274 BOTANICAL GAZETTE [OCTOBER 
Parkinsonia Texana formation—The coarse gravelly débris 
slopes in the Eagle pass quarter of the province are occupied 
by this species. It is an habitually procumbent or straggling 
and spreading shrub which, while occupying the ground fully, is 
a very diffuse covering. 
Leucophyllum Texanum formation.—This species occurs in solid 
mass in the more arid parts of the province, especially within 
twenty miles of the Rio Grande on both the Texan and Mexican 
sides. The hairy felt covering of the foliage gives it a light 
tone which is very striking to the eye, 
Opuntia Lindheimeri formation—As an ecological type it is 
scarcely consistent to class this succulent with dwarf lignescent 
vegetation, and as suchatype it is discussed elsewhere (see 
under ‘succulent vegetation”). But the “prickly pear” is SO 
thoroughly a part of the bush vegetation of the Rio Grande 
country as to necessitate its mention here. The association of 
this species with mesquite has been remarked. Together they 
have preempted a vast amount of territory, and have introduced 
economic problems of no small concern, as well as interesting 
ecological phenomena. 
On the upper course of the Rio Grande, within the Rio 
Grande plain, the chaparral formation assumes a more xero- 
phytic aspect, and species of the trans-Pecos chaparral appear. 
A transverse section across the Rio Grande embayment from 
northeast to southwest would encounter this same transition to 
the extreme xerophytic type on the Mexican side, where the 
central plateau and sierra region begins, that is, at the outer rim 
of the embayment. 
THE TRANS-PECOs CHAPARRAL.—The area covered by this 
type of chaparral embraces, besides trans-Pecos Texas, the north 
Mexican plateau and the arid lower Sonoran zone plateaus of 
New Mexico, Arizona, and southern California. Its northeastern 
extension includes the western part of the Edwards plateau and 
the southern slope of the Staked plains. 
In the trans-Pecos region the formation occupies the cafion 
sides and bluffs from the Pecos westward, the rougher southern 
