250 THE FORMATION 



phil/ious to mesophytic, and, in dry regions, xerophytic conditions. When 

 the process of drying out occurs rapidly, as in a single summer, the original 

 formation is destroyed, and the new vegetation consists largely of ruderal 

 plants. A peculiar effect of climate occurs in regions with poor drainage, 

 where the result of intense evaporation is to produce alkaline basins and salt 

 lakes, in which the succession becomes more and more open, and is finally 

 represented by a few stabilized halophytes, or disappears completely. 



Fig. 63. A typical gravel slide (talus) of the Rocky mountains, 

 before invasion. 



305. Succession by animal agency. Successions of this class are alto- 

 gether of secondary importance, the instances in which animals produce de- 

 nudation being relatively few. Such are the heaps of dirt thrown up by 

 prairie dogs and other burrowing animals, upon which ruderal plants are 

 first established, to be finally crowded out by the species of the original for- 

 mation. Buffalo wallows furnish examples of similar successions in which 

 the initial stages arc subruderal, while overstocking and overgrazing fre- 

 quently produce the same result with ruderal plants. 



306. Succession by human agency. The activities of man in changing 

 the surface of the earth arc so diverse that it is impossible to fit the resulting 

 successions in a natural system. While man does not exactly make new 

 soils, he exposes soils in various operations: mining, irrigation, railroad 



