264 BOTANICAL GAZETTE | OCTOBER 
cafions and the bottoms of dry ones contain those species of the 
central erosion formation whose range is to the southwest and 
west. In the main, however, the dry cafions and lower slopes 
are beset with a chaparral formation, to be_discussed separately 
XEROPHYTIC AND SEMI-MESOPHYTIC POST OAK FORMATIONS.— 
The post oak formation furnishes a striking instance of the close 
relation existing between soil structure and its vegetation cover- 
ing. Where there are exposures of coarse sand beds and 
gravelly clays, at least as far west as the 100th meridian, there 
occurs an arborescent formation whose main elements are the 
two species of oak Quercus minor (post oak) and Q, Marilandica 
(black jack). Very extensive areas of this formation occur also 
on the western margin of the east Texas forest belt. In the 
west Texas region the areas are in the main as follows: (1) 
the extension of Fayette sands into the Rio Grande plain ; (2) 
the gravelly débris and sands of the granitic area and its borders 
(extending for example westward beyond Fredericksburg) ; (3) 
the sands and gravels of the Carboniferous area (Brownwood 
and Palo Pinto countries); (4) the upper cross timbers (the 
lower cross timbers to the east of our area are of the same 
formation); (5) river terrace gravel beds, such as those about 
Austin. 
The character of the formation varies with the underground 
water conditions. Where the porous textured sand or gravel 
beds lie so that the soil water level is easily available, the 
formation is a compact forest with a mixture of semi-mesophytic 
trees filled in by a lower zone of shrubby species. Such is the 
character of the formation in the Fredericksburg district and in 
the upper cross timbers. More frequently, however, the soil 
water conditions are not so favorable, and as a consequence the 
formation more nearly reflects the normal climatic conditions, 
as in the granite country. Then it is no longer a compact 
forest, but an open timbered grass plain where the trees are of 
low stature and broad spreading crown, each with the individu- 
ality of orchard trees. As a consequence of this open forma- 
tion, such a forest displays no cooperative capacity in producing 
. 
