1901] VEGETATION OF WESTERN TEXAS 213 
this crumbling rock, even permitting a soil accumulation. The 
existence of similar conditions in the exposed formations in the 
Grand prairie results in carrying the timber vegetation over the 
hills and bluffs and bordering escarpments of this province far 
to the northward of the main body (fig. zz). This timber forma- 
tion is an open stunted forest covering, which suffers extensive 
interruptions, but continues in spite of this to be the dominant 
vegetation feature of the areas just described. On some high 
arid divides it becomes reduced almost to shrubby dimensions. 
Floristically, the chief elements of this vegetation are Lower 
Sonoran, of which many species range southwest into the Mexi- 
can mountains, or westward across the continent. Inthe western 
part of this province the presence of western mountain species is 
very marked. At the east there remains a strong element com- 
mon to the Austro-riparian and Carolinian zones; but, aside 
from these indications of relationship, the province has a strong 
individuality. This is well shown by the fact that Austro-riparian 
and Carolinian species (when not present in the cafion timber) 
are vicariously represented by species stamped with the climatic 
characteristics of the region. Perhaps the most noteworthy case 
is that in which Juniperus Virginiana is succeeded by Juniperus 
sabinoides, the latter being a most characteristic xerophyte. 
As to the content of the forest formation, this may vary from 
a pure formation of one species to a more or less uniform mix- 
ture of 50 or 60 per cent. of all the species. This variation 
is generally due to variations in soil condition. For example, a 
pure formation of Juniperus sabinoides may be due to an extensive 
exposure of very friable limestone like the Glenrose beds. In 
some cases the prevalence of a single species is due to the 
accident of its having gained the ground first. The following 
species formations are the most notable in this province: 
Juniperus sabinoides formation —This is the most important 
formation, partly because of its ecological relations, partly from 
its economic value. The formation is popularly called ‘cedar 
brake.” Such brakes may be almost impenetrable over many 
Square miles. The tree branches diffusely from the ground up, 
