GLACIAL EPOCH UPON THE EARLY HISTORY OP MANKIND. 53 
wenty-four million years is all the time which geologists can 
have at their disposal.* With him Alfred Russell Wallace is 
in substantial agreement, maintaining from the rate of probable 
deposition of geological strata that thirty million years is really 
all the time that geologists require. 
The approximate correctness of these recent calculations 
cannot well be doubted, since they rest upon very substantial 
data from which speculative considerations are largely eliminated. 
Professor Darwin’s calculations are based upon the known 
influence of the tides in retarding the revolution of the moon 
upon its axis, while Lord Kelvin’s confident assertion rests upon 
the known laws governing the radiation of heat from the solar 
system, and Wallace’s conclusions are largely derived from the 
new light which recent studies have shed upon the rate of 
erosion which is going on upon the surface of the earth at the 
present time. Extended and careful investigations show that 
the Mississippi is depositing sediment in the Gulf of Mexico at 
n. rate which would require the removal of one foot of soil from 
the entire area of the Mississippi basin, stretching from the 
Rocky to the Alleghany mountains, in less than 5,000 years. 
This would lower the level of the whole American continent 
200 feet in 1,000,000 years. If this process continues without 
interruption not much will be left of North America after 
3,000,000 or 4,000,000 years. Other river systems are much 
more active on account of the steeper gradient of their channels. 
The Po, for example, is lowering its basin at the rate of one foot 
in 700 years. 
It is true that geologists have not readily accepted these 
narrow limits imposed upon them by the astronomers and 
physicists, and they are attempting by various lines of argument 
to obtain an extension of credit to the extent of 100,000,000 
years or so. But even with such an extension the case is very 
different from what it was when 306,000,000 years could be 
spoken of as a “ mere trifle.” In the readjustment of the ratios 
■of geologic periods under these new limits the Glacial Epoch is 
brought down to comparatively recent time. 
Dana’s estimate of these ratios are twelve for the Palaeozoic 
Period, three for the Mesozoic, and one for the Cainozoic, which 
includes the whole of Tertiary and post-Tertiary time. If we 
accept Lord Kelvin’s estimate of the whole time at the disposal 
of geologists, these ratios would give us 18,000,000 for Palaeozoic 
time, 4,500,000 for Mesozoic, and 1,500,000 for Cainozoic time, 
* See Annual Address as above. 
