1906] SHANTZ—VEGETATION OF THE MESA 181 
Coleosanthus umbellatus, Oryzopsis micrantha, Elymus condensatus, 
Filix jragilis, and Bryum argenteum. The thicket was rather open, 
as may be seen at once by the light readings, the ratio of which is 
3y: This ratio often becomes ,';, but is usually less for the greater 
part of the formation. 
Along the ridges north and east of Colorado Springs there is certain 
evidence that the thicket formation is slowly pushing its way out 
into the grass formation. Of the shrubs Cercocarpus parvijolius 
seems best adapted for this invasion, and it is several miles in advance 
of any of the other dominant species (jig. g). Nearer the mountains 
a ee RE ae ele ae tele 
_ Fic. 9-—Cercocarpus parvifolius invading the Bouteloua formation; pine forma- 
tion on the horizon 
evidence is also found of the invasion of the grass formation by the 
thicket formation. This invasion, however, is not rapid. As soon 
as the young shrub is established or has grown to sufficient size to 
Produce shade, the grass formation gives way rapidly to species 
