PHYSICAL STATE OF THE INTERIOR 33 



present condition of the earth's interior is, we encounter great 

 difficulties. On this subject three principal hypotheses have been 

 proposed. ( 1 ) That the earth is practically a liquid body, covered 

 only by a relatively thin solid crust. (2) That it is substantially 

 solid, and may or may not contain localized enclosures of molten 

 matter within it. (3) That it has a very large solid nucleus and a 

 solid crust, and interposed between the two a layer of fused mat- 

 ter, upon which the crust floats in equilibrium. 



In the present extremely imperfect state of knowledge it is not 

 possible to decide definitely between these conflicting theories. 

 The first, or "thin crust theory," is now almost entirely abandoned, 

 for the known facts do not require us to believe that the increase 

 of temperature downward keeps on indefinitely. It may well be 

 the case that the globe has a uniform temperature from the centre 

 to within a few miles of the surface. If this be true, we should 

 observe the same increasing heat, the more deeply the crust is 

 penetrated. Again, there is reason to believe that most rocks ex- 

 pand on melting, and thus the great pressures found deep within 

 the earth would necessitate a higher temperature to melt a given 

 rock at a given depth than at the surface. We do not know how 

 far the temperature must be raised to overcome the increased press- 

 ure. More important are the astronomical objections, according 

 to which the behaviour of the earth is like that of a rigid and not 

 of a fluid body. 



The second hypothesis, that the earth is solid to the centre, is 

 held by many geologists and by most astronomers. The evidence 

 in its favour is chiefly the fact already stated, that the globe be- 

 haves, in its astronomical relations, like a rigid solid. This view 

 is not at all incompatible with the belief that considerable masses 

 of fused material may be contained at various depths within the 

 earth. 



The third hypothesis, which postulates the presence of a fused 

 layer between the crust and the nucleus, is held by many geolo- 

 gists and astronomers. According to this view, the earth first 

 solidified at the centre from pressure, and it has even been sug- 

 gested that the molte?i (as distinguished from the nebulous) part 



