246 



THE FORMATION 



cious sinter, etc., made by iron and lime springs and by geysers, and they 

 must be changed by decomposition into soils proper to be comparable with 

 alluvial soils. 



297. Succession in aeolian soils. The only wind-borne soils of geological 

 importance at the present time are those which form dunes, both inland and 

 coastal. Aeolian deposits consist largely of rounded sand particles, which 

 are of almost uniform size in any particular dune, but vary greatly in dunes 

 r>i different ages. The reaction of the pioneers on dunes plays an important 



Fig. 62. Talus arising from the decomposition of granite; the gravel 

 is covered with a formation of foHose lichens (Parmclia-chalicium), 

 the second stage of the primary talus succession; the herbs are pioneers 

 of the next stage. 



part in building the latter, but the immense dunes of inland deserts, which 

 are entirely destitute of vegetation, seem to indicate that its value has been 

 overestimated. The first stages in dune successions are dissophytic, i. e., the 

 plants grow in a soil of medium or high water-content, but in an atmosphere 

 that is extremely xerophytic. The ultimate stages vary widely in accord- 

 ance with the region in which they occur ; they may be xerophytic heaths or 

 mesophytic meadows and forests. Because of their striking character and 

 economic significance, dunes have received much attention, with the result 



