216 Prof. J. D. Dana on some Results of 



greater density, therefore pressure may bring about the condi- 

 tion of the solid. The fact that ice, which has less density than 

 water, changes to water under pressure, has been appealed to in 

 support of the conclusion. The pressure to which the material 

 within the earth is subjected is so great that experiment can 

 never imitate it or directly test its effects. Beneath only one 

 hundred and fifty miles of liquid rock it would be not less than 

 one million of pounds to the square inch. Less than this may 

 have been sufficient to produce crystallization, and so give rigi- 

 dity to the viscous rock-material, or, at least, after the cool- 

 ing the earth has undergone. The rigidity of slowly solidified 

 rock is beyond that of glass or steel, or the degree which, 

 according to Sir William Thomson, must exist in order that the 

 earth should be as completely free as it is from tidal movements 

 in its mass. 



4. According to the above, the solid part of the globe consists, 

 as regards origin, of three parts : — 



(a) The central mass, consolidated by pressure ; the solidifi- 

 cation centrifugal, or from the centre outward. 



(b) The crust proper consolidated by cooling; the solidifica- 

 tion centripetal, or from the surface inwards. 



(c) The outer crust or superficial coatings (the supercrust), 

 made chiefly by the working over and elaborating of the mate- 

 rial of the surface through external agencies, aided by the ever- 

 acting lateral force from contraction, and including all terrains 

 from the Archaean upward. 



5. As to the thickness of the viscous layer and the overlying 

 crust, or the depth of the later undercrust seas, I have nothing 

 to offer. The Appalachian subsidence might have been accom- 

 plished with but seven or eight miles of depth underneath. 



The undercrust fire-seas would have their heat from time 

 to time supplemented through the movements of the crust. 

 But the ordinary oscillations of the crust were so extremely slow 

 and so ineffectual in producing heat, and the greater mountain- 

 making movements occurred at so very long intervals (many 

 millions of years), and then were so very limited in area com- 

 pared with the earth's surface, that this cause could not have 

 prevented a gradual narrowing of their limits with the progress- 

 ing refrigeration. But even after the general union of the 

 crust and nucleus, giving the earth trap-like " rigidity," had 

 taken place, leaving only local fire-seas, the connexion may not 

 have been so complete that it would not sometimes yield 

 enough to the slow working of lateral pressure to permit 

 oscillations of nearly continental extent, like those of the post- 

 Tertiary. 



