EVOLUTIONAL GEOLOGY. 309 



enjoyed a happy existence till it fell into the hands of mathematicians, 

 when it fared very badly, and now lies in a pitiable condition neg- 

 lected of its friends.^ 



For it seemed proved to demonstration that the contraction conse- 

 quent on cooling- was wholly, even ridiculously, inadequate to explain 

 the wrinkling. But when we summon up courage to inquire into the 

 data on which the mathematical arguments are based, w^e 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 ex^3eriments made under such pressures as we 

 can counuand in our laboratories down to the very center of the earth, 

 where the pressures are of an altogether different order of magni- 

 tude; so with a still more important coefficient, that of expansion, 

 our knowledge of this quantity is founded on the behavior 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 tremen- 

 dous 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 Rev. O. Fisher is 

 so trifling that it would be passed through by all very deep mines. 

 Mr. C. Davison, however, has shown that it will lie considerably 

 deeper, if the known increase of the coefficient of expansion with rise 

 of temperature be taken into account. It is possible, it is even likely, 

 that the coefficient of expansion becomes vastly greater w^hen regions 

 are entered where the rocks are compelled into the solid state by pres- 

 sure. So little do we actually know of the behavior of rock under 

 these conditions that the geologist would seem to be left ver}- much to 

 his own devices; but it would seem there is one temptation he must 

 resist — he may not take refuge in the hypothesis of a liquid interior. 



We shall boldly assume that the contraction at some unknown depth 

 in the interior of the earth is sufficient to ati'ord the explanation we 

 seek. The course of events may then proceed as follows: The con- 

 traction of the interior of the earth consequent on its loss of heat 

 causes the crust to fall upon it in folds, which rise over the continents 

 and sink under the oceans, and the flexure of the area of sedimentation 

 is partly a consequence of this folding, partly of overloading. By 

 the time a depression of some 30,000 or 40,000 feet has occurred along 

 the ocean border the relation between continents and oceans has become 

 unstable, and readjustment takes place, probably by a giving way of 

 the continents, and chiefly along the zone of greatest weakness, i. e., 

 the area of sedimentation, which thus becomes the zone of mountain 



^ With some exceptions, notably Mr. C. Davison, a consistent supporter of the 

 theory of contraction. 



