1920] PALMER, LIGNEOUS FLORA OF THE STAKED PLAINS 91 



to abandon the country, both in area occupied and in number of forms, 

 there appears to be a steady if slow increase in progress at the present time. 

 The southern portion of the Great Plains, occupying western Texas, rises 

 gradually or by a series of step-like plateaus from the lower Rio Grande 

 valley, with a maximum elevation of only a few hundred feet above sea- 

 level, to the high plains of the Panhandle, parts of which exceed four thou- 

 sand feet in altitude. The first of these, if we exclude the Rio Grande Plain, 

 which properly belongs to the Gulf Coastal Plain, is the Edwards Plateau, 

 set off from the lowlands by the bold Balconies Escarpment; the second is 

 the Staked Plains, lying to the northwest of the rocky plateau and rising 

 gradually from it. The boundary between these two regions must, there- 

 fore, be somewhat arbitrarily drawn, but in general it may be regarded as 

 following the line of contact between the limestones of the Comanchean 

 Series and the overlying Tertiary or Quaternary deposits, principally of 

 gravels, sands and other unconsolidated materials, that constitute the 

 suHace formations of most of the Staked Plains area. This line runs in a 

 southwesterly direction from near the town of Big Spring, in Howard 

 County, to the valley of the Pecos River, in Crane County, which it follows 

 thence at an angle of about forty-five degrees northwestward into New 

 Mexico. On the eastern side the plateau rises abruptly from the lower 

 bordering plain by a high escarpment or line of clifiFs, the crown of which is 

 formed by a hard, thick stratum, usually of the recent, non-marine lime- 

 stone or "caliche.*' The more resistant nature of this upper layer is 

 responsible for the bold character of the cliffs, and it is locally known as 

 the "cap-rock" or "rim-rock," The escarpment is also well defined on the 

 north, where it forms a wall of the deep valley of the Canadian River, 

 which separates the Staked Plains from the next successive, higher stage, 

 the Panhandle High Plains. On the west it is bounded by the valley of the 

 upper Pecos River, in New Mexico. The margins of the plateau are in 

 places deeply incised by the canyons of streams which have their sources in 

 the highlands. The most important of these are on the eastern side the 

 various branches of the Colorado, Brazos and Red Rivers, and on the west 

 those of the Pecos. It is in these canyons and along the ravines and broken 

 ground bordering them that most of the ligneous plants are found, and 



confined 



the 



Viewed as a w^hole the Staked Plains is a high, mesa-like tableland, with 

 a remarkably level surface over its interior, and nearly destitute of running 



streams. 



ranging 



to twenty-five inches, and its elevation and exposed position in the vast 

 plains stretching unbrokenly from the Arctic Ocean to the Gulf of Mexico, 

 gives it a somewhat rigorous climate, marked by great extremes of temper- 

 ature, both seasonable and diurnal. Dry winds of high velocity prevail 

 during the greater part of the year, and under their influence the potential 

 evaporation is much greater than the total average rainfall. In addition 

 to the normal severity and wide variations of the climate its effect upon life 



