DEPOSIT. 49 



The location of the talus is important in determining its water relations, as 

 well as its possible population. Along banks that are being undercut, the 

 material is swept away, but when the current leaves the shore the talus is 

 often built up in the water. This happens not infrequently on lake-shores 

 as well. In both cases the excessive holard which results initiates the succes- 

 sion with a hydrophytic or amphibious stage. When the talus accumulates 

 on land, as is the rule, the initial holard is typically less than the normal for 

 the particular soil in the climatic region concerned. This arises from the 

 looseness and unevenness of the talus and the corresponding ease of evapora- 

 tion from the soU. In desert regions this tendency becomes decisive, and the 

 colonization of the south and west slopes is extremely difficult. 



Bare areas due to volcanic deposits. — Volcanic agencies bring about deposits 

 of lava, of cinders so-called, of ash or dust, and of sinter. Deposits of ash 

 have already been considered briefly under "Wind." The local deposition of 

 ash is less influenced by wind, and the depth of accumulation is often very 

 great, sometimes reaching 50 to 100 feet. On the cones themselves it is fre- 

 quently much greater. Coarser material — cinders, rocks, and enormous 

 stones — are also blown from craters in great quantities and fall near the cone 

 or upon its slopes. The lava and mud expelled from volcanoes flow in streams 

 from the crater. Rivers of lava have been known to reach a length of 50 miles 

 and a width of half a mile. In flat places the stream spreads out and forms 

 a lava lake which hardens into a plain. Mud volcanoes are small, geyser-Kke 

 structures which discharge mud. They build up small cones, which are usually 

 grouped and cover considerable areas with their deposits. The deposits due 

 to volcanoes or geysers regularly result in the destruction of vegetation, but 

 this effect may be produced alone, as a consequence of the emission of poison- 

 ous gases, steam, hot water, or hot mud, of fire-blasts or the heating of the soil. 

 Such bare areas are characteristic features of Yellowstone Park (plate 8,a,b). 



All volcanic deposits are characterized by great sterility. They are usually 

 small in extent, and hence easily accessible to migrants. The ease of invasion 

 depends largely upon the coherence of the deposit. Invasion takes place 

 more readily in ash and cinders than upon lava, unless they are quite deep. 

 Mud deposits would apparently be invaded most readily. The seres of vol- 

 canic deposits have been little studied, but it is known that they are relatively 

 long. As would be expected, this is particularly true of lava, though climate 

 exerts a decisive effect, as is shown by the invasion of lava fields in Iceland 

 and Java. 



Ponding and draining. — These constitute a second pair of related but oppo- 

 site topographic processes. Flooding or ponding is ahnost inevitably fol- 

 lowed by draining, and the drainage of an area may be obstructed and result 

 in flooding at almost any time. Ponding usually produces extreme water con- 

 ditions, with a corresponding effect upon succession. Drainage reduces the 

 depth of water, or extreme holard, and accordingly shortens the development. 

 Although opposite as processes, the two may produce exactly the same initial 

 area. This is probably a frequent occurrence. Ponding may be so shallow 

 as to permit the immediate entrance of hydrophytes or of amphibious plants. 

 Drainage may reduce the level of pond or lake without completely emptying 

 it, and thus produce similar depths for invaders. Here again it is evident that 

 the water relation of the new area is decisive, and not the originating process. 



