Editorial Comment. 269 
of plants and animals, to be about 20,000,000 years, with- 
in limits of error perhaps ranging between 15,000,000 and 
80,000,000 years. Again, in the Toronto meeting of the Brit- 
ish Association, he has more recently expressed the same opin- 
ion, which indeed he reached many years ago from his inves- 
tigations of the rates of secular cooling of the earth and the 
sun. This estimate, nearly agreeing with another b}^ Clarence 
King from similar physical data, has generally been regarded 
b}^ geologists as too short for the processes of sedimentation 
and erosion, and for the evolution of floras and faunas, of 
which the earth's strata bear record. More probably, as ra- 
tios and computations by Dana, Walcott, and other gef)logists, 
somewhat harmoniously indicate, the duration of time since 
the beginning of life on the earth has been some three to five 
times longer than Kelvin's estimate, or from sixty to a hun- 
dred million years. 
The larger figures imply for the dawn of life, to the devel- 
opment of the Cambrian and Silurian faunas, probably 
50,000,000 years; thence to the end of Paleozoic time, perhaps 
30,000,000 years; onward through Mesozoic time, about 
15,000,000 years: and through the Tertiarj- era, about 5,000- 
000 years. The comparatively very short Quaternary era, hav- 
ing, in its organic evolution, as shown by the marine mollus- 
ca, no higher ratio to Tertiary time than 1:50, may therefore 
have occupied only about 100,000 years. 
With these ratios in mind, the reader of the last number of 
the Journal of Geolor/y is surprised to find, in an article by 
Mr. F. B. Taylor, the very large estimate of 75,000 to 150,000 
years for the formation of the series of terminal moraines 
amassed by our continental ice-sheet at times of halt or re- 
advance interrupting its final retreat between Cincinnati and 
the strait of Mackinac. But, according to the ratios of 
the several stages or epochs of the Glacial period, well deter- 
mined by Chamberlin, Leverett, Calvin, Bain, and others, the 
Wisconsin epoch, during which these moraines were formed, 
was no more than a tenth or twentieth part of the whole 
Ice age. 
It seems needful, therefore, to look for other explanations 
of the regular recurrence of morainic accumulation, whereby 
it may have taken place at much shorter intervals than those 
