120 BOTANICAL GAZETTE [AUGUST 
whose vertices lie at San Antonio, Del Rio, and Brownsville.. 
At the north, along the base of the triangle from San Antonio 
to Del Rio, the plain ends abruptly at the southern margin of 
the Great plains region, which is here marked by a sudden down- 
fall, the Balcones fault escarpment, which here has an altitude 
of about 1000 feet. From the foot of this escarpment the plain 
slopes gradually to Gulf level. The Rio Grande plain of geolo- 
gists, called also the Rio Grande embayment, is described as a 
constructional plain lying between this escarpment of the plains 
and the east front of the Mexican cordilleras. Its surface con- 
sists of the sheet flood débris of these two border regions, and 
its individuality, as distinct from the Atlantic coast plain east- 
ward, lies in its construction and its surface weathering under far 
drier conditions than those which prevail eastward. 
The sheet flood débris from the margin of the plains does 
not cover all of the Rio Grande plain as here defined. The flat 
coast prairie with its compact clay structure still extends along 
a narrow belt toward the lower Rio Grande. From Brownsville 
northward to the middle of the region extend vast sand plains,. 
tongues of which reach well up toward the escarpment border. 
The flood débris lies in coarser or finer beds over the northern. 
half of the plain, with exposures of arid clays, flat silt plains, or 
ridges of coarse gravel. Add to these features the basalt extru- 
sions both along the northern margin and in the southern sand 
plains, and the streamway erosions through the various deposits,. 
and we have the factors which not only combine to determine 
different types of grass formation, but have also figured promi- 
nently in favoring the encroachment of the woody vegeation,. 
chiefly the chaparral formations. 
CLIMATIC CONDITIONS.—The temperature conditions are of 
great significance to vegetation in this province, but only indi-- 
rectly do they react upon the character of the grass formations. 
This indirect control consists chiefly in permitting the occur- 
rence of woody species that require high annual temperature 
(Mimoseae for example), which, with certain artificial barriers- 
removed, the burning of the grass notably, are capable of. 
