LOCAL DISTRIBUTION OF SPECIES. 3D 
CERCIDIUM TORREYANUM. 
Before taking up this species, it is necessary to describe certain physio- 
graphic or soil areas which are referred to in connection with its local 
distribution. 
The west part of the Laboratory domain is divided naturally into four 
physiographic, or soil areas as follows: 
(1) The area in the northwest part of the Laboratory domain, thence 
extending some distance north and west, composed of gentle ridges or 
swells, with a definite substratum of caliche. It is the outer margin of 
the Tucson slope. Larrea is the characteristic plant and, on the drier 
portions, Redellia coopert. 
(2) The second area lies immediately south of the first, and extends 
from the main wash at the west base of Tumamoc Hill westward beyond 
the Laboratory domain to the foothills of the Tucson Mountains, certain 
ones of which, composed of rhyolite, are probably its sources. Its flat 
surface, with abrupt gravel bluffs along its washes, is characterized by 
coarse, reddish soil containing a large proportion of hard angular frag- 
ments. Caliche is present, but lies deeper than in the first area. This 
second area is marked by the presence in large numbers of Franseria 
deltordea and Opuntia fulgida, as well as the absence, over considerable 
stretches, of everything else except Parkinsonia microphylla. 
(3) The third area occupies the southwest part of the Laboratory 
domain, bounding the foregoing area on the south and extending along 
the main wash southwestward and southward to the wash which enters 
the Santa Cruz just south of Sentinel Hill. Its nearly flat or gently 
undulating surface is composed for the most part of fine, silty, and some- 
what sandy soil with caliche absent near the surface. The soil has appar- 
ently been deposited within comparatively recent time by the main 
wash. Though less homogeneous in its soil and plant cover, and possibly 
in its moisture supply, than the two preceding areas, it is nevertheless 
fairly well marked by distinctive physiographic features. The most char- 
acteristic plants are Ephedra trifurca and mesquite. 
(4) The fourth area includes the practically level bottom of the main 
wash, beginning at the point where this is deflected northward by Tumamoc 
Hill, and extending to the vicinity of St. Mary’s Hospital. Caliche is 
absent, except on areas apparently long abandoned by the water on its 
way toward the river. Wash species proper, as Cercidium torreyanum, 
Prosopis velutina, and Acacia greggu, dominate here. 
Within the Laboratory domain the boundaries of all four areas are 
for the most part very definite; they are indicated on the map (plate 
16) by broken lines. Small areas marked B are basaltic outcrops with 
a very distinct soil and might be designated collectively as a fifth physio- 
graphic area. The part of the map marked West Slope Tumamoc Hill 
represents an area of several different kinds of rock and soil and does 
not belong within the areas above described. 
Cercidium torreyanum is almost entirely confined to the washes; only 
in the second physiographic area does it occur side by side in the same 
