1906] SHANTZ—VEGETATION OF THE MESA 187 
in this region. It is the only tree which supports a rich lichen 
flora. 
Four important formations are found meeting and mixing here: 
the oldest, the foothill woodland, which has almost disappeared; 
the pine formation, which is slowly advancing; the thicket forma- 
tion, which is also gradually advancing; and the grass formation, 
which gives way as the others advance. The thicket formation at 
this point entirely lacks the oaks, which fact is probably due to 
grazing. 
In another part of the grass formation there is evidence of a slight 
advance of the foothill woodland. Young trees of Juniperus mono- 
sperma have established themselves in a few places on the mesa. 
In other places Stipa Scribneri is entering the plains region along 
with the thicket formation. The preponderance of evidence, how- 
ever, seems to be in favor of a recession of this formation, and there 
is good reason to believe that it was at one time more extensive than 
at present. 
Many species which seem to be most at home in the mountain 
formations also push down into the Bouteloua formation. Among 
these may be noted Achillea lanosa, extending down the draws, 
Geranium caespitosum, Gilia pinnatifida, G. aggregata, Campanula 
petiolata, and many other species, 
Succession. 
PRIMARY SUCCESSION. 
On rock. 
What the primary succession has been in this region cannot be 
determined. The succession on rock undoubtedly began with the 
lichen. On the most exposed rocks of the lime ridge Staurothele 
wmbrina and Lecanora previgna are practically the only lichens 
found. On the other rocks the lichens are much more mixed and 
there seems to be good evidence of the accepted succession for 
lichens: first the crustose; then the more foliose forms like Lecanora 
rubina and L. rubina opaca; and finally Parmelia conspersa. On 
the mesa, where the rocks range from 5%" in diameter to coarse 
Sravel, Parmelia conspersa is the most important lichen, although 
‘™odina oreina and Lecanora calcarea are also common. Placo- 
