igoS.j THE PHYSICS OF THE EARTH. 273 



determined by these forces, which have produced the mountains 

 and river systems of the world. 



34. We cannot prove by experiments on rock twenty miles 

 thick that it will leak under the pressure of the ocean, but we can 

 observe the surface movements in earthquakes such as occur in 

 Alaska, where lava is being expelled from under the ocean and 

 pushed under the land. 



35. This movement is everywhere in the same direction, whether 

 in Alaska, Japan, the Antandes, South America, or elsewhere — 

 namely from the ocean towards the land. The reason of this is that 

 much steam is formed under the oceans, but scarcely any under 

 the land, and hence it pushes up the crust along the edge of the con- 

 tinents and finally almost walls them in with mountains, as was 

 long ago pointed out by Dana. 



36. The old theory of secular cooling and contraction of the 

 globe is false and misleading, and all who have carefully examined 

 it agree that it is totally inadequate to account for terrestrial 

 phenomena. In fact so far from contracting it seems certain that 

 the earth is actually undergoing a slow secular expansion. 



-^y. The Rev. O. Fisher and Major Button were among the earliest 

 to reject this theory as incapable of explaining mountain ranges. 

 But it is remarkable that after the contraction theory was proved 

 to be unsatisfactory, it continued to be used in all works on geology 

 and kindred sciences, and indeed still is accepted by those who 

 adhere to the antiquated doctrine that the earth is a failing structure. 

 Such views had some justification a quarter of a century ago; today 

 they are absolutely without excuse. 



38. There are the best grounds for accepting the doctrine of 

 Isostacy, as approximately true for the earth at all times ; conse- 

 quently there are no sensible stress-differences, or tendencies to 

 flow, except in the layers just beneath the crust. At greater depths 

 the matter of the earth is made solid by pressure, being at the centre 

 about three times more rigid than nickel steel. Hence deep down 

 the earth is now and always has been quiescent. The only layer of 

 the earth which is plastic and perhaps viscous is that just beneath 

 the crust ; this layer flows under the tremendous forces at work in 

 earthquake movements. It is the movement of this molten rock be- 



