down through the talus, finally reaching the forested area 
at its foot. On the other hand, soil found on the north 
crest is soon cheeked in its descent by the forest. A certain 
amount of it collects in depressions and irregularities of the 
rock surface and along the edges of joint planes, affording 
a foothold for crevice plants and matted vegetation. 
(b) The plant associations. 
The pioneer stages at the summit 
consist of lichens and mosses on the bare rock surface, and 
on the ground. Among the commoner crustose lichens appear 
such forms as Lecanora cinerea , Rhizocarpon petraeum and 
PIacodium aurellium . Following these come foliose species, 
Parmelia conspersa (Khrh.) Ach. being the most conspicuous 
and covering a large proportion of the rock surface; Physcia 
obscura (Schaer.) Hyl., Lecanora garovaglii and Gyrophora 
hyperborea Ach. are less common. Where a slight amount of 
soil has accumulated on the rock surfaces, fruticose types 
appear, common species being Cladonia rangiferina (L.) Web., 
Claddmia sylvatica (L.) Hoffm., and Stereocaulon paschale 
(L.) Hoffm. 
Associated with the lichens in the pioneer rock suc¬ 
cessions are moss species, such as Grimmia apooarpa , Poly - 
trichum piliferum Schreb. and Tortella tortuosa (L.) Limpr., 
the two latter being the commonest. One of the early mat 
