CONDITION OF THP: FLOOK OF THE OCEAN. 321 



stones, in lavas, and hypog-ene rocks — ])y the action of water as a sol- 

 vent and su))hn]ant — warrant the belief that the centrosphere is largely 

 made up of metals and metalloids with imprisoned gases. It is admit- 

 ted that the vast nucleus has a very high temperature, but so enormous 

 is the pressure of the superincumbent crust that the melting point of 

 the substances in the interior is believed to be raised to a higher value 

 than the temperature there existing — the centrosphere in consequence 

 remains solid, for it may be assumed that the melting point of rock- 

 forming materials is raised by increase of pressure. Astronomers, 

 from a study of precession and nutation have long been convinced that 

 the centrosphere must be practically solid. 



Recent seismological observations indicate the transmission of two 

 types of waves through the earth — the condensational-raref actional 

 and the purely distortional — and the study of these tremors supports 

 the view that the centrosphere is not only solid, but possesses great 

 uniformity of structure. The seismological investigations of Profes- 

 sors Milne and Knott point also to a fairly abrupt boundary or transi- 

 tion surface, where the solid nucleus passes into the somewhat plastic 

 magma on which the firm upper crust rests. 



In this plastic layer or shell — named the tektosphere — the materials 

 are most proba])ly in a state of unstable equilibrium and bordering on 

 fusion. Here the loose-textured solids of the external crust are con- 

 verted into the denser solids of the nucleus or into molten masses, at 

 a critical point of temperature and pressure; deep-seated rocks maj^ 

 in consequence escape through fissures in the lithosphere. Within 

 the lithosphere itself the temperature falls off so rapidly toward the 

 surface as to be everywhere below the melting point of any substance 

 there imder its particular pressure. 



Now, as the solid centrosphere is slowly contracted from loss of heat, 

 the primitive lithosphere, in accommodating itself — through changes 

 in the tektosphere — to the shrinking nucleus, would be buckled, 

 warped, and thrown into ridges. That these movements are still going 

 on is shown by the fact that the lithosphere is everywhere and at all 

 times in a slight but measural)le state of pulsation. The rigidity of 

 the primitive rocky crust would permit of considerable deformations 

 of the kind here indicated. Indeed, the compression of mountain 

 chains has most probably been brought about in this manner, but 

 the same can not be said of the elevation of plateaus, of mountain plat- 

 forms, and of continents. 



P'rom many lines of investigation it is concluded, as we have seen, 

 that the centrosphere is homogcMieous in structure. Direct observa- 

 tion, on the other hand, shows that the lithosphere is heterogeneous in 

 composition. How has this heterogeneity been brought about? The 

 original crust was almost certainly composed of complex and stable 

 silicates, all the silicon dioxide being in combination with bases. 

 SM 99 21 



