640 GEOLOGY. 



gone from time to time through the ^ction of hfe will be discussed as 

 the earth's history is followed in the second volume. It may suffice 

 here to note briefly the chief ways in which the atmosphere has probably 

 been modified by the agency of life, not only as regards its quantity 

 but also as regards the proportions of its constituents. 



The consumption and restoration of carbon dioxide. — As the funda- 

 mental food of the organic world, carbon dioxide has suffered enormous 

 consumption in the course of the geological ages, and is now reduced 

 to the very small proportion of .0004 or .0003 of the whole. At the 

 outset it was probably one of the most abundant constituents; possibly 

 even the chief one. It has been partially restored, concurrently with 

 its consumption, by animal respiration, by certain classes of plant 

 action, and by combustion and other forms of inorganic combination. 

 This restorative action has been incomplete at all known stages of the 

 earth's history, and hence there has been constant loss of carbon dioxide. 

 The inorganic processes which have also profoundly affected both the 

 consumption and restoration of carbon dioxide are here neglected and 

 discussed elsewhere. 



The freeing and consumption of oxygen. — The oxygen of the atmos- 

 phere is actively consumed by animals and by plants, but on the other 

 hand, it is set free abundantly by green plants, and hence its amount 

 has probably fluctuated from time to time according to the state of 

 balance between the organic processes of its production, and those of 

 its consumption. The consumption of oxygen by organic processes is, 

 however, little more than a reversal of the previous process by which 

 it was set free ; for instance, green plants in forming their food set free 

 the oxygen of the carbon dioxide used for the purpose. When the 

 organic substance so formed is ultimately consumed through plant or 

 animal action or by inorganic means, an equivalent amount of oxygen 

 reunites with the carbon to again form carbon dioxide. And so if the 

 whole of the organic matter is returned to the inorganic state, no more 

 oxygen is consumed than had been before set free in the process of 

 forming the organic matter. But, as a matter of fact, a large amount 

 of organic matter has not gone back completely to the inorganic state, 

 and this residue constitutes a factor of no small importance in the geo- 

 logical record. 



The organic residue. — There is a certain portion of vegetation that 

 is not consumed by animals or by other plants, and that escapes com- 



