170 



struction; for with the advent of taller, shade-producing forms 

 they are speedily eliminated. There may be three more or less 

 distinct lichen sub-stages, viz., crustose, foliose, and fruticose; 

 but as a rule the last two are more or less completely telescoped 

 into one. Of the foliose lichens the most conspicuous in the 

 trap-rock succession is Parmelia conspersa, a form which often is 

 so abundant as to almost obscure the rock surface over consider- 

 able areas. But sometimes other foliose species are equally 

 important, notably Dermatocarpon miniatum, Umhilicaria penn- 

 sylvanica, and Parmelia caperata; while not infrequently Ste- 

 reocaulon paschale, one of the most characteristic of the fruticose 

 lichens along the trap ridges, usurps the soil prepared by the 

 crustose lichens. Associated with these lichens, and equally 

 capable of thriving wherever they can secure foothold, are a few 

 mosses, e. g., Grimmia Olneyi and Hedwigia ciliata. 



The rapidity with which not only the changes just described, 

 but subsequent changes as well, are brought about is influenced 

 of course to a greater or less extent by the nature of the site — the 

 degree of exposure to sun and wind, steepness of the rock surface, 

 etc. Succession almost invariably proceeds more rapidly along 

 the lower slopes of a hill than near its crest, due to the lesser 

 exposure here. The relative abundance of seepage water as 

 the bottom of a hill is approached, also furthers rapid succession. 



Thus far observations have been restricted to the vegetation of 

 the rock face. Attention must now be directed to another phase 

 of the trap rock succession, viz., succession, in the crevices. 

 Crevices due to various causes are found in greater or less abun- 

 dance in practically all exposed rocks. Trap rocks especially, on 

 account of the peculiar manner in which they were formed, are 

 characterized by the presence of numerous fissures. In' these 

 fissures, and in hollows of the rock surface, dust and sand collect, 

 thus favoring the conservation of moisture and making it possible 

 for plants to develop whose roots or rhizoids require a soil. The 

 pioneer crevice plants are fruticose lichens and mosses. Of the 

 lichens, the majority belong to the genus Cladonia, e. g., C. rangi- 

 ferina, C. uncialis, C. furcata, C. sylvatica, C. pyxidata. Of the 

 mosses, Ceratodon purpureus, species of Polytrichum (P. commune, 



