GEOLOGY OF THE INNER EARTH——-GREGORY. 31 74 
meteorites of an average weight of 2.31 pounds, and 5 were siderolites 
(or meteorites containing a large proportion of both silicates and 
nickel-iron) of an average weight of 54 pounds.* Therefore, accord- 
ing to this test the stony materials would appear to be the more 
abundant. But if all known meteorites are considered, the iron group 
far outweighs the other; for the iron meteorites in the British Mu- 
seum collection weighed 11,873 pounds, as against a total weight of 
only 865 pounds of stony meteorites. The available evidence suggests 
that the stony meteorites fall the more frequently on the earth, but 
the meteoric irons come in such large masses that they outbalance the 
showers of the smaller stones. 
We might have expected help from another source in examining 
what hes below the Archean rocks. Can not the relative proportions 
of the stony and metallic constituents in the earth help us? Unfor- 
tunately, this proportion is as uncertain as that of stony and iron 
meteoritic material. The best-established fact about the interior of 
the earth is that its materials are much heavier than those of its 
crust. The specific gravity of the earth as a whole is about 5.67; the 
specific gravity of the materials of the crust may be taken as about 
2.5, while that of the heavier basic rocks is only about 3.0. Hence 
the earth as a whole weighs about twice as much as it would if it 
were built of materials having the same density as those which form 
the crust. 
Two explanations of the greater internal weight of the earth have 
been given. According to one, the earth is composed throughout 
of the same material, and the internal mass is only heavier because 
it is compressed by the weight of the overlying crust. Laplace esti- 
mated that the material would gradually increase in density from the 
surface to the center, where its specific gravity would be 10.74, and 
the calculations of Schlichter show that condensation due to com- 
pression may be adequate to account for the greater internal weight. 
According to the alternative or segregation theory, the difference 
in density is explained as due to a difference in composition; the 
interior of the earth is thought to be heavier owing to the concentra- 
tion of metals within it. The probability of this metallic interior 
has been advanced from several lines of evidence; and the assumed 
metallic mass has received from Posepny the name of the “ bary- 
sphere.” or heavy sphere. According to this view the earth is essen- 
tially a huge ball of iron, which, like modern projectiles, is hardened 
with nickel; and it is covered by a stony crust, the materials of which 
were primarily separated from the metallic mass, like the slag formed 
on a ball of solidifying iron in a puddling furnace. 
@The weights are given in pounds avoirdupois. For the calculation I am 
indebted to Mr. W. R. Wiseman, of the Geological Department of Glasgow 
University. 
