34 



tlie south aud west. The east slope consists of a broad border of black 

 alluvial soil along the river, then a strip of sauds 60 miles wide, aud 

 then black ''hog wallow'" prairie on the northern edge of the sands. 

 Each of these soil formations has its peculiar grasses. At San Diego, 

 which is on the western border of the arable soil, the dominant species 

 are seed mesquite {Boutcloua texana) (fig. 4) and two gramas, B. trijida 

 and B. hromoides. The occurrence of the gramas as the dominant spe- 

 cies seems always to mark the transition from arable to pasture lands. 

 Proceeding toward Laredo on the line of the Mexican National Eailroad, 



the soil changes abruptly from 

 gray-black to brick-red, and soon 

 becomes almost pure sand. On 

 these sands, as on the sands of 

 Cameron County, the vegetation 

 is largel}" bunch grass. There 

 is still an open strip about 35 

 miles wide near Hebronville, but 

 it is only a question of a few years 

 before the brush and cactus will 

 have advanced from both direc- 

 tions to take complete possession 

 of it. 



The valley of the Rio Grande 

 is arid, receiving only a small 

 amount of rainfall, poorly dis- 

 tributed, throughout the year. 

 The soil is shallow, and sterile 

 because of an insufficient supply 

 of moisture. The vegetation con- 

 sistsofdensechaparral, with close 

 thickets of various kinds of cac- 

 tus, and the grasses are few and 

 scattering. There is almost no 

 water back from the river, and 

 thebrackish or salty underground 

 flow lies from 200 to 300 feet be- 

 low the surface. The carrying 

 capacity of thechaparral covered 

 valley lands is never more than 

 30 Lead to the square mile. Grasses exist so sparsely that the destruc- 

 tion of brush and cactus bj^ fire is almost out of the question, and could 

 only be accomplished by sacrificing the grass accumulation of several 

 years. The only i)racticable method of improving the conditions would 

 be the introduction of forage shrubs from similar arid regions abroad, 

 the construction of reservoirs or tanks to catch and hold the storm waters 

 which descend as torrential rains, and the irrigation of the narrow val- 



FlG. 4.— Seed mesquite {Boutelaua texana). 



