294 



THE FORMATION 



Cetraria, Acarospora, Lecanora, Lecidea, Bticllia, etc. The stratum itself is 

 physically very different and constitutes a distinct habitat. These groups 

 are really small formations, which are quite distinct from the surrounding 

 forest or meadow. This is proven conclusively in many places in the moun- 

 tains where areas of the characteristic lichen formations of cliffs are carried 

 by the fall of rock fragments into forest and meadow, where they persist 

 without modification. This also shows clearly that the groups on scattered 

 rocks in the same area are to be regarded as examples of the same cliff for- 

 mation, except- where the differences are evidently to be ascribed to develop- 



Fig. 75. 

 mosses. 



Relict lichen formation in a spruce forest, invaded by rock 



ment and not to alternation. Where these rock formations can not be traced 

 to cliffs or magmata with certainty, they must be considered as antedating 

 the vegetation in which they occur. Often, indeed, especially in igneous 

 areas, they are relicts of the initial stage of a primary succession.* Finally, 

 they prove their independence of the forest or meadow formation by initiat- 

 ing a distinct succes.sion within these. Crustaceous groups or formations 

 yield to foliose ones, and these in turn give way to formations of mosses, 

 particularly in the forest where the effect of the diffuse light is felt. From 

 the above, the following rule of formational limitation is obtained : any area, 

 which shows an essential difference in physical character, composition, or 

 development from the surrounding formation is a distinct formation. 



