350 ASSINNIB0INE AND SASKATCHEWAN EXPEDITION. 



of oak and other forest trees, not crowded together, but 

 scattered over the surface at a considerable distance from 

 one another, without any low shrubs or underbrush 

 between them, is the characteristic of the border of the 

 Prairie region."* 



The growth of timber in the river bottoms in the 

 United States disappears altogether at the borders of the 

 Plains about the 98° of longitude, f In Eupert's Land, 

 south of the 52nd parallel, trees cease in the river bot- 

 toms except near the Upper Qu'appelle Lakes, in lon- 

 gitude 104° ; but they occur only on the northern as- 

 pect of the south side of the deep valley west of longi- 

 tude 102°, or a few miles west of Fort Ellice. 



The true limit of the Plains in Eupert's Land, east of 

 the South Branch, is well shown by the Grand Coteau 

 de Missouri. The country east of that natural boundary 

 may be classed as Prairie country, over the greater por- 

 tion of which forests of aspen would grow if annual fires 

 did not arrest their progress. The plateau of the Grand 

 Coteau forms the true Plains of Eupert's Land, where 

 both soil and climate unite in establishing a sterile region. 

 Mr. J. D. Whitney considers the absence of forest on the 

 rich prairies of the United States to be mainly due to 

 the physical nature of the soil ; " the extreme firmness of 

 the particles of which* the prairie soil is composed, is pro- 

 bably the principal reason why it is better adapted to the 

 growth of its peculiar vegetation than to the development 

 of forests." J The origin of the groves scattered over the 

 Prairies is traced to ridges of coarse material, apparently 

 deposits of drift, on which, from some local cause, there 



* Report on the Geological Survey of the State of Iowa, by James 

 Hall, State Geologist, and J. D. Whitney, Chemist and Mineralogist, 

 t Ibid. 



X Geology of Iowa. 



