Igor] VEGETATION OF WESTERN TEXAS 277 
adaptation seems to lie in the great quantity of resinous matter 
in the leaves, The odor from this gives notice of the nearness 
of a greasewood formation. 
THE TRANS-PECOS TYPE OF CHAPARRAL EAST OF THE PECOS.— 
As already noted, this chaparral spreads east of the Pecos upon 
the western margin of the Edwards plateau, especially its plains 
area, and upon the southern border of the Staked plains. The 
easterly extension of the more arid lower Sonoran area is indi- 
cated in this chaparral. A typical case may be cited in the 
vicinity of Big springs in Howard county (fig. 76). This is a 
region covered by the plains débris. The arid mesas are cov- 
ered by a characteristic formation of chaparral in which not less 
than twenty species occur, among which are more than 50 per 
cent. of the characteristic trans-Pecos species, 
THE Rio GRANDE CHAPARRAL IN CENTRAL TEXAS.—The chap- 
arral of the Rio Grande plain terminates abruptly at the foot 
of the Balcones escarpment, where the xerophytic forest of the 
erosion area begins. A few species, however, mingle with the 
timber species, and occasionally (as in the case of Condalia obo- 
vata, Zizyphus obtusifolia, and others) forms a characteristic 
chaparral bush or thicket. On the grass plains of the granite 
country a much more pronounced encroachment of woody vege- 
tation is taking place. As already pointed out, the mesquite 
has spread over most of the province. With it have gone the 
prickly pear and several shrubby species which together are 
building up a compact chaparral formation. The Rio Grande 
Mimoseae, except Prosopis, and Caesalpineae, and all others of 
the more characteristic species, do not appear in this chaparral. 
FORMATIONS OF SUCCULENT OR WATER-STORAGE VEGETATION. 
This third general type of formation stands out in strong 
contrast with the two preceding types of grass and woody vege- 
tation. The contrast lies in the fact that whereas the preceding 
types have ‘adjusted themselves to xerophytic conditions by 
retaining the least amount of sap-bearing tissue, this type has 
Sone exactly to the opposite extreme. The areas occupied by 
