THE EQUATORIAL ZONE. 291 



thread, they tumble from rock to rock, or creep through 

 the turf-like patches of Arctic plants ; then, joining com- 

 pany with other springs, the stream becomes a torrent, 

 which makes a deep bed for itself down the side of the 

 mountain, till, lower down still, many of these ravines unite 

 to form deep valleys, which are enclosed by rocky ramparts 

 of many hundred feet in height.* Now, it is on the im- 

 mense ridges which separate these valleys from each other 

 that the Cinchona, or Peruvian bark-tree, is found growing 

 in the midst of thick forests." The bark is chiefly obtained 

 from the varieties of four different kinds the grey, red, 

 yellow, and white Cinchonas. The Cinchona belongs to 

 the Natural Order of Madderworts (Rubiacea) ; it is very 

 European in its appearance, and makes a striking contrast 

 to the tropical forms around it. (Plate XYI.) 



"The Indians who are employed in collecting the bark 

 are often exposed to considerable danger," says Humboldt, 

 "in climbing to the summits of the highest forest-trees, 

 in order to obtain an extended view, from which they may 

 distinguish the scattered, slender, and aspiring trunks of 

 the Cinchona, by the reddish tint of their large leaves." 



* " Plusieurs centaines de metres d'elevation." A metre is rather more 

 than three English feet. 



