568 RY Ee COHAMBEREEN, 
oped in the combustion-tube, and does not appear to exist as a free 
gas in igneous rocks, it is not likely that this constituent of the atmos- 
phere has come directly as an exudation from the interior of the globe. 
It is to be sought, rather, in a dissociation or decomposition of com- 
pound gases by physical or organic agencies. Originally, enough 
oxygen was derived from water vapor, by physical means, to permit 
the beginning of plant life; after vegetation appeared, an abundant 
source of oxygen was found in the carbon dioxide. 
The average gas content of igneous rocks, as determined by the 
analyses now made, may be used to test the competence of the rocks - 
to yield the present atmosphere. Taking the average volume of 
nitrogen per volume of rock to be 0.05, which is probably nearer the 
truth than the figure 0.09 given in table 8' (owing to leakage of air), 
it would require the liberation of all the nitrogen in the outermost 
7o miles of the earth’s crust to produce the nitrogen in the present 
atmosphere. For an estimate of the amount of igneous rock necessary 
to yield the carbon dioxide which is now locked up in limestone and 
coal deposits, we may take Dana’s figure of 50 atmospheres of this 
gas, and an average of 2.16 volumes of carbon dioxide per volume of 
rock. ‘To produce these 50 atmospheres of carbon dioxide, it is 
found that a thickness of 66 miles of crust would have to be deprived 
of its carbon dioxide’—a figure which corresponds fairly well with the 
estimate for nitrogen. If the water of the rocks be placed at 2.3 per 
cent., a depth of 70 miles would supply the hydrosphere. 
On the planetesimal hypothesis, gas has been supplied from the 
interior to the atmosphere ever since an early stage of the earth’s 
growth, probably from the earliest stage at which an atmosphere could 
be held, which may be placed at the time when the earth’s radius was 
about 2,000 miles. From this it appears that only a small fraction 
of the full gas-producing possibilities of the rocks of the earth was 
required to supply the atmosphere. The fact that gases are still 
being given forth through volcanoes, and that the ejected lavas still 
have gas-producing qualities, makes it clear that all the resources 
of the interior are not yet exhausted. The working qualities of the 
planetesimal hypothesis, therefore, do not seem to be found wanting 
in either past possibilities of supply, present output, or prospective 
reserve. 
t Ante, p. 545. 2 The limestones, of course, are not here included. 
