DAE DRILL AND GEOLOGIC TIME 739 
in parts; hampered by inequalities of the underlying surface 
and by the detritus it shears off trom these, must have had. a 
very slow, though irresistible, progress; and that, accepting the 
existence of such an ice-sheet and taking account of this slow 
rate of progression, the contained erratics, whose origin can be 
identified by the situations in which they are found and the dis- 
tances they have traveled, will afford a better means of making 
an approximate minimum estimate of the duration of the Pleis- 
tocene period than any other at our command. By this we can 
assure ourselves with almost absolute certainty that a single ice 
invasion could not have taken place carrying a single erratic 
from north of the lakes to the southern limit of the drift in less 
than four or five thousand years, and this without taking any 
account of the time required for the change of climate, the 
gradual gathering of the ice, its recession, the probable slower 
motion of the erratics than of the ice mass as a whole, or its 
retardation by friction, as evidenced by its facetted and striated 
surfaces. Taking all these into the reckoning, we ought, it 
would seem, to triple or quadruple the time; and if, instead of 
taking the highest estimates of glacier motion, we accept the 
more reasonable and probable ones, the period will be still more 
prolonged. It is difficult to see how, under these circumstances, 
a single ice invasion could have begun and run its course within 
the limits of less than thirty or forty thousand years; and if we 
accept Dana’s estimate of the glacier flow at one foot a day for 
a maximum, and one foot a week as a possibility, we would have 
to carry our figures very much higher. We have, however, 
according to some of the highest authorities, Chamberlin, Lev- 
erett, and others, five separate ice invasions to account for, 
besides interglacial periods of possibly equal or greater duration ; 
and this greatly magnifies the necessary estimate of time for the 
whole glacial period. One of these ice invasions appears to have 
transported bowiders one thousand miles, which, at the liberal 
rate of two feet per day, would require 7200 years, and the 
others, from the average extreme distance to which erratics were 
transported, will equal certainly 500 miles, which would require 
