586 THE ICE AGE IN NORTH AMERICA. 



the junction of these arcs is at Salamanca, New York, almost 

 exactly on the meridian of Toronto. It is therefore a plau- 

 sible hypothesis that the lower till at Toronto was deposited 

 by the Keewatin ice-sheet near its eastern margin and that 

 it withdrew some time before the Labradorian sheet reached 

 that point. 



This opens up a wide field of speculation connected with 

 our theories of the cause of the spread of the various ice-sheets. 

 On the theory that elevation of land is the prime cause it 

 would appear that the rise of land proceeded in a wave from 

 west to east. The Keewatin center therefore rose first and 

 sent out its ice-sheets far south to Kansas, and east to Pennsyl- 

 vania. Then as it began to sink under its accumulating load 

 of ice, the eastern or Labradorian center began to rise and in 

 due time started its glaciers to meet the vanishing ones from 

 the Keewatin center. But, possibly long before Toronto was 

 reached by the Labradorian ice-sheet, the Keewatin glacier had 

 retired from its eastern limit amid conditions of climate that 

 were essentially preglacial. For it must be borne in mind that 

 the retreat of the ice can only take place when the climate is 

 abnormally warm. Indeed such warmth would seem to be 

 essential for the melting of the ice. 



An interesting direct .proof of this was found by Dr. Hoist 

 in southern Sweden, where he excavated gravel beds in front 

 of the principal moraine which contained remains of plants 

 and animals characteristic both of warm and cold climates 

 in close connection, and which must have been contemporane- 

 ous. Similar facts were reported to me by Professor Tscher- 

 naschev from Finland. It is also well known that large species 

 of oysters lived long after the glacial epoch in Maine, especially 

 at Damariscotta, which do not survive except on our southern 

 coasts. It is in point also to instance the spread of the masto- 

 don, the mammoth, the rhinoceros and even the hippopot- 

 amus under the conditions which prevailed in northern 

 Europe and Asia during the glacial period. 



