Davis.] 22 [Nov. r6. 
that of the i)Ost-glacial period. Yet I believe still further that 
the only logical position of a glacialist at present is that taken 
by Chamberlin before the International Geological Congress at 
Washington in 1891 ; it is j)roper enough to state that such and 
such observations lead to the belief, more or less provisional, that 
the ice-sheet advanced twice or thrice, for example, over a certain 
region, and that the interval between the advances was long or 
short compared to post-glacial time ; it is at the same time 
entirely unwarrantable at the present stage of glacial investiga- 
tion to divide ])Ost-tertiary time into a definite number of geolog- 
ical periods or epochs, as if no more and no fewer occurred, and as 
if the subject were settled. Such a conclusion is an unsafe j)ro- 
vision with which to undertake field work, because it will inevita- 
bly lead to special interpretations of observed phenomena. The 
proper division of post-tertiary time into glacial and interglacial 
epochs is still an unsolved problem ; the very criteria by which 
the division or unity may ultimately be decided are still in discus- 
sion ; but the unity of the glacial invasion seems to me one of the 
least likely solutions, in as much as every year is adding to our 
knowledge of the extraordinary complexity of its record. 
Mr. Uphaiu refers in one part of his theory to the relative dura- 
tion of the glacial Lake Agassiz and the post-glacial Lake Michi- 
gan ; following Andrews in concluding that the latter has endured 
about ten thousand years, while the former endured only one 
thousand years. This ratio is determined by the amount of shore 
work done along the margins of the lakes. The conclusion can 
not be accepted as final, because it is not yet shown that the rate 
of work in the two lakes was the same : just as the conclusions 
as to the age of Niagara Falls were all shown to be faulty when 
it was made probable that the volume of falling water, and hence 
the recession of the falls also, was variable. It is possible that 
Lake Agassiz was frozen over during a considerable part of the 
year, and that its shore work was thus greatly retarded. It is 
also possible that a considerable part of the area of Lake Agassiz 
was for much of its life occupied by a lobe of ice, thus diminish- 
ing the swing of its waves. Hence a conclusion that rests on the 
very brief duration of Lake Agassiz as a postulate is not an una- 
voidable conclusion, while our knowledge of the duration of that 
remai-kable lake is open to varying interpretations. And in re|)ly 
to Mr. Upham's answer on this point that certain experts think 
