assumed that the 

 rate at which the fusion-point rises with increased 

 pressure is constant, and follows the same law as is 

 deduced from experiments made under such pressures as 

 we can command in our laboratories down to the very 

 centre of the earth, where the pressures are of an alto- 

 gether different order of magnitude ; so with a still more 

 important coefficient, that of expansion, our knowledge of 

 this quantity is founded on the behaviour of rocks heated 

 under ordinary atmospheric pressure, and it is assumed 

 that the same coefficient as is thus obtained may be safely 

 applied to material which is kept solid, possibly near 

 the critical point, under the tremendous pressure of the 

 depths of the crust. To this last assumption we owe the 

 terrible bogies that have been conjured out of " the level 

 of no strain." The depth of this as calculated by the 



* With some exceptions, notably Professor George Darwin and 

 Mr. C. Davison, who have consistently supported the theory of 

 contraction. 



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