242 



THE FORMATION 



em 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. GO. A lichen formation (Lecanora-Physcia-pctrium), the first 

 stage of the typical primary succession (Lecanora-Picca-sphyrium) of 

 the Colorado mountains. 



292. Succession through volcanic action. The deposition of volcanic 

 ashes and flow.s of lava are relatively infrequent at present, occurring only 

 in the immediate vicinity of active volcanoes, chiefly in or near the tropics. 

 Successions 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 xcrophytes, but 

 the soil differences will determine that these shall be sandbinders in the for- 

 mer, and rock-weathering plants in the latter. 



