WVPOTHESLS OF CAUSE OF GLACIAL PERIODS 679 
atmospheric hypothesis.t This will be more evident as we touch 
on the time rates. 
4. The change in the eccentricity of the earth’s orbit which 
Croll has made the foundation of his beautiful hypothesis of 
glaciation, if not found competent to produce general glaciation 
itself, might still be effective in producing climatic changes of 
less degree, and might superpose important modifications upon 
the series postulated by the atmospheric hypothesis. It may 
be remarked in passing, however, that the computed variations 
of eccentricity of distant periods of the past do not rest on so 
firm a mathematical basis as is currently supposed. 
It is obvious that these and other possible agencies might 
work concurrently with the atmospheric influences, or antago- 
nistic to them, in either case distorting and masking the normal 
rhythmical expression which a purely atmospheric series would 
assume. 
DO THE TIME RATES FALL WITHIN WORKABLE LIMITS? 
The working capabilities of a glacial hypothesis are some- 
what severely conditioned by its time factors. It must not only 
present a satisfactory correlation between the time of occurrence 
of glaciation and that of the assigned cause, but the rhythmical 
action of the cause must be consonant with the rhythmical history 
of glaciation. That the Pleistocene glaciation followed the 
Ozarkian or Sierrian stage of elevation at an appreciable dis- 
tance, I hold to be demonstrated by the relations of the glacial 
deposits to the eroded topography of that period. On the other 
hand, there is no evidence of a prolonged interval, geologically 
speaking. The atmospheric hypothesis demands that the accel- 
erated erosion due to elevation (or rather to the dissection that 
followed elevation) should have continued long enough to 
remove about three times the present atmospheric content of car- 
bon dioxide before glaciation could begin, following Arrhenius’ 
computations. This removal could only be accomplished by the 
excess of consumption of carbon dioxide over supply and there is 
‘See GILBERT’S review in last number of this JOURNAL, p. 621. 
