A STUDY OF THE VEGETATION OF THE MESA REGION 
EAST OF PIKE’S PEAK: THE BOUTELOUA FOR- 
MATION. 
Il. DEVELOPMENT OF THE FORMATION. 
H. L. SHANTZ. 
(WITH SIX FIGURES) 
IN an earlier paper™ the writer has discussed the structure of the 
Bouteloua formation, and to this publication the reader is referred 
for general introductory matter. Space will not permit the inclusion 
_ here of lists of species in the formations of minor importance, which 
_. have to do with the development of this formation. The foothill 
age ee Aen tee Nt pe ee ae Gee ee 
thicket formation, the plains ruderal formation, and the plains bank 
formation are each made up of many species. Only the facies and a 
few of the more important principal species can be mentioned. 
Invasion by formations. 
FOOTHILL THICKET FORMATION. 
This formation extends along the eastern base of the mountains 
and down along the ridges and gullies far out on the plains. It 
forms a distinct zone at the base of the mountains (fig. 8) and here 
occurs in its best developed form. In most places under natural 
conditions there seems to be an ecotone, a place of equal aggressive- 
hess, between this formation and the Bouteloua formation. 
Facies: Cercocarpus parvifolius H. & A., Rhus trilobata Nutt., Quercus 
nhovo-mexicana (DC.) Rydb., Q. utahensis (DC.) Rydb., Q. Gambellii Nutt. 
Principat Spectes: Rubus deliciosus James, Holodiscus dumosus (Nutt.) 
Heller, Ribes cereum Dougl., R. leptanthum Gray, R. pumilum Nutt. 
In addition to those named there are about one hundred principal and second- 
ary species. 
This formation and the Bouteloua formation seldom mix, because 
where the shrubs grow the facies of the grass formation cannot exist. 
Nearer the mountains and along the hillsides they alternate sharply. 
While the climatic and soil conditions are identical, the differ- 
* Bor. Gazette 42:16-47. 1906. : 
179} (Botanical Gazette, vol. 42 
