INITIAL CAUSES OF SUCCESSION 205 



areas, deposit is quite distinct from erosion. Running water may 

 build flood plains, and sand bars may be built in a similar way by 

 waves and tides. ^Yind, glaciers, and gravity are all agents of de- 

 posit since they must deposit the materials which they have moved 

 by erosion. Volcanic dust is sometimes carried by the wind and de- 

 posited in such quantities that existing vegetation is buried and a 

 bare area produced. Lava flows and other volcanic deposits form 

 bare areas in the vicinity of their source. The water from hot 

 springs and geysers often forms deposits of considerable extent. 

 Water from hot springs is often heavily charged with lime and this 

 is deposited as travertine. Rock Island in Utah Lake near Provo, 

 Utah, was built up entirely by deposits of travertine from hot 

 springs and extensive terraces in Yellowstone National Park have 

 been built up by the same means. The typical deposit from the 

 hot water of geysers, on the other hand, is siliceous sinter or gey- 

 serite. Both travertine and geyserite are rock and the successions 

 that take place on them are similar to those on other types of rock. 



The elevation or subsidence of a land mass, if great enough and 

 rapid enough, will produce a bare area. There are also many 

 climatic and biotic factors that may produce bare areas, though 

 these are, for the most part, secondary rather than primary bare 

 areas. For example, a drought may be severe enough to kill all 

 vegetation on an area, a wind storm may destroy a forest, a snow 

 slide may destroy the vegetation on a mountain side, a hail storm 

 or a late frost may kill herbaceous plants, or lightning may start a 

 fire which will destroy a forest (Fig. 90). In arid regions evaporation 

 may cause a pond or lake to completely disappear and so expose a 

 bare area of soil. iSIan is the most destructive of the biotic factors 

 and destroys vegetation over the largest areas (Fig. 91). Other 

 animals such as rodents, ants, or prairie dogs often destroy the plant 

 life of small areas. Parasitic plants such as the dodder or a disease- 

 producing fungus may become so abundant as to completely destroy 

 the plants on a limited area. 



Bare areas may be classified as sand, clay-gravel, rock, and water. 

 Sand consists essentially of disintegrated rock. It is usually com- 

 l)osed largely of grains of quartz and silicates but may have other 

 minerals and even organic matter mixed with it. A clay-gravel sub- 

 stratum also consists of disintegrated rock. The soils classed under 

 this head are extremely variable since they consist of varying pro- 

 portions of sand, gravel and clay. Bare rock also varies greatly, 

 both in hardness and in chemical composition. Water is of course 



