684 Ll. C. CHAMBERETN: 
inter-glacial epochs, or at least some of them, for there is abundant 
evidence that the land was not then so greatly elevated as in the 
Ozarkian or Sierrian period. 
Instead, therefore, of combining 20 per cent. increase of land 
area with 100 per cent. increase of elevation, and these with the 
coéperating 20 per cent. reduction of sea area, the destruction 
of sea-shelves, and the restraining effects of lowering tempera- 
tures, as we are entitled to in bringing down the rich Tertiary 
atmosphere to the lean conditions of the glacial period, let us 
content ourselves with some modest fraction of these intensifying 
combinations. If we only assume that the agencies of deple- 
tion were superior to the agencies of return by the amount 
of Tor per cent., (heydepletion requisite to" bring on a glacial 
epoch, starting with atmospheric conditions like those of the 
present, would be effected in less than 10,000 years. If, there- 
fore, we over-generously allow as much time for deglaciation as 
for reglaciation, an interglacial epoch might not require the 
operation of the postulated agencies for more than 20,000 years, 
so far as they themselves are concerned. The development of 
the ice-sheet might take more time, but we have little or no 
data for estimating this. If 10,000 years additional is allowed 
for this the total remains at the modest figure of 30,000 years. 
It would not seem to be pushing the data previously given to 
extremes to postulate a larger percentage of difference between 
depleting and repleting agencies than Io per cent., which would 
make the requisite atmospheric depletion possible in a shorter 
period. It is probably not extravagant to assume that the differ- 
ence might rise to 20 per cent., in which case the requisite 
time would be brought down to extremely modest limits. It is 
difficult to see how anyone who studiously considers the 
phenomena of the Toronto interglacial epoch could assign to it 
a duration less than is compatible with these agencies, as here 
interpreted. It would seem, therefore, that the hypothesis is 
not excluded from the working category by inadaptibility to the 
time rates of the phenomena which it seeks to elucidate. 
There is not likely to be any serious question respecting the 
