| 
} 
1 
GED agg ae ee 
asorienalave 
+ enna oenae 
1901] VEGETATION OF WESTERN TEXAS 109 
not only are the fluctuations extreme, but that a large percent- 
age of the season’s rainfall may be precipitated in two or three 
heavy thunderstorms. The effect of the consequent erosion is 
of significance for vegetation under certain soil conditions, as 
for example in the Red beds province, and in other places 
where too heavy pasturing has bared the ground of the soil- 
Fic. 4.— Types of rainfall in the Texas region: /, Gulf; 7/, Great Plains; ///, 
Yuma; /V, Mexican; xxx, western margin of Gulf; ..... southern margin of 
great plains; _.__., northern margin of Mexican.— From Hill’s Phys. Geog. Texas. 
building grasses, as in the upper Rio Grande province. From 
these irregularities it follows that the vegetation has adapted 
itself to continuous: drouths, and to sudden transitions from 
growing to dormant periods and back again. 
SoiL mMoristurE.—The relation of rainfall to vegetation after 
its precipitation is of importance, and is largely a result of 
geological structure and relief. A single illustration will suffice. 
In traversing the, Rio Grande plain from Uvalde to Eagle pass 
