212 BOTANICAL GAZETTE [SEPTEMBER 
a mountainous country. Here are found in full operation those 
agencies which have worn so much of the surface elsewhere 
from a higher to an approximately level lower plane. The 
Cretaceous limestones, which on the grass plains portion of the 
Edwards plateau lie still unbroken, have here been cut through 
2.— Juniperus sabinioides formation; a prominent feature in the xerophytic 
Fic, 
timber region of the Edwards plateau province. 
and exposed on all sides, thus not only offering lines of penetra- 
tion for tree roots, but moisture facilities not available from an 
unbroken level surface. By this erosion, also have come about 
unequal conditions as between the summits and slopes of the 
hills, and the gorges and cafions between them. Certain Creta- 
ceous formations, friable on exposure to weathering, outcrop on 
the hill slopes or summits, thus exposing an unstable founda- 
tion for vegetation. On these outcrops, often entirely bare, 
certain very hardy species can secure a foothold, and so anchor 
