1901] VEGETATION OF WESTERN TEXAS 205 
fairly constant condition. As to their affinity floristically, these 
annual species, while containing a considerable element common 
to Texas prairies south and southwest, are those which range 
over the whole stretch of plains to Nebraska and the northwest, 
thus indicating more positively the Upper Sonoran character of 
the province (fig. 78). 
The true value of the zonal areas of the Staked plains is not 
strongly brought out, because the dominant grass vegetation is 
not so sensitive to temperature differences. Yet it seems pretty 
certain, though not yet established, that some of the very 
important species of grasses found in southern and trans-Pecos 
Texas and north Mexico do not extend upon the Staked plains, 
but that rather the floristic content of the formation agrees with 
that of the plains northward. 
The grass formation is broken on the sand plains of this 
province because of the shifting character of the sands, This 
sand zone according to Dr. Havard, is entered upon going 
about twenty miles west of Odessa. ‘It extends thence south 
and east nearly to the Pecos and north to the very center of 
the plains. In this zone are the sand hills, a dreary chaotic 
belt of reddish sand, tossed by the wind into hillocks, cones, 
and ridges of various sizes and shapes.’’7 
The lower belt of the sand zone marks the transition from 
Staked plains to the Cretaceous Edwards plateau. This is the 
only portion of the sand plains of which we have data as to the 
plant formation. Dr. Havard says of it: ‘‘The only grasses 
seen were a stout Andropogon (near A. furcatus) three to five 
feet high, with running roots holding the loose soil in their 
meshes; a Sporobolus (probably a form of S. eryptandrus), like- 
wise erect and tall; and a large form of Cenchrus myosuroides.”’ 
This indicates that the characteristic plains formation is entirely 
wanting. The herbaceous annuals are also those which prefer 
sandy soils. It appears from Dr. Havard’s report that woody 
vegetation is the more dominant formation. 
HE STOCKTON PLATEAU.—This is a subdivision of the 
7 HAVARD, Contrib. U.S. Nat. Mus. 8: (no. 29) 1885. 
