ISi SIXTH UiiPORT — 183G. 



arctic circle down to the tabic landb of Mexico, almost without 

 varying their climate ; and that were the altitude of the ridge 

 between the tropics great enough and sufficiently continuous to 

 join the temperate zones of North and South America, we might 

 expect to find many species of quadrupeds and birds common 

 to both; but it so happens that in the Isthmus of Panama, nearly 

 at the place where the elevation for the accomplishment of such 

 an union would require to be greatest, the Cordilleras are de- 

 pressed to a height not exceeding 500 or 600 feet, and still 

 further south there is a plain extending from sea to sea between 

 Rio Naipipi and the Gulf of Cupica*. It is not, however, as 

 Humboldt has remarked, the altitude of the mere peaks which 

 is to form an element in an inquiry of this kind, but rather the 

 heights of the backs of the mountains over which the passes 

 from one side of the chain to the otlier are usually made. But 

 we have no positive information respecting the height of the 

 passes of the Rocky Mountains, and even the altitude of the 

 base of the range above the sea, which forms a material item in 

 the computation of the absolute elevation of the peaks, has not 

 been calculated from barometric measurement, but merely by 

 vague estimates of the descent of rivers. Major Long assigns 

 to this base a height of 3000 feet, while Lieutenant Pike with 

 less probability more than doubles that altitude. 



The Rocky Mountains are bounded on the Atlantic side by vast 

 plains, having a gradual inclination to the eastward, and forming, 

 from the 50th degree of latitude down to the Gulf of Mexico, a 

 water-shed traversed by the Red River, the Arkansas, Missouri, 

 and Mississippi. A zone to the westward of the latter river 

 about 200 miles broad is well wooded, but the remainder con- 

 sists of sandy and naked prairies, whose surface though gently 

 undulated presents as few landmarks to guide the traveller on 

 his way as he would meet with in the middle of the occanf. 



Between the 50th and 54th parallel lie plahis of similar 

 character, crossed by the forks of the Saskatchewan, which falls 

 into Hudson's Bay; and still further north the Peace River 

 flows towards the Ai'ctic sea through a fertile tract generally 

 level, and inclosing portions of prairie land, but more en- 

 croached on by pine forests than the southern plains. The 

 valley of the Mackenzie beyond the Gist parallel, instead of 

 being separated from the Rocky Mountains by an intervening 

 level tract of land, skirts their bases until it issues in the Arctic 

 sea. We thus perceive that to the eastward of the Rocky Moun- 



• Humboldt. 



t The eastern banks of the Mississippi arc in general thickly wooded, but in 

 the State of Illinois there are some considerable tracts of i>rairie lands. 



