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1906] SHANTZ—VEGETATION OF THE MESA 33 
secundiflorus society; the crests of the south Mesa by the P. angusti- 
jolius society; and the west crests in either situation or on the west 
side by the Arenaria Fendleri society. The alternation between 
the first two societies is very marked and may be explained partly 
by the facts of development. P. secundiflorus occurs on soil that 
is less disintegrated, on coarse gravel, or coarse gravelly loam. P. 
angustijolius occurs in older gravelly loam, where the water content 
is from 2-4 per cent higher; it seems to be more at home on the 
plains, while P. secundiflorus thrives best in the foothill region. These 
societies also alternate with the Arenaria Fendleri society. 
There are in this aspect no well-developed societies on the hill- 
sides. The flora of the slopes however, is rather rich and varied. 
Practically all of the societies are represented here and one may 
also find many of the secondary species. Crest forms are especially 
abundant on the south and east slopes. 
The alternation between the Astragalus bisulcatus society and 
the A. Drummondii society is partly due to the difference in soil. 
A. bisulcatus occurs in clay and is best developed on south and west 
exposures, usually at the base of the hills. North and west exposures 
are the most suitable for A. Drummondii, which likewise occurs 
at the base of the hills. 
On the lower land surrounding the Mesa the flora is largely 
of the Astragalus bisulcatus society and of the Sophora sericea society, 
together with the following communities: Lappula occidentalis, 
Quincula lobata, Opuntia polyacantha, Malvastrum coccineum, and 
Salvia lanceolata. This vegetation is in the Ft. Pierre clay, a heavy 
soil with a water content of 13-17 per cent., 8-10 per cent. of which 
is not available. 
As seen by this arrangement, there is zonation exhibited by 
these societies. This, however, is not very well marked, and the 
alternation within the zones is much more distinct than the zones 
themselves. 
The reason for this alternation within the formation is to be found 
in the physical nature of the soil. The soil of the Mesa is a gravel, 
mixed with a limited amount of humus and silt, or even clay, and 
is derived entirely from partly decomposed granite and plant remains. 
The soil is pervious and there is consequently very little run-off, 
