MORAINES OF RECESSION 455 
would then average about 164 feet a day. And further, this 
retreat would have to take place in the face of the continual 
advance of the ice, so that the ice would have to melt back 
probably considerably more than 164 feet a day. Sucha con- 
clusion is manifestly absurd and its absurdity is only increased 
when we reflect that this is the average per day for the whole 
year. During the winter months there must have been not only 
no great amount of melting, but more or less readvance because 
melting ceased; and it is probably true that for some months in 
the spring and fall the forces of advance and retreat were about 
at a balance. Only for three or four months in summer would 
the forces of retreat be effective, so that substantially the whole 
annual retreat would have to take place during the summer and 
the rate of retreat would have to be at least four or five times 
the daily average for the whole year, or 700 or 800 feet or more 
per day. It does not help the matter much to change the period 
to 100 or even 300 years. For it would still be necessary to 
postulate a high rate of retreat—seventy or eighty feet per day 
during the effective melting season. If the period were 3000 
years the rate would still be seven or eight feet a day at that 
season. With the period at 6000 years the rate would be three 
and a half to four feet a day, and at 12,000 years one and three- 
fourths to two feet a day. 
We may suppose, if we choose, that at every turn of retreat 
the ice-sheet became completely disintegrated and broken up, at 
least over a wide marginal belt, on account of the suddenness 
and intensity of the increased warmth. But such a supposition 
savors of catastrophism and does not seem to be in the line of 
probable truth. The equilibrium between glacial accumulation 
and ablation must have been a very delicate one. There could 
be no wide departure from a balance of forces without a corre- 
sponding great change in the extent of the ice-sheet. Surely the 
climatic conditions which permitted the ice-front to stand fora 
long time at Fort Wayne were not greatly different from those 
that permitted it to stand at Defiance or at Toledo. There is no 
need of supposing that sudden or violent climatic changes produced 
