170 PHYSICAL GEOGRAPHY. 



beds of lignite are known to exist, which are probably 

 also of the tertiary era.* 



In the whole width of the Mississippi basin, from the 

 falls of St. Anthony downwards, no primitive (or hypo- 

 genous) rocks appear, except in the low ridges of the 

 Ozark Mountains, which have a hilly prominence, owing 

 to the excavation of the valley ; but their summits scarcely 

 rise above the plane of the general slope, supposing that 

 it were extended with an even descent to the Atlantic. 

 They range from Red River to St. Louis, parallel to the 

 Alleghanies, and consequently make an angle with the 

 Rocky Mountain chain. Coal measures crop out on their 

 flanks. 



Of the forty degrees of latitude which intervene between 

 the Gulf of Mexico and the Arctic Sea, the valley of the 

 Mississippi occupies, as we have seen, about one half; and 

 the whole drainage of that portion west of the Alleghanies 

 is accumulated in one great channel, which is directed 

 southwards and a little eastwards. From the head of Lake 

 Superior northwards, there is a series of great transverse 

 excavations, occurring in succession, on to Great Bear 

 Lake, which lies under the Arctic circle ; and it is re- 

 markable that nearly all the lake basins f of these valleys 

 commence in the silurian strata, and are continued into 

 or entirely across a belt of primitive, or hypogenous, and 

 metamorphic rocks J, which extends from Lake Superior 



* Vide Journ. p. 194. 



-j- This grand series of lakes forms the line of canoe navigation 

 from Canada northwards ; and the fact of its position in the fracture 

 between limestone and granite, was perceived and recorded by Sir 

 Alexander Mackenzie. 



I My imperfect acquaintance with the science of geology renders 

 me incompetent fully to appreciate the worth of the several systems 

 that profess to explain the mode in which the beds forming the crust 

 of the earth have been formed ; neither have I exclusively adopted 



