THE CAUSE OF THE GLACIAL PERIOD. 527 



the Glacial period. From a glance at the map it would seem, 

 therefore, that the greatest area of snow-fall was somewhere 

 in the vicinity of Lake Superior, and that a secondary area of 

 large snow-fall was in the vicinity of Labrador ; for the south- 

 ern boundary of the glaciated region consists essentially of the 

 arcs of two circles whose centers would fall within the areas 

 indicated. 



In speaking of these two areas as centers of radiation for 

 the glaciers of the great Ice age in North America, it is not 

 affirmed that the movement received no impulse from still 

 farther north. It is not improbable that the upper portion 

 of Baffin Bay was filled and crossed by the glaciers still 

 lingering over the continental area of Greenland, and that 

 this Greenland ice aided in the movement which covered the 

 northern part of the United States with its icy mantle. But 

 it was probably by reaction rather than by direct action that 

 aid came from that quarter. The accumulations to the north 

 prevented an outflow in that direction, and so compelled a 

 southerly movement from the vicinity of the Laurentian 

 highlands. It is not, however, probable that any Greenland 

 ice ever reached the United States. 1ST one of the bowlders 

 so common in the United States are, so far as known, more 

 than a few hundred miles distant from their parent ledges. 

 There was doubtless all the while an ice-stream down Baf- 

 fin Bay toward the Atlantic Ocean, with a movement into 

 it from both sides. But even if this were not the case, the 

 areas south of Hudson Bay and in Labrador would still be 

 the predominant influence in determining the southern out- 

 line of the glacial boundary. The snow that piled up from 

 year to year over those centers would be compelled to move 

 off in the lines of least resistance. Now, ice can be an ob- 

 struction to other ice as well as to water ; and what the 

 Greenland glacier probably did was to close up the upper 

 portion of Baffin Bay, so that the excess of snow-fall over 

 the subcenter referred to as north of Lake Superior could 

 not move off to the northeast, but was compelled to spend 

 all its force in a southerly movement. It is evident also that 



