66 A Study of the Vegetation of 
of succession. Starting from the vantage ground of a crack or 
crevice, the small tufts collect wind-blown soil and lichen remains 
about them and building upon this substratum increase their | 
area. Well-worn rocks are frequently covered with the black 
mats of the Grimmia consocies. 
Less xerophytic but far more abundant is Tortula ruralis. 
Growing in places less dry than those occupied by Grimmia, it 
surrounds the base of rocky outcrops and occurs on thin stony 
soil where its soft cushiony mats may cover areas of many square 
meters. While Tortula is very abundant on basaltic ledges it is 
sometimes partly or wholly replaced, especially about quartzite 
rocks, by species of Polytrichum, especially P. piliferum and P. 
juniperinum. 
In other situations, and especially on canyon walls with north 
and northeast exposure other mosses are abundant, often covering 
large areas of sheer rock surface or growing luxuriantly over the 
thin stony soil. The following lists include the most important 
of these together with a number of lichens which usually occur 
with the mosses. Some of these lichens are less xerophytic than 
the foliaceous forms already mentioned. Indeed, they are the 
most exacting members of the foliose lichen community. 
Brachythecium. albicans Ceratodon purpureus 
Brachythecium erythrorrhizon Encalypta ciliata 
Camptothecium aeneum Grimmia apocarpa 
Camptothecium lutescens Mnium affine 
Cladonia pyxidata Ochrolechia upsaliensis 
Diploschistes scruposus Peltigera canina 
Poa-Polygonum Associes 
In the thin soil between the fragments of broken rock, an area 
over which Tortula ruralis and’other mosses for a time have 
control, bunches of June grass and plants of Polygonum majus 
and Plantago purshi begin to appear (Fig. 18). Changes of 
light and root competition react unfavorably upon the mosses, 
while shallow-rooted plants are well adapted to dominate just 
66 
