122. BOTANICAL GAZETTE [ AUGUST 
THE FLOOD DEBRIS PLAIN.—The flood débris from the Cre- 
taceous formations of the Edwards plateau covers approximately 
the upper half of the Rio Grande plain, the altitude being from 
500 to goo feet. The level stretches of this part of the plain are 
covered with finer silt débris, the mesquite-chaparral plains. 
There are arid clay hills in the Eagle pass region, and coarse 
gravel and stony slopes nearer the escarpment, besides basalt 
cones or ridges and the outlying block of rough hills (Anacacho 
mountains ). 
Floristically the grasses are chiefly of genera making up the 
buffalo grass range of the plains northward. On the rougher areas, 
especially westward, the extreme xerophytic conditions give the 
aspect of the arid plains. The associated species are chiefly lignes- 
cent perennials, or perennials with thick fleshy or tuberous roots, 
such as Jatropha spathulata sessiliflora on stony or gravelly soil, /. 
macrorliza on loose silty soil, and numerous other Euphorbiaceae 
and Nyctaginaceae peculiar to warmer lower Sonoran areas. 
The grass formation on these areas has been very much 
reduced by over-pasturage, so that during drouth periods vast 
tracts lie quite bare of grass vegetation. In this condition pas- 
tures not wholly beset with chaparral have the appearance of 
fields lying beaten and fallow. So great has been the depletion 
of grasses that during certain dry years it was stated that from 
fifteen to twenty-five acres of land were required to pasture a 
single cow. What permanent effects on the grass formation will. 
result upon areas so. denuded it is not yet possible to say, except 
that the chaparral will cover the entire plain. The grasses have 
great recuperative power, and it is said that after periods of 
abundant rainfall the earth is covered again with a close grass 
formation. No doubt a period of rest from excessive pasturage 
would enable them to recapture fully much lost ground. Of 
course, with the presence of the chaparral and the new relations 
it involves, the original grass formation of open sunny plains 
will suffer some material changes. This subject offers a field for 
ure investigations. 
ITH, JARED G.; Grazing problems of the southwest. Bulletin 16, Division of 
5 eee U. S. Department of Agriculture. 1899 
