PIV POUL SiO OL ICA OS OF GLACIAL PERIODS 579 
THE FORMATION OF ORGANIC COMPOUNDS AS AN AGENCY OF 
ENRICHMENT AND DEPLETION 
The familiar fact that plants produce complex carbon com- 
pounds at the expense of the carbonic acid of the air, and that 
animals, aided by plants, by combustion, and by decay, decom- 
pose these compounds, and return a portion to the air as carbonic 
acid need not be dwelt upon. These reciprocal processes con- 
stitute a cycle which, in so far as it is mutually compensatory, 
affects the constitution of the atmosphere only in temporarily 
locking up carbon in the transient organic matter. The cycle, 
however, is not complete at any time, and has fallen far short of 
being complete at certain times. A portion of the carbon com- 
pounds are not reconverted into carbonic acid, and this residuum 
has been sealed up in the strata, and represents so much of 
depletion of the atmosphere. When this residuum was large 
there was a hastening of the process of robbing the atmos- 
phere. When it was small it put less tax upon the agencies of 
supply. In its concrete application, the hypothesis recognizes 
one notable period of residual accumulation, the Coal Measures. 
Subordinately it recognizes others, as the Huronian and the late 
Cretaceous. Perhaps the Coal Measure period is the only one 
in which the excess of carbon composition over decomposition 
was so great as to seriously influence the constitution of the 
atmosphere, considered by itself alone, though this is open to 
question. A computation of the carbonic acid locked up in 
coal and similar carbonaceous deposits compared with that 
locked up in the limestones shows that the former is greatly 
inferior to the latter, from which it is inferred that the organic 
factor has been much the less influential in producing variations 
of atmospheric constitution, per se, than through its relations to 
the carbonates. 
Respecting the organic cycle itself, it is obvious that when 
the sum total of vegetable and animal life increases, the amount 
of carbonic acid locked up in the living organisms is increased, 
and wice versa. The total mass of all the vegetable and animal 
living matter on the earth is some small fraction of the total 
