34 THE AGE OF THE EARTH 



Rev. 0. Fisher is so trifling that it would be passed 

 through by all very deep mines.* Mr. C. Davison, how- 

 ever, 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 when regions are entered, where the rocks 

 are compelled into the solid state by pressure. So little 

 do we actually know of the behaviour of rock under these 

 conditions that the geologist would seem to be left very 

 much to his own devices ; but the evidence of earthquakes 

 shows 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 

 afford the explanation we seek. The course of events 

 may then proceed as follows. The contraction 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 con- 

 tinents and sink under the oceans, and the flexure of the 

 area of sedimentation is partly a consequence of this fold- 

 ing, 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 building. (Fief. 



o \ o 



2.) It may be observed that at great depths readjust- 

 ment will be produced by a slow flowing of solid rock, and 

 it is only comparatively near the surface, five or ten miles 

 at the most below, that failure of support can lead to 

 sudden fracture and collapse ; hence the comparatively 

 superficial origin of earthquakes. 

 * See Note on Mountain Chains in " Figure of the Earth," p. 59. 



