THE DATE OF THE GLACIAL PERIOD. 585 



Both the fauna and the flora of the lower, or Don, beds indi- 

 cate a much warmer climate than those of the upper, or Scar- 

 boro beds. In the Don beds there are found leaves and wood 

 of maple, elm, ash, hickory, basswood, and even of pawpaw 

 and osage orange which now flourish only in latitudes several 

 degrees south of Toronto. Also, of the mollusks found in the 

 Don beds, four of the species are not now found in the St. 

 Lawrence basin, but only after passing the watershed which 

 separates it from that of the Mississippi. 



On the other hand, the upper, or Scarboro sands and 

 clays are wanting in the species indicating a warmer climate 

 but abound in both a flora and a fauna suggestive of Labra- 

 dor and of the region north of Lake Superior. 



In the opinion of Professor Coleman these facts cannot 

 be accounted for except on the supposition that the earlier 

 ice-sheet retired from practically the whole region to the 

 northward before the latter one began its advance; which 

 certainly looks very reasonable at first sight. But there are 

 a number of considerations, too much overlooked, which 

 perhaps permit a contrary conclusion. 



1. We are not warranted in assuming that the advance of 

 the ice was simultaneous from the Keewatin and the Labra- 

 dorian centres. On the contrary it seems certain, as has been 

 shown above, that the advance from the Keewatin center 

 was much earlier than from the other. The so-called Kansan 

 till underlies the Illinoisan for several hundred miles east of 

 the Illinoisan border. For example the Illinoisan ice crossed 

 the Mississippi at Burlington, Iowa, and advanced many 

 miles westward. But Kansan ice had at an earlier time 

 spread eastward so as to carry Lake Superior copper as far 

 as Warren in Western Pennsylvania. In a previous chapter 

 (p. 527) attention is called to the fact that the boundary of 

 the glaciated areas in the central and eastern parts of the 

 United States consists of the arcs of two circles with their 

 centers respectively in Labrador and the Lake Superior region. 

 This will appear at a glance by consulting the map. Now, 



