Address to Section C, Geology. 415 
collection weighed 11,873 lbs., as against a total weight of only 865 lbs. 
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 lies below the Archean rocks. Cannot the relative proportions 
of the stony and metallic constituents in the earth help us? 
Unfortunately, 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 
do 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 
estimated that the material would gradually increase in density: from 
the surface to the centre, where its specific gravity would be 10:74, 
and the calculations of Schlichter show that condensation due to 
compression 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 concen- 
tration 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 ‘ barysphere,’ 
or heavy sphere. According to this view the earth is essentially 
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. 
It has been objected that the weight of the earth is not great 
enough for much of it to be composed of metallic iron or of meteoritic 
material. The specific gravity of iron under the pressure at the 
earth’s surface is about 7:7, and it would be even greater when 
compressed in the interior. But the barysphere is doubtless im- 
pregnated with much stony material that would lessen its weight. 
An estimate by Farrington (1897) of the average specific gravity of 
the meteorites of which the fall had been recorded is only 3°69. 
According to the Rev. EK. Hill (1885) the mean specific gravity of 
all the meteorites in the British Museum was 4:5; and, though 
Mr. Hill duly considered the effect of compression, he concluded that 
‘the density of the earth is perfectly consistent with its being an 
aggregation of meteoric materials.’ Moreover, within the metallic 
barysphere there may be a core of lighter material; for earthquake 
