i9o8.] THE PHYSICS OF THE EARTH. 199 



Zi'ciglit of continents and inoiDitains. Xeither the uplift of moun- 

 tains about the sea coasts, nor the earthquakes occurring in these 

 regions can be explained by flowage beneath the crust, because the 

 movement is positive rather than negative, as required by this theory. 

 Whilst the investigation of Professor Sir George Darwin there- 

 fore does not give us a clue to the observed movements, it is never- 

 theless very valuable as furnishing an indirect confirmation of the 

 present theory that mountain formation depends on the sea. Ob- 

 servation shows that the movements are positive, and as the theory 

 of flowage indicates that they should be negative, we may infer 

 that whatever be the stress-differences existing beneath the earth's 

 crust, the movements thus produced are insensible compared to those 

 depending on the expulsion of lava from under the sea by world- 

 shaking earthquakes. 



III. The Xew Physical Theory of Earthquakes and Moun- 

 tain Formation Based on the Secular Leakage 

 OF THE Ocean Bottoms. 



§ 22. On the Plastic and Perhaps Viscous Layers Just Beneath 

 the Earth's Crust. — We have now examined at length the arguments 

 in regard to the constitution of the earth's interior, and have shown 

 that although as a whole the earth is solid, owing to the pressure to 

 which the matter is subjected, there is a plastic layer just beneath 

 the crust which in earthquakes is made to flow and behave almost 

 as a viscous fluid. In this layer just beneath the crust either the 

 pressure is not great enough to produce entire solidity, with the 

 existing temperature, or else the solid is made to flow by the break- 

 ing down of the elasticity under the action of the earthquake forces, 

 w^hich are powerful enough to disturb the whole world. 



Although the matter in this substratum appears to have some 

 rigidity, it seems probable that it has not the requisite elasticity to 

 behave as a perfect solid. W"e know that the layer must be nearly 

 solid, because, if it were not so, there would be a greater tendency 

 of the mountains to subside than actually is observed. The stress- 

 difference in the layers just beneath the crust must be very con- 

 siderable ; and yet this plastic matter is so stiff that it does not flow 

 and allow the mountains and plateaus to sink in. 



