THE POLAR WORLD. 



forest are continually Avasted by extensive fires, kindled accidentally or inten- 

 tionally, which spread with rapidity over a wide extent of country, and con- 



tinue to burn until they are extinguished b}- a heavy rain. Sooner or later a 

 new growth of timber springs up, but the soil, being generally enriched and 

 saturated with alkali, now no longer brings forth its aboriginal firs, but gives 

 birth to a thicket of beeches {Betula alba) in Asia, or of aspens in America. 



