602 THE GLACIAL LAKE AGASSIZ. 



uevertheless true tliat great contrasts exist between the eastern region, with 

 its plentiful raintall, and the dry western plains, as also between the almost 

 tropical southern margin of the United States and the tundras beneath the 

 Arctic Circle. In traveling from the once wholly forest-covered country 

 of the eastern States across the prairies to the far western plains, bearing 

 cacti and sagelirush, there is observed a gradual change in the flora, until 

 a very large proportion of the eastern species is left behind, and their places 

 are taken by others capable of enduring more arid conditions. Likewise 

 in going from St. Augustine or New Orleans to Chicago, St. Paul, Wimiipeg, 

 and Hudson Bay and Strait, the palmettoes, the evergi'een live oak, bald 

 cypress, southern pines, and the festooned Tillaudsia or "Spanish moss," 

 are left in passing from the southern to the northern States ; and instead we 

 find in the region of the Laurentian lakes the bur or mossy-cup oak, the 

 canoe and yellow birches, the tamarack or American larch, the black spruce, 

 balsam fir, and the white, red, and Banksian pines, while farther north the 

 white spruce, beginning as a small tree in northei'u New England and on 

 Lake Superior, attains a maj estic growth on the lower Mackenzie in a more 

 northern latitude than a large part of tlie moss-covered Barren Grounds 

 which reach thence eastward to the northern part of Hudson Bay and 

 Labrador. Thus, although no grand topographic barrier, like a high moun- 

 tain range, impassable to species of the lowlands, divides this great region, 

 the transition from a humid to an arid climate in passing westward, and the 

 exchange of tropical warmth for polar cold in the journey from south to 

 north, are accompanied by gradual changes of the flora by which in the 

 aggregate its aspect is almost completely transformed. 



In the central part of this large area, the basin of the Red River of 

 the North, with my geologic exploration during a half dozen summers, I 

 have given careful attention also to the geographic limits and relative 

 abundance of the species making up the flora. It has been interesting to 

 find there the intermingling and the l)oundaries of species whose principal 

 homes or geographic range lie respectively in the directions of the four 

 cardinal points, east and west, south and north. 



