294 THE FORMATION 
Cetraria, Acarospora, Lecanora, Lecidea, Buellia, 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. Relict lichen formation in a spruce forest, invaded by rock 
mosses. 
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 succession 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. 
