6 The American Geologist. Tmnuary, 1905 
the other hand, there is no known ground other than mere 
convenience for supposing an original homogeneity either 
of the nebula or of the earth. 
The problem of the distribution of density in the earth 
is one of the most important in all geophysics. It is as sig- 
nificant for geodesy and terrestrial magnetism as for geology. 
That Paplace's empirical law represents it approximately is 
generally acknowledged, but it appears substantially certain 
that this is merely an approximation without theoretical 
value. Only extended researches, however, can replace it by 
one better founded. 
The solidity of the earth is now very generally accepted, 
though Descartes's hypothesis of its fluidity, invented to sat- 
isfy his erroneous theory of vortices, died hard. Lord Kelvin 
showed from tidal phenomena that the efifective rigidity of the 
earth is about that of a continuous globe of steel. Professor 
Newcomb pointed out that the Chandlerian nutation leads to 
the same conclusion and an almost identical value of the 
modulus of rigidity, and professor George H. Darwin dem- 
onstrated that, if the earth is a viscous fluid, its viscosity 
must be some 20,000 times as great as that of hard brittle 
pitch near the freezing point of water. From the point of 
view of modern physical chemistry and in consideration of 
professor Arrhenius's opinions, the matter requires further 
consideration.- In particular it is most important to know 
whether the earth is substantially a crystalline solid or an am- 
orphous substance, for many modern physical chemists con- 
sider amorphous matter as liquid. This opinion is far from 
being established, however, and recent experiments by Mr. 
Spring show that mere deformation at ordinary temperatures 
attended by only a very small absorption of energy, suffices 
to convert crystalline metals into substances exhibiting 
characteristics of amorphous bodies. Since Xordenskiold's 
great discovery of great masses of terrestrial iron, or, rather, 
nickel steel, in Greenland, and the wide distribution since 
proved for similar metal imbedded in igneous rocks, a great 
amount of evidence has accumulated that a large part of the 
earth is composed of material indistinguishable from that of 
metalHc meteorites. Meteoric iron is of course a highly crys- 
talline material. 
