Igor ] VEGETATION OF WESTERN TEXAS 271 
fewer than forty species associate in this formation, in many 
cases a limited area being tolerably uniformly covered by at 
least half of all the species (figs. 7-10). On the other hand, a 
single species constitutes almost the entire formation over exten- 
Sive tracts. 
Fic. 18.—Summit of the Staked plains, 3,700 ft. alt.; grass formation with 
Yucca glauca. PranBts)d mrpters 4 Fornt_es c 7 Can vv . 
tow weaat i 
On the basis of temperature, there is a distinction of species 
into those of semitropical and even tropical affinities and those 
able to endure the freezing winter temperatures of the warm 
temperate zone. The former occur chiefly in the lower Rio 
Grande country, the latter constitute the chaparral of the northern 
half of the plain, spreading also by some species into central 
Texas. Geological structure and conditions of soil act power- 
fully to determine the facies of the formation upon any given 
area. Thus, gravelly slopes, denuded of finer sediment, may 
