242 — THE FORMATION 
ern coast is rising at the rate of five or six feet a century. There can be little 
question that such changes of level will produce marked changes in vegeta- 
tion, but the modification will be so gradual as to be scarcely perceptible in a 
single generation. It is probable that the forests of the Atlantic coastal 
plains are the ultimate stages of successions initiated at the time of the final 
‘elevation of the sea bottom along the coast line. 
Fig. 60. A lichen formation (Lecanora-Physcia-petrium), the first 
stage of the typical primary succession (Lecanora-Picea-sphyrium) of 
the Colorado mountains. 
292. Succession through volcanic action. The deposition of volcanic 
ashes and flows of lava are relatively infrequent at present, occurring only 
in the immediate vicinity of active volcanoes, chiefly in or near the tropics. 
Successicns of this sort are in consequence not only rare, but they are also 
relatively inaccessible to investigators. They have been studied in a few . 
cases, for example, those of Krakatoa by Treub, but this study has been con- 
fined to the general features of revegetation. Ash fields and lava beds are 
widely different in compactness, but they agree in having a low water- and 
nutrition-content. The pioneer plants in both will be intense xerophytes, but 
the soil differences will determine that these shall be sandbinders in the for- 
mer, and rock-weathering plants in the latter. 
