TTE GASES WEN: ROCKS 567 
lated by the planetesimal hypothesis, the rock-producing portion is 
supposed either to have aggregated into planetesimal bodies, or to 
have been gathered, molecule by molecule, into the nucleus of the 
earth. The planetesimal bodies gathered in gas molecules of the 
atmospheric class both by chemical union and by surface adhesion or 
occlusion. As the earth grew by sweeping in the planetesimals, 
whatever gases they contained became entrapped in the body of 
the growing planet and well distributed throughout its mass. At 
first, the gravity of the earth may possibly have been able to hold only 
the gases brought in by planetesimal aggregates of rock material 
and those that became impounded in it by impact, but at a later stage, 
when increased mass enabled it to hold gaseous molecules, gases may 
have been added to the atmosphere directly from the nebula, and 
these, by chemical reactions, may have become united with the sur- 
face rocks. As soon as vulcanism commenced, a system of exchange 
was set up. While gases were being fed to the atmosphere by volcanic 
action, water, carbon dioxide, oxygen, and nitrogen were being buried 
with the surface rock material, partly by chemical union and partly 
by mechanical entrapment, as the growth by infalling matter continued. 
It is thus quite easy to understand how the earth came to be affected 
by these gases throughout its mass, and how they came to exist there 
in all available forms of retention. 
While the carbon monoxide and methane derived from rocks by 
heating 7m vacuo are doubtless chiefly produced from the carbon diox- 
ide and water present in the rock material, there seems good reason 
to suppose that similar reactions took place within the earth, as the 
surface material became buried and heated, and hence that carbon 
monoxide and methane exist, as such, in the earth’s body, and are to 
be reckoned among the natural gases of the rocks. 
RELATIVE TO ATMOSPHERIC SUPPLY 
The fact that many of the igneous rocks are able to yield hydrogen 
from reactions between water and ferrous compounds, at high tem- 
peratures, indicates that the material of the earth’s crust is in a con- 
dition of partial oxidation only. Near the center of the earth there is 
probably very little oxygen, and even up to the surface, barring the 
weathered mantle, the rocks are suboxidized. Yet the earth is sur- 
rounded by an oxygenated atmosphere. Since oxygen is not devel- 
