654 TC CHA MEE RIETIN 
atmosphere and to offer for trial a competitive hypothesis or 
group of hypotheses. To admit a competitive hypothesis to the 
working list is a concrete form of embodying a doubt respect- 
ing existing hypotheses and serves better than any abstract 
skepticism to keep alive the sources of doubt. I assume that 
the system of multiple working hypotheses is accepted as 
furnishing the most wholesome conditions for research, and that 
any additional hypothesis not in itself incredible will be wel- 
comed. 
If we compute the mass of the several constituents of the 
present atmosphere, and estimate the rate at which they are 
being consumed in alterations of the superficial rocks, we find 
that the carbon dioxide will last but a short period unless there be 
some source of supply. A group of careful estimates by different 
methods gives results ranging from five thousand to eighteen 
thousand years’ with a weighted mean of about ten thousand 
years. Only the alteration of the crystalline terranes was 
admitted to the computations. The estimates assumed the 
degradation rates of current geological opinion. Granting these 
may be too high, and multiplying the results accordingly, it still 
appears that we are confronted by the early exhaustion of a vital 
factor of the atmosphere, if there be no compensating source of 
supply. 
There is, however, an immediate source of compensation. 
The ocean is an atmosphere in storage. It is not improbable 
and of slightly extending and modifying the treatment on some points, but it still 
remains merely synoptic. The treatment of the periodicity of Pleistocene atmospheric 
changes is especially incomplete, but this is only a particular case under a general 
hypothesis whose value does not necessarily hang upon this individual application. 
I desire to add that most of the questions involved in the paper have been dis- 
cussed with scientific friends and with the advanced graduate students of my classes 
during the past two years, and that I have, received from them much valuable aid. 
Computations and quantitative estimates have been made by F. R. Moulton, H. L. 
Clarke, A. W. Whitney, J. P. Goode, H. F. Bain, Samuel Weidman, C. F. Tolman, Jr., 
N. M. Fenneman, and C. E. Siebenthal, which I desire to gratefully acknowledge. 
The main points of the paper were presented to the Geological Club of the University 
of Chicago, October 1896. 
™Made by A. W. Whitney, H. Foster Bain, J. P. Goode, Samuel Weidman, C. F. 
Tolman, Jr., and the writer. 
