1917] VESTAL—FOOTHILLS VEGETATION 361 
larger-foliose, also gray-green, Parmelia conspersa (Ehrh.) Ach., 
which may be said to be the dominant and most frequent lichen of 
exposed rock. Here also occur the gray-green crustose-foliose 
Rhizocar pon geographicum (L.) DC., with Rinodina oreina persisting 
from the preceding stage, and a few other species, including one or 
two of Parmelia, Gyrophora erosa, and the large and peculiar 
Gyrophora vellea (L.) Ach., in crisped-margined dirty-gray plates 
©.25-2 inches in diameter, attached centrally beneath. These 
Fic. 2.—Lichens on steep rock-wall of rather sheltered side-canyon opening north- 
ward into Boulder canyon; dark masses at joints are cushions of Selaginella. 
established stages appear to be self-perpetuating so long as physical 
conditions of dry rock are unchanged. 
Less exposed, but usually dry, surfaces, such as shaded rocks, overhangs, 
surfaces dripping for some time after rains, recesses in joints, etc., hav 
characteristic lichen assemblages and definite successions. The earlier lichens 
include the yellow-green crustose Acarospora xanthophana (Nyl.) Fink (charac- 
teristic and abundant), with Lecanora rubina, Rhizocarpon geographicum, and 
Rinodina radiata. Acarospora persists in the established stages, which also 
show the bright orange- sor crustose-foliose Caloplaca elegans (Link.) Th. Fr. 
(characteristic and abundant), Parmelia conspersa, P. sulcata Tayl., P. conspersa 
var. stenophylla? Ach., peters vellea, and other species. Considerably 
less of the rock surface remains uncovered with lichens here than in the xero- 
phytic situations, and the number of species is larger, although the area of this 
