igoS.J THE PHYSICS OF THE EARTH. 211 



plateau is comparable with that in Asia, and it is easy to see how 

 the relief of the Pacific on our side may have taken the form of a 

 table-land of greater width but smaller height. The numerous 

 parallel mountain chains west of the Rocky ^Mountains show the 

 nature of the mighty forces at work, and prove that this uplift was 

 the work of the Pacific Ocean. 



§ 27. The forces which have raised the mountains and plateaus 

 of the globe are identical with those zchich have raised the conti- 

 nents above the sea, and all these forces depend on the leakage of the 

 oceans. — The geological evidence of the slow operation of the forces 

 which have uplifted the plateaus and mountains shows the immeas- 

 urable ages during which they have been at work. Sometimes large 

 portions of a continent have risen for a time, and again slowly 

 subsided, and thus have arisen the phenomena noted in the sedi- 

 mentary rocks studied in geology. These gentle movements often 

 are without violent earthquake shocks, because the yielding is very 

 gradual, and the crust is slowly raised up and down without breaking. 

 It is only where the expulsion of lava from under the sea is rapid 

 and violent that breaking develops at such rate as to form mountain 

 chains and plateaus. The uplift of a" plateau also requires a large 

 amount of material. Where the process is gentle and gradual a 

 whole continent may be slowly uplifted, and this process evidently 

 has raised the low broad plains above the water. The cause of 

 epeirogenic and of orogcnic movements is everyn'here one and the 

 same. The movements take different forms according to the sud- 

 denness with which the forces act ; but both depend on the leakage 

 of the oceans, and not at all on the secular cooling of the globe, the 

 effect of which is insensible.^ 



' Since this was finished the writer has carefully recalculated the shrink- 

 age of the earth's radius in 2,000 years, and finds that it can not exceed 1.5 

 inches. This takes no account of the increase of the interior heat of our 

 globe due to radio-activity. If this latter effect were taken into account 

 probably there would be no shrinkage whatever. Quite independently of 

 these effects, however, there is an actual expansion of the globe due to the 

 leakage of the oceans. 



In the same way it is found, by the application of Fourier's theory of 

 heat to the cooling at the surface, that the total shrinkage in the length of 

 a continent such as North or South America, assumed to be equal to the 

 terrestrial radius in length, is less than 1.5 inches. This again takes no 



