4.9 4, TRANSACTIONS OF SECTION C, 
their proportions in other bodies and in meteoric swarms is problematical. There 
appear to be no satisfactory data for an estimate of the relative abundance in 
space »f the iron and stony meteoric material. Stony meteorites have been seen 
to fall far more frequently than iron meteorites ; but the largest known meteorites 
are of the nickel-iron group, although this material, in moist climates, very soon 
decays. The most reliable indication as to the relative amounts of the stony and 
nickel-iron meteorites is given by a comparison of the weight of the two types of 
material in meteorites of which the fall was seen. According to Mr, Fletcher's 
list of the meteorites in the British Museum up to 1904, the collection included 
319 specimens of which the fall is recorded: of them 305 specimens were stony 
meteorites of an average weight of 2°63 lb., 9 were iron meteorites of an average 
weight of 2°31 lb., and 5 were siderolites (or meteorites containing a large pro- 
portion of both silicates and nickel iron) of an average weight of 54 lb.’ There- 
fore, according 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 Museum collection weighed 
11,873 lb., as against a total weight of only 865 lb. 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. ‘I'he 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 mags is only heavier because it is compressed by the weight of the over- 
lying 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 concentration of metals within it. The 
probability of this metallie interior has been advanced from several lines of 
evidence ; and the assumed metallic mass has received frem 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 
impregnated 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. E, 
Hill (1885) the mean specific gravity of all the meteorites in the British Museum 
1 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. 
