LOCAL DISTRIBUTION OF SPECIES. 33 
(b) Detached lower talus groups, where coarse angular fragments are 
accumulated by the washing out of the fine material, are found near the 
foot of the slopes, dotting areas thickly strewn with basalt fragments 
imbedded in dark, fine, retentive soil. They usually lie in the bottom 
of depressions or along the lower course of arroyos. ‘This material has 
been brought down from above in times past. Some of these areas are 
good reservoirs for the storage of flood-waters. 
(6) Gully groups border the arroyos or gullies, and a collection of such 
groups forms a belt of unusual density following a dry watercourse, or 
a system of washes. Even where such a course is merely a band of sand 
it often has the power to attract to its sides the only plants of the neigh- 
borhood. Such arroyos often have the steep rocky banks which this plant 
affects, and it is quite probable that the roots also profit by the periodic 
flow. 
(7) Deltoid groups are probably the most significant of all. The smaller 
gullies on the slopes are not always continuous, but sometimes lose them- 
selves on a pile of rock débris deposited by floods. On the south slope, 
where they occur, their sides give a warm, southerly declivity, and the 
bowlder-filled soil is thoroughly saturated by every generous flood that 
comes along, while it receives the full value of even a tiny rill. All these 
habitats point to a high degree of warmth as essential, coupled with the 
best moisture conditions obtainable. 
The sahuaro is absent from the middle portions of talus slopes, and, 
speaking generally of mature plants, from cliffs, evidently because of 
lack of soil. It is comparatively scarce on northerly slopes, and present 
in intermediate numbers on east and west slopes. It is absent, either 
absolutely or comparatively speaking, from level land and from slopes 
cemented by caliche. 
The outline of the sahuaro groups is often determined by the physi- 
ography. ‘The belts of unusual density following arroyos and gullies are 
very evident, almost general, and follow the direction of these. Certain 
well-marked cases occur in depressions, where no water-channel is in 
evidence, but where the grouping takes the form of a belt running down 
hill, clearly marking seepage channels, either present or past, according 
to the general age and grouping of the plants. On the other hand, the 
long axis of certain other groups lies along the contour. ‘These will be 
found to grow along the sides of more or less horizontal terraces. A 
succession of such terrace groups, but, more generally, irregular rock- 
outcrop groups, may be seen to make a horizontal belt of unusual density, 
running about certain hillsides, notably Sentinel Hill on the south, clearly 
traceable to geologic strata of great comparative resistance to erosion. 
Protruding tuff-beds were found to be as prolific as outcrops of the different 
kinds of basalt. 
