- ( .)-2 The American Geologist. May, 1806 
der to ;i more western position. Tins assumption would imply, 
therefore, that about 750 laminae of clay had been deposited 
during so many warm periods, each of several years' duration, 
while the ice-sheet had advanced about twenty-live miles. 
A better hypothesis would be to consider the periods to 
correspond to the annual seasonal changes which must have 
affected the climate then as well as now. During the long 
frigid winter the ice remained frozen, and but little glacial 
rock-flour was supplied to the lake to be scattered over its 
bottom by the gentle currents which carried it to the most 
western parts. But when the hot summer sun beat down on 
the ice, it rapidly melted; the lake became warm* r, under- 
mining the glacier ; small icebergs broke off and floated out 
into the lake ; and through these means the exact conditions 
were produced for the formation of our variegated, laminated 
lake deposit. Under this theory the ice-front advanced about 
twenty- tive miles during a period approximately 750 years in 
length. I know that I am treading on dangerous ground in 
attempting to determine the rate of advance in the ancient 
ice-sheet, lately designated the Kansan, by means of the 
lamination of an extraglacial lake deposit; but, by placing 
the facts before the public, every glacialist can decide for 
himself what the value of the evidence may be. 
Many will consider 750 years as too long a time for so short 
an advance, but it must be remembered that, although in the 
region of the glacial lake Agassiz the general retreat of the 
last great ice-sheet of North America has been calculated to 
have been as*much as 750 miles in 1,000 years, all the phe- 
nomena of the Kansan drift sheet indicate much less vigor, 
both in forward movement and in rapidity of melting and so- 
called retreat. Furthermore, the retreat of a glacial front 
under warm climatic conditions must have been much more 
rapid than the advance under conditions just barely frigid 
enough to cause a general advance. The time was approach- 
ing when the climate would become so mild as to drive back 
the ice border, never again to reach its old limits. As I may 
attempt to show in some future paper, the rate of retreat, 
after the ice-front had again passed to the east of Freeport, 
was apparently about a quarter of a mile in a year, or twenty- 
five miles in a hundred years. At that later time the ice Avas 
