400 Tl. C. CHAMBERLIN 
high elevation of the land, and slowly at times of low altitude, 
grand averages being always understood. 
If the atmosphere were once excessively burdened with car- 
bonic acid and its later history has been merely a progressive 
depletion, these stages of rapid consumption only introduced 
specially rapid reduction of the superabundant supply, and the 
effects on tangible geological processes may have been quite 
beyond detection. But if, on the other hand, the atmosphere 
was limited in amount at the beginning and has been gradually 
supplied as well as gradually consumed throughout the ages and 
has been susceptible to serious change, an unusually rapid con- 
sumption of the carbon dioxide at the stages of land elevation 
would result in appreciable depletion of the atmosphere unless 
the supply were correspondingly increased. On the other hand, 
at those stages in which the continents were reduced well toward 
sea level and the land areas were diminished by the incursion of 
the ocean, the consumption of the carbonic acid would be 
checked, and if the supply were not correspondingly reduced, 
reénrichment in carbonic acid would follow. Under this hypoth- 
esis, the history of the atmosphere involved alte:nate enrich- 
ment and depletion. 
The carbon dioxide is critical because of that peculiar ther- 
mal capacity by virtue of which it retains the heat of the sun to 
a relatively extraordinary degree, a capacity which is shared by 
water vapor, but which is possessed in very low degree by 
oxygen and nitrogen. The amount of aqueous vapor, however, 
is dependent upon temperature, while the carbon dioxide is 
stable and active at all terrestrial temperatures. Whenever, 
therefore, there is a notable percentage of carbon dioxide in the 
atmosphere, it performs a most important function in conserving 
the heat of the sun and raising the temperature of the lower 
atmosphere and of the earth’s surface. By this rise it increases 
the aqueous vapor in the atmosphere, which in turn aids the 
carbon dioxide in retaining the heat of the sun} the two acting 
conjointly. On the other hand, when the carbon dioxide is 
reduced to a small factor, the heat of the sun 1s less effectually 
