HVPOTHESES BEARING ON CLIMATIC CHANGES 677 
If we push the doctrine thus far it is important to assign 
J causes for the fluctuations of supply and exhaustion of the 
atmosphere, to give the doctrine a working form and to devise 
means of putting it to the test. Concerning external sources of 
enrichment we know so little that we can scarcely say that there 
is a leaning of probabilities either toward or against practical 
uniformity. The internal supplies were probably correlated in 
- some measure with igneous extravasations— not that such extrav- 
asations were the sole mode of liberating gases, but that other 
modes probably worked concurrently with them. The escape 
of gases was probably also correlated with crustal move- 
ments, especially those that compromised the continuity of the 
surface rocks, particularly the profound crushings which mining 
and the microscopic study of the hypogene rocks reveal. In 
these phenomena therefore, may be found a rational basis for 
inferring the times of probable atmospheric enrichment. For- 
mulated as a proposition, it may be postulated that special 
enrichment coincided with special igneous extravasation and 
crustal disruption, taking the earth as awhole. The supply may 
be assumed to have been uniform in so far as these and other 
means of liberation were on the average uniform. 
The phases of depletion are susceptible of more satisfactory 
treatment. In the first place, the depletion was differential. 
The loss of nitrogen was doubtless slight, because of its chem- 
ical inertness, and hence, though the supply may have been 
small, the nitrogen grew to ultimate dominance. The depletion 
of oxygen through the alteration of surface rocks was notable, 
but less than that of carbon dioxide. As a result the latter 
became the minimum factor of the atmosphere and the critical one. 
The enormous reserve supplies of water rendered its consump- 
tion inconsequential. 
In the second place, the depletion was conditioned upon the 
exposure of the surface rocks to atmospheric alteration. This 
in turn was conditioned upon topography. In stages of ele- 
vation the water table of the land is depressed and the zone 
of atmospheric penetration is deepened. At the same time the - 
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