32 THE AGE OF THE EARTH 



spherical, but will rise or fall beneath an imaginary 

 spherical or spheroidal surface according as they occur 

 beneath the continents or the oceans. No doubt at 

 some depth within the earth the departure of isothermal 

 surfaces from a spheroidal form will disappear ; but con- 

 sidering the great breadth both of continents and oceans 

 this depth must be considerable, possibly even forty or 

 fifty miles. Thus the sub-continental excess of tempera- 

 ture may make itself felt in regions where the rocks still 

 retain a high temperature, and probably not far removed 

 from the critical fusion-point. The effect will be to 

 render the continents mobile as regards the ocean floor ; 

 or, vice versa, the ocean floor will be stable compared with 

 the continental masses. Next it may be observed that 

 the continents pass into the bed of the ocean by a some- 

 what rapid flexure, and that it is over this area of flexure 

 that the sediments denuded from the land are deposited. 

 Under its load of sediment the sea-floor sinks down, sub- 

 siding slowly, at about the same rate as the thickness of 

 sediment increases ; and, whether as a consequence or a 

 cause, or both, the flexure marking the boundary of land 

 and sea becomes more pronounced. A compensating 

 movement occurs within the earth's crust, and solid 

 material may flow from under the subsiding area in the 

 direction of least resistance, possibly towards the land. 

 At length, when some thirty or forty thousand feet of 

 sediment have accumulated in a basin-like form, or, 

 according to our reckoning, after the lapse of three or four 

 millions of years, the downward movement ceases, and 

 the mass of sediment is subjected to powerful lateral com- 

 pression, which, bringing its borders into closer proximity 

 by some ten or thirty miles, causes it to rise in great folds 

 high into the air as a mountain chain. 



It is this last phase in the history of mountain-making 

 which has given geologists more cause for painful thought 



