THE ORIGIN OF THE EARTH. 7 



The supposed crust. — It was formerly held that the next step was 

 the crusting-over of this molten globe by surface cooling, and that 

 this was followed by a progressive thickening of the crust. It was a 

 common view that this state continued even down to the present day, 

 and that the earth still consists of a liquid interior inclosed in a solid 

 shell. The term " crust of the earth" arose from this belief, and it is 

 still much used as the most convenient term for the outer part of the 

 earth, even by those who do not attach the original meaning to it. 

 Against this view it has been urged that solid lava is heavier than 

 molten lava and should therefore sink to the center as fast as it formed. 

 In reply it is urged that, in such a molten globe, the heaviest material 

 would gather at the center, with successive layers of lighter and lighter 

 material above it, the lightest of all being at the surface, so that, even 

 though the solidified portions of this lightest layer were heavier than 

 their own liquid stratum, and so would sink through it, they would 

 still be lighter than the heavier liquid layers below, and would thus 

 be arrested by them. These sunken portions might be remelted for 

 a time, but the process would at length so cool the whole outer layer 

 that remelting would cease, and a subcrust would form which, in time, 

 would be built up to the surface and give a complete crust formed of 

 the outermost layer of light material. In pursuance of this view, it 

 was supposed that the granitic group, the lightest class of igneous 

 rocks, formed the primitive surface, but this special view has been 

 weakened by recent studies of the oldest known rocks. 



Astronomical argument for solidity. — Arguments, against a crust 

 resting on a liquid bed have been brought to bear by physicists and 

 astronomers who, reasoning from the phenomena of the tides, of the 

 precession of the equinoxes, of variation of latitude and of nutation, 

 have urged that the earth must be essentially solid; must, indeed, 

 have a practical rigidity of a high order. The support of the con- 

 tinental platforms at a height of 12,000 to 18,000 feet above the ocean 

 basins, as well as the support of the great plateaus and mountain 

 ranges superposed on these, present other grave objections to the 

 hypothesis of a liquid interior. To these last objections, the answer 

 has been made that the continents, plateaus, and mountains are inher- 

 ently lighter than the material of the ocean bottoms, and this seems 

 to be essentially true. To the astronomical argument it has been 

 replied, in part, that a stiff liquid inclosed in a thick shell and rota- 



