LOCAL DISTRIBUTION OF SPECIES. 39 
(3) Passing to the Gila fine sandy loam, which comes next to the river 
sands, it is seen at once that its dense growth of well-developed plants has 
not suffered, for a long time at any rate, from the effects of periodical 
floods. The willows and cottonwoods are good-sized trees, 20 to 30 feet 
in height, the arrow-weed forms dense thickets, and Sueda moquini 
occurs, in places growing to such a size as to form woody stems some 2 
inches in diameter. As here observed, the vegetation of the Gila fine 
sandy loam may be characterized, first, by the presence of most of the 
species of the Pecos sand in a much more advanced stage of development; 
second, by the presence of other plants adapted to its peculiarities of 
texture and drainage, but incapable of successful resistance to floods; 
and third, by the absence of various species that affect the heavier soils 
beyond. 
(4) The Maricopa silt loam and Maricopa sandy loam, which grade 
into each other, constitute the distinctive habitat of a dense mesquite 
forest, which upwards of 30 years ago covered a large part of the valley 
and was grubbed out by the Mormon settlers. All the best of this land 
is now divided into productive farms, but enough of the native vegeta- 
tion remains to make it plain that in earlier years there were, as now, 
two well-marked associations of plants on this area, which practically 
corresponds with the flood-plain of the river. These are essentially 
identical with the two associations of the flood-plain of the Santa Cruz 
River, already described, namely, the mesquite forest association, con- 
sisting of a thick growth of mesquite, catclaw, and a few shrubs, with a 
lower growth of Atriplex canescens and a fairly numerous complement 
of herbaceous perennials and annuals; and the association of the salt- 
bushes, here exceedingly well represented and covering large areas, miles 
in extent, on which at the time of observation Atriplex nuttallu and 
Sueda moquint were the almost exclusive occupants of the soil. Even 
these species fail on some of the worst spots marked on the alkali map 
of Lapham and Neill (J. c., p. 24). On one adjacent to the Fort Thomas 
Canal there was a good deal of bare ground and much of the Sweda was 
dying. 
No more perfect correspondence of soil and association could well be 
found than exists here, the mesquite forest occupying the flood-plain, 
with its fine silt or sandy loam and a sufficient supply of water, and the 
Atriplex and Sueda forming the association of the salt-spots where the 
percentage of alkali is too high for most other plants. 
(5) The Maricopa sand, which succeeds the preceding soils as we ap- 
proach the lower slopes of the mountains, is characteristic in texture 
and topographical position, and also presents a characteristic vegeta- 
tion. Here the creosote-bush begins, Yucca elata and certain cacti make 
their appearance, and Afriplex polycarpa attains its best development. 
