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. 
207. 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 
of 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 foliose lichens (Parinelia-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 regicn 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 
