618 LAG. CLLAMIB LACE LN 
able content. It seems apparent that a process which sets free 
carbon dioxide in so large a proportion to the total atmospheric 
content would be competent to vary that content notably, if, 
as contended, the process is subject to notable variations, and 
especially if, as also contended, the reciprocating process of fixa- 
tion varies coincidentally with it so that the two mutually inten- 
sify each other’s effects. 
Dr. Arrhenius? has estimated that the addition of two or 
three times the present amount of carbonic acid to the atmos- 
phere would give the genial climate in the arctic regions which 
the Tertiary flora indicates. Even if this estimate should be 
much too small, it would not seem to be beyond the competency 
of a great limestone-making epoch to enrich the atmosphere in 
carbonic acid sufficiently to thermally blanket the earth effect- 
ively and to so retain, distribute, and equalize the temperature 
as to render all latitudes available to vegetal and animal life. 
The notable feature connected with the extension of life to the 
high latitudes is the marvelous equalization of temperature. 
White and Schuchert’ have recently given great emphasis to this 
by showing that in the Potomac epoch an almost identical flora 
flourished in north Greenland and in Virginia. A simple increase 
of solar heat, distributed as it is today, does not meet the 
demands of the problem. An equalizing and distributing factor 
seems to be indicated. And this, eminent physicists from Tyn- 
dall to Arrhenius encourage us to believe may be found in a 
change of the atmospheric constitution in the critical item of 
carbon dioxide, a change of no excessive amount and without 
serious variation in other constituents, except as they follow 
from this incidentally. 
If the freeing of carbonic acid incident to limestone deposi- 
tion were a potential factor in the equalization and amelioration 
of climate which permitted the extension of warm-temperate 
life to high latitudes, such extension should be coincident with 
the great limestone-forming epochs. Such appears to be the 
™Phil. Mag., S. 5. Vol. XLI, No. 251, April 1896, pp. 237-279. 
2 Bull. Geol. Soc. Am., Vol. IX, pp. 343-368. 
