HUMUS AS A «EOGRAPHICAL AGENCY. 265 



among the agencies which determine the aspect and the future 

 of a country, i.e., among the geographical agencies. Owing to 

 its role having been overlooked, or not fully appreciated, many 

 phenomena failed to find their correct interpretation. 



In modifying the physical and chemical constitution of the 

 soil, it affects the superficial erosion of the rocks. Here, by 

 soluble acids, it furthers the disintegration of the solid rock, 

 and impoverishes the superficial layers. Elsewhere, by covering 

 the soil, it protects it from extreme variations of temperature 

 and moisture, prevents the transportation of disintegration 

 products, and checks erosion. In those light soils where it 

 accumulates in proper quantities, it binds together and acts 

 against a too rapid mechanical washing away. 



Humus forms enormous deposits, particulai'ly in peat-bogs, 

 in tundras, and in steppes (Tchernozoni). The coal-beds, too, 

 we may in most cases conceive as derived from former infra- 

 aquatic marshes of geological ages. 



No less striking and varied is its action on vegetation. For 

 instance, it changes a forest into a sphagnum bog or a heather 

 tract, and again the bog into heather. It transforms pasture 

 into marsh, marsh into infra-aquatic bog, this into sphagnum 

 moor, etc. Elsewhere it develops slowly forest at the expense 

 of steppe. 



The geographer must take humus into account and under- 

 stand its influence if he desires to interpret aright the relief 

 and nature of the land, its vegetation and fauna, the economic 

 and social history of man. 



VOL. XVII. PART 11. 



