98 The American Geologist. February, i904. 
great excess of this chemically active substance on the one 
hand or its great depletion on the other, and how the ocean has 
acted as a reservoir and equalizer. In the case of nitrogen, 
there would seem to have been little loss, on account of its 
chemical inertness. The relative proportion of oxygen is an 
interesting question. The suggestion of several writers, in- 
cluding Lord Kelvin, that the presence of free oxygen in the 
air is due to the action of sunlight on plants, is opposed 'by the 
requirement in oxygen of the earliest animal life. The relative 
intensity of oxydation processes in early as compared with 
later geologic periods is a new problem under the new hypoth- 
esis. Such knowledge as we have relating to geologic climates 
seems to indicate that the atmosphere conditions of Postarchean 
time were not radically unlike those of to-day. But the primi- 
tive atmosphere must have been very dififerent, as theoretically 
it consisted of the gases of lower molecular velocities. The 
carbon dioxide would seem to have been the first of the atmos- 
pheric constituents to be retained on the surface of the grow- 
ing globe. The enlargement of the earth and of its attractive 
power added the lighter gases to its envelope, and the carbon 
dioxide has been relegated to a minor place in volume. The 
active consumption of the gas in carbonation of the crystalline 
rocks has been a positive reduction of vast amount. The nitro- 
gen has sufifefed little loss, on account of its inertness, and has 
become the preponderating element. The amount of water 
exhaled by the earth has been immense but it has been mostly 
condensed to liquid, and has always been a small constituent of 
the atmosphere and locally variable, depending on temper- 
ature. It could not freely escape beyond the earth's control 
because of its easy condensation to a liquid. The quantity of 
water seems to vary proportionally with the carbon dioxide, as 
the latter is an independent controlling factor of the atmos- 
pheric temperature. This matter will be discussed later under 
climate. Hydrogen and helium have never been important 
constituents of the atmosphere because their high molecular 
velocities have probably carried them beyond the earth's con- 
trol ; although it may be possible that free hydrogen in the air 
unites with oxygen or other elements. 
Under the new hypothesis the atmosphere becomes a sub- 
ject of geology. Its origin and its history as well as its pres- 
