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P. pilifenim, P. jumperiniim) , Leiicohryum glauciini, and Dicra- 

 num scopariiim are prominent. Closely following these, and 

 indeed often contemporaneous with them are certain Pterido- 

 phytes and the advance guard of the Angiosperms — the group 

 which ultimately is destined to predominate. The appended 

 list includes a few of the more conspicuous herbaceous vascular 

 plants characteristic of crevices in trap. 



Selaginella rupestris Danthonia spicata 



Woodsia ilvensis Krigia virginica 



Andropogon scopariiis Lechea temdfolia 



Agtdlegia canadensis Opiintia vulgaris 



Aristida dichotoma Poa compressa 



Campanula rotundifolia Potentilla argentea 



Corydalis sempervirens Rumex Acetosella 



Saxifraga virginiensis 

 It will be noted that most of the species here mentioned are 

 perennial, and necessarily all are xerophytes. 



Notwithstanding that crevice plants and rock face forms are 

 contemporaneous, the crevice vegetation, for the sake of con- 

 venience, may be regarded as the second stage in the succession 

 on trap (fig. i). On steep slopes this condition may be protracted 

 indefinitely. But as a rule a third stage is soon inaugurated by 

 the encroachment of the crevice vegetation on neighboring 

 portions of the rock face; and where, as on gentle slopes, the soil 

 collects not only in crevices but in shallow depressions of any 

 sort, the surface of the rock may soon become clothed with a 

 more or less continuous plant cover. The spreading out of the 

 crevice colonies is accelerated by the continued accumulation 

 alongside of windblown particles of inorganic matter and frag- 

 ments of vegetable debris. It is hardly necessary to more than 

 suggest the improved condition of the rock as a habitat for 

 plants which results from the development of a soil. The usual 

 pioneers on such an area are the lichen and moss species already 

 present in the crevices, and it is a common occurrence to find 

 flat rocks and gentle slopes completely overgrown by loose masses 

 of Cladonia, thin mats of Ceratodon, or dense colonies of Poly- 

 trichum. But the prestige of these lower forms is short lived, 



