LOCAL DISTRIBUTION OF SPECIES. ot 
between the bushes, or at least not filling up. Where filling-up occurs, 
other plants come in. On the flood-plain, a filled-up area, Larrea is 
replaced by mesquite, and on the residual rocky slopes, where there is 
no abrasion going on, it is largely replaced by other plants. Compared 
with the mesquite, which is preeminently a plant of deposition areas, 
the creosote-bush is distinctly one of erosion areas. 
From extended observation, not only here but elsewhere in Arizona, 
the distribution of the creosote-bush may be said to present the following 
essential facts: 
(1) Larrea is largely a plant of transported as against residual soil; 
it is preeminently a plant of the gentle undulating slopes that lie at the 
base of the mountains throughout the southwest, giving out on the steeper 
slopes. above. 
(2) In contrast with Prosopis, a plant of the flood-plains, periodically 
subject to overflow and deposition, Larrea stands as the distinctively 
character plant of the adjoining erosion areas. Probably parallel is the 
fact that while the mesquite grows on sand, silt, and fine adobe, con- 
taining no caliche but relatively much humus, the creosote-bush grows 
in greatest numbers on dry, more or less gravelly soil, with little humus, 
and often a hardpan of caliche. 
(3) As compared with plants of the hill, Larrea is further a plant of 
deep as against shallow soil, expressing in a rough way proportional 
distance of bed-rock from the surface. It is also a plant of unstable talus 
areas, as long as there is soil present, and finally, it is a plant of the rela- 
tively highly stable mesas, which are nevertheless slowly wearing down, 
on the tops of hills and ridges. 
(4) As to certain observed relations to other plants, it frequently 
occurs conspicuously with Festuca octoflora, but loses its hold rapidly 
on areas dominated by [ranseria deltotdea, and over wide areas, where 
both may occupy ground of their choice, it is roughly inversely propor- 
tional to Encelza in its distribution. 
(5) It is not limited by altitude, except within very wide limits, nor 
is it manifestly affected by aspect. Thus the local distribution of Larrea 
presents a complex problem, upon which at present most light is gained 
by consideration of the sharp contrast it presents to various other species, 
notably Prosopis and Encelza, in its requirements. 
CEREUS GIGANTEUS. 
From the map (plate 15) it is at once evident that the sahuaro decidedly 
prefers southerly aspects; if other conditions could be eliminated it would 
increase in density directly as aspect grows more southerly. It is also 
seen, by reference to the map, that within limits the density of this species 
increases with the narrowing of intervals between the contour lines, that 
is, with increasing steepness of slope. In this, however, the substratum 
