188 BOTANICAL GAZETTE [SEPTEMBER 
dium elegans, which is also common on rock, seems to require pro- 
tection and is probably one of the later species to appear. The 
same may be said of Lecanora subjusca allophana, one of the impor- 
tant lichens, which succeeds best in crevices. 
The rock lichens occurring within this region belong to the 
primitive lichen formation. 
Factes: Parmelia conspersa (Ehrh.) Ach., Rinodina oreina (Ach.) Mass. 
PRINcIPAL spEctEs: Lecanora calcarea (L. )Sommerf., L. subfusca allophana 
Ach., L. previgna (Ach.) Nyl., L. rubina (Vill.) Ach., L. rubina opaca Ach., 
Placodium elegans (Link) DC., Buellia petraea montagnaei Tuck., Lecanora 
previgna revertens Tuck., L. xanthophana dealbata Tuck., Staurothele umbrina 
(Wahl.) Tuck. 
SECONDARY SPECIES: Placodium cerinum (Hedw.) Naeg. & Hepp., Aceto- 
spora chlorophana (Wahl.) Ach., Biatora crenata dealbata Tuck., Heppia 
Despreauxii (Mont.) Tuck., Placodium vitellinum (Ehrb.) Naeg. & Hepp., 
Umbilicaria rugifera. 
This formation occurs on all exposed rocks, with the possible 
exception of the Permian, which in most places disintegrates too 
rapidly to support a lichen flora. With the exception of the last 
four species, all of the species occur on exposed surfaces. The last 
four and Placodium elegans prefer shaded or at least somewhat Pro 
tected situations, Throughout the mesa this formation has been 
almost completely replaced by the Bouteloua formation. On hills 
and more exposed rocky situations it is sometimes as important as 
the grass formation with which it alternates. With the more comr 
plete disintegration of the rocks this formation will entirely disappe™ 
On alluvium. 
Uncertain as is our knowledge of the primary succession on rock, 
it is much more certain than our knowledge of the primary succe> 
sion on alluvium. A careful study of the formation, and in partic: 
ular those places which are least covered with vegetation, seems to 
aid in forming an idea of this primary succession. 
Near Eastonville, in the region lying between the Bouteloua 
formation and the invading pine formation, an open area is being 
invaded by the following species: Potentilla coloradensis, Thermops® 
rhombijolia, Erigeron glandulosa, Paronychia Jamesii; followed by 
Arenaria Fendleri, Muhlenbergia gracilis, Bouteloua oliogostachy4, 
