Lae ite bh AND GEOLOGIC TIME 731 
have, amongst recent writers, R. Bell,t who thinks the estimates 
of the above-mentioned authors mot excessive. Chamberlin? 
estimates the lapse of time since the Kansan epoch to be equal 
to fifteen times the lapse since the last epoch, which would give 
a high figure for the whole period on any tenable estimate of 
this last factor, and to this is to be added an undetermined num- 
ber of years for the pre-Aftonian (Albertan ) stage., -Penck, at 
the recent Toronto meeting of the British Association, is reported 
to have allowed at least 500,000 years for the glacial epoch, 
including all the interglacial stages, and very secenitly Be 
Taylor,3 in an article on the moraines of recession of the latest 
(Wisconsin) ice-sheet, gives as his estimate of the time required 
for the retreat of this single ice invasion from the latitude of 
Cincinnati to the straits of Mackinac, a period of from 75,000 
to 150,000 years. Adding to this an equal lapse of time for its 
advance, and we have an estimate of 150,000 to 300,000 years 
for the whole time occupied by this most recent member of the 
drift. 
In addition to the actual periods of the ice occupancy of the 
territory we have to reckon in the interglacial epochs of which 
there is considerable evidence, and which must materially add 
to the length of Pleistocene time. I have not seen many esti- 
mates of the time required by these, the most noteworthy cne 
being that of Professor McGee, of the time required for the 
deposition of the forest bed overlying the earlier Iowa till. 
Taking for his unit the period of written history, the very 
least figure he gives for this formation is about 112,000 years, 
and this is far exceeded by his estimate of its possible maximum 
duration. The thickness of some of these intercalated beds 
would naturally indicate a considerable period for their deposi- 
tion, but McGee’s estimate certainly seems an extreme one, and 
is the more noticeable when considered in connection with the 
relatively very short allowance of time he has given for the ice 
invasions themselves, even allowing, as we should, that he is 
* Bull. Am. Geol. Soc., I, 295. 3JouR. GEOL., V, 1897, July-August. 
2Jour. GEOL., IV, 875, 1896. 
