Southeastern Washington and Adjacent Idaho. 4I 
clearly by the edge of the great lava sheet. Exceptions to this are 
seen especially along river canyons where xerophytic forest trees, 
such as pines, may follow down the canyons, and extend out upon 
the basaltic soils. Indeed, in the canyon of the Palouse River a 
belt of such woodland extends clear across the prairie community 
and drops out only at an altitude of 1,450 feet, and where it meets 
the desert scrub formation. But because of the peculiar rigors 
that forest frontiers meet, tree species invade the prairie with 
extreme slowness, notwithstanding the fact that there may be 
sufficient soil moisture in such situations for the maintenance of 
a woodland cover. 
On its lower and western border the prairie-plains formation 
gives way to the desert scrub formation. No sharp geographical 
boundary can be drawn between the two formations. Under the 
stress of low available soil water, the rainfall being here only 12 
to 14 inches, soil depth plays a determining role. The prairies, 
with the loss of their less xerophytic components, extend far 
westward and to lower elevations upon deep soil. On the other 
hand, the desert scrub formation, occupying the shallow soils of 
the scab-lands and rim-rock, may extend far into the prairies. 
Indeed, about Rock Lake and thence eastward along Pine Creek, 
the scab-land sage is represented clear to the Idaho boundary. 
Likewise, it occupies a zone in the warm Snake River Canyon, 
extending through the prairie region. 
As would perhaps be anticipated from the nature of the transi- 
tion zone occupied by the Agropyron-Festuca association, its com- 
position in the drier portion is strikingly different from that of 
the well developed upland part. The latter is characterized not 
only by a greater number of important non-grassy species and a 
denser plant cover as well as by the absence of certain character- 
istic desert plants, but also by a change in habit of a dominant 
grass, Agropyron spicatum (including var. inerme), which under 
the less xerophytic conditions abandons its bunch habit in part 
and becomes a sod former. Because of these differences the 
formation may be separated into a more xerophytic part, the 
Agropyron consociation, and a less xerophytic Festuca con- 
sociation. 
4I 
