108 SUCCESSION 



and of lowlands and shores, where sedimentation or ele- 

 vation constantly occur. The principal physical phenomena 

 which bring about the formation of new soils are: (1) ele- 

 vation , (2) volcanic action, (3) weathering, with or with- 

 out transport. 



1. Succession through elevation. Elevation was 

 of very frequent occurrence during the earlier, more plastic 

 conditions of the earth, and the successions arising as a 

 result of it must have been important features of the vege- 

 tation of geological periods. Today, elevation is of much 

 less importance in changing physiography, and its operation 

 is confined to volcanic islands, coral reefs and islets, and to 

 rare movements or displacements in sea coasts, lake beds, 

 shore lines, etc. There has been no investigation of the de- 

 velopment of vegetation on islands that are rising, or have 

 recently been elevated, probably because of the slow growth 

 of coral reefs and the rare appearance of volcanic islands. 

 On coral reefs, the first vegetation is invariabl.y marine, but 

 as the reef rises higher above the surf line and the tide, the 

 vegetation passes into a xerophytic terrestrial type adapted 

 to an impervious rock soil, and ultimately becomes meso- 

 phytic. In volcanic islands, unless they are mere rocks 

 over which the waves rush, the succession must always be- 

 gin with a xerophytic rock formation. The best known ex- 

 ample of a rising coast line is found in Norway and Sweden, 

 where the southeastern 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 vegetation, 

 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. 



2. Success ion 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. 

 Successions of this sort are in consequence not only rare, 



