450 PLANT ZONES ON MOUNTAINS. 



will perhaps be a dry atmosphere and very scanty 

 rainfall. These conditions, it so happens, are charac- 

 teristic of the climates upon all the world's greatest 

 mountain regions, notably for instance, upon the Andes, 

 the Himalayas, the Rocky Mountains, the Sierra Madre, 

 and many other important chains. 



Furthermore from the very nature of things, almost 

 every variety of soil and aspect is to be found upon 

 extensive mountain ranges. The surface soil is washed 

 down by rains and streams into valleys and hollows and 

 forms more or less extensive areas of land of super- 

 excellent fertility, while in the immediate neighbour- 

 hood will very likely be extensive tracts of gravel 

 and rock, where trees and plants flourishing in those 

 descriptions of soil find their natural habitat: conse- 

 quently almost every species of vegetation finds some 

 spots adapted to its necessities, where it can grow and 

 flourish. Hence the magnificence of the timber and 

 other growths which are constantly met with upon the 

 mountain sides, often strikes one with surprise, especially 

 when the apparently barren situations, where they are 

 often seen growing, appear to be destitute of any regular 

 surface soil, and consisting almost entirely of dry sand 

 and gravel, or the detritus fallen from cliffs and other 

 rocks. In such situations, where it sometimes seems 

 difficult to imagine how they can find sufficient nutriment 

 to sustain life, splendid trees are often seen, the secret 

 of their growth being that the fertile portions of the 

 soil are mixed with or overlain by an exterior coat of 

 gravel, etc., which is constantly being carried down 

 from above, while rich veins of mould exist beneath, 

 where it has been washed in amongst the stones, into 

 which the roots penetrate sometimes to very great 



