1901} VEGETATION OF WESTERN TEXAS 283 
ecological conditions prevail. In the wearing away of the 
mountains in this dry climate the débris is carried downward to 
fill the intervening basins. The finer débris being carried nearest 
the basin center, on the slopes at the outer rim of the basin lies 
the coarser débris, an unconformable mass of coarser or finer 
stony talus. This talus débris may extend far up the slopes of 
mountains, which receive but a few inches of rainfall annually, 
especially those in the Great bend, and thus carry the formation 
of cactus, yucca, and agave with it. These “yucca belts,” as 
Mr. Hill calls them, when projected upon a map appear as mar- 
ginal lines of basins and to encircle the various mass elevations. 
The formation extends even to the east of the Pecos cafion along 
the coarse débris from the Balcones escarpment. The abundant 
occurrence of Yucca Treculeana and several species of cacti on 
the timberless cafion sides as far east as the Colorado may be 
regarded as an extension of the same formation. 
As to the prominence of the several types in the formation, 
as already stated, viewed from a distance the yucca vegetation 
seems the only prominent one. It is in reality so only in some 
instances. In other cases other types play the chief rdle, so 
that several variations exist, as illustrated by the following: 
Open forest-like formations of the palmo or Yucca macrocarpa.— 
This largest of our yuccas is very abundant in the Great bend 
region, covering the long arid slopes with an open formation of 
yucca trees ‘with caudex ten to twenty feet high and one to 
two feet in diameter” (Havard). 
Lechuguilla (Agave heteracantha) and sotol (Dasylirion Tex- 
anum) formation.—This is a very characteristic formation of very 
rough stony slopes in which the lechuguilla forms a most obnox- 
ious feature because of its agressive short bayonet-like leaves. 
This formation is very common as far east as the Nuéces cafion. 
Formations in which globular and cylindrical cacti predominate.— 
These include several species of Mamillaria (among them forms 
called “devils pin cushion”), Cereus (notably the strawberry 
cactus, C. stramineus), Echinocactus (especially the “ Turk’s 
head,” £. longihamatus), and Anhalonium. Such formations 
