234 JOURNAL OF THE ARNOLD ARBORETUM [vol. i 



fourth to a third of the total for the year may fall in less than twenty-four 

 hours. This circumstance, together with tlic wide diurnal and seasonal 

 variations in temperature, has had a nuirkerl effect ui)on both tlie topogra- 

 phy and the vegetation. Under tliese conditions erosion is, of course, going 

 on quite rapidly: and a very thin mantle of resi(hial soil covers most of the 

 table-land, while upon the slopes and narrower ridges the naked rock is 

 everywhere exposed. 



In their lower courses some of the larger streams traverse broad valleys 

 l)ounded bv hnes of hills and chfTs. The vallevs, whicli are sometimes 

 several miles in width, are locally called canyons and the highlands sepa- 

 rating them are known as divides. Through these wide canyons the streams 

 meander and below the level of the old flood planes they have in many cases 

 carved secondary canyons with low rocky terraces or bluffs which in i:)laces 

 serve to confine their waters even in times of greatest flood. Some of the 

 old canyons are quite out of proportion to the present volume of the streams 

 and aj)pear to have been tlie work of an earlier period when they carried a 

 much larger volume of water. The streams confined to their recent chan- 

 nels rarely ai)proach the outer barriers of the old valley, so that high i)er- 

 pendicular bluffs contiguous to such barriers are unusual along their lower 

 courses. 



The settlements and roads follow the canyons and in traveling up them a 

 broken sky-line of peaks, domes and flat-topped hills is in siglit on either 

 hand. Gradually these converge as you ascend until l>y th(M'r near api)r(>ach 

 they form a narrow gorge or true canyon, dividing frequently into many 

 branches. Scaling the walls of these canyons you emerge upon the rocky 

 table-land of the divide, dissected into innumerable sharp ridges, knolls and 

 mesa-like hills, their summits mostly conforming to a general level deter- 

 mined by an unusually hard, thick stratum or cap-rock. Where the cap- 

 rock has been eroded away locally and softer strata exposed gentle slopes 

 are formed. On such hills is often found a stunted growth of Texas Red 

 Oak and a Junij)er with large copper-colored fruit (J2i}iiprr)is Plnchotii), 



Viewed as a whole the Edwards Plateau niay be described as a sparsely 

 timl)ered region. Although, excei)t in the protection of the u])])er canyons 

 and along the margins of the larger streams, very little of the growlh would 

 be designated as forest according to usual standards, tlie num])er of woody 

 species is ciuite large and many of them become ar})oresccnt under favorable 

 conditions. The contrast of the treeless, grassy plains to the north makes 

 this diversified ligneous flora appear more remarkable. 



Up the wide canyons of the lower river courses the Chai)arral and Mes- 

 quite flora of the Rio Grande Plain has pushed, occupying most of the open 

 ground. Closely investing the streams, especially along the rocky terraces 

 and about the larger water holes, there is usually a narrow fringe of trees. 

 Pecan, Live Oak, llackl)erry {Cellis laevigata var. texana) and the Cedar 

 Elm ( Uhnus crassiJoUa) are common here. Close along the margins of the 



streams a xariety of the Black Willow (Salix nigra var. Lindheimeri) and 

 the Small-fruited Walnut {Jnglans rupcstris) are frequently found. But 



