Relations of the Atmosphere. 355 
of this was doubtless absorbed at an early period in the his- 
tory of our globe, since the limestones of the Eozoie are of 
great thickness, and those of more recent times have been in 
part formed by the solution and re-deposition of portions of 
these older limestones, . 
The question now arises, whence came this enormous vol- 
ume of carbonic dioxide which, since the dawn of life on our 
planet, has been fixed in the form of carbon and carbonates? 
phenomena the principal agent which restores to the atmosphere 
animal life 
1s so often disen gaged from the earth, both in voleanic and non- 
Voleanic regions (having a similar origin to the chlorhydrie, 
namely, the decomposition of saline compounds of aqueous 
origin),* it by no means meets the requirements of the problem. 
* Hunt, Chem. and Geol. Essays, pp. 8 and 111. 
