MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 559 



can be done at present is to picture as clearly as possible the different 

 hypotheses that have been entertained, and the different consequences 

 that logically flow from them. The most important factor in the case 

 is the original amount and distribution of internal heat. 



ALTERNATIVE VIEWS OF ORIGINAL HEAT DISTRIBUTION. 



The hypothetical modes of origin of the earth will be treated in the 

 historical section. Suffice it here to say that one view is that the earth 

 was once gaseous, passed thence into a liquid, and later into a solid state. 

 Under this view, there are two hypotheses as to the original distribution 

 of internal heat, dependent on the mode of solidification. According 

 to the one, solidification began at the surface after convection had brought 

 the temperature of the whole mass down nearly to the point of conge- 

 lation; according to the other, solidification began at the center at a 

 high temperature, because of pressure, and proceeded thence outwards. 

 The former only has been much developed in the literature of the sub- 

 ject, though the latter is now generally regarded as the more probable. 



Another view of the globe's origin is that the earth was built up 

 gradually by the infall of matter, bit by bit, at such a rate that though 

 each little mass became hot as a result of its fall, it cooled off before 

 others fell on the same spot, the rain of matter not being fast enough 

 to heat up the whole mass to the melting-point. Under this view, the 

 internal heat arose chiefly from compression due to the earth's gravity. 



A clear conception of the three hypotheses of thermal distribution 

 which rest on these two views of the origin of the earth is important to 

 the further discussion. 



I. Thermal distribution on the convection hypothesis. — It was 

 formerly the prevailing opinion that the molten condition of the earth 

 persisted in the interior until after the crust had formed, and that solidi- 

 fication proceeded from the surface downwards. It was a natural 

 corollary of this view that, previous to the beginning of solidification, 

 convection stirred the liquid mass from center to circumference and 

 equalized the temperature so that the whole mass cooled down equably 

 until it approached the point of solidification and became too viscous 

 for ready convection. The temperature should, therefore, have been 

 nearly the same from center to surface at the stage just preceding in- 

 cipient solidification. This conception forms the basis of most discus- 



