1910] LIVINGSTON—SOIL MOISTURE 243 
may be briefly described as follows. A more complete description 
is included in the author’s article above cited. 
1. The soil of Tumamoc Hill is a heavy clay, underlaid by 
practically impervious rock at a depth of but ro-5o cm., and much 
broken into small pockets and irregular masses by outcropping 
rock or large rock fragments. Fully 50 per cent of the entire 
gross volume of the soil is made up of large and small rock masses. 
A subterranean water table does not exist here, so far as is known, 
and, from the impermeability of the underlying rock both to water 
and to plant roots, it is safe to conclude that the water supply for 
this soil is derived exclusively from precipitation. The unsifted 
soil possesses a water-holding power of 48 per cent of its dry weight. 
The usually pronounced slope of the soil surface here is partially 
offset by the numerous small catchbasins formed by rock frag- 
ments, so that rain water stands in small pools over the hill and does 
not drain away quite so rapidly as though the surface were more 
even, 
2. The soil of the Larrea slope, so named from the prevalence of 
the creosote bush thereon, occupies the gentle slope which surrounds 
the hill. This soil is underlaid at a depth of 20-30 cm. by the 
limestone hardpan (caliche) so common in this region. The surface 
soil is more loamy than that of the hill, with an admixture of 
about half its volume of fine angular gravel; that near the caliche 
contains even more gravel and rock fragments, sometimes rounded 
pebbles. The slope, the smoothness and impermeability of the 
hardpan, the presence of rock fragments, and the loamy nature of 
the soil itself, all tend to make this a very well-drained soil. It, 
like the hill soil, is not influenced by subterranean influx of water. 
Unsifted soil from near the surface possesses a water-holding power 
of only about 20 per cent. This low retaining power is not here 
due to general coarseness of the soil, but to the large proportion 
of angular gravel, which of course holds practically no water. 
3- The soils of the wash, a broad streamway from the Tucson 
mountains, which cuts through the general valley floor (Larrea 
Slope in this vicinity) near the northern base of the hill, vary from 
gravel through sand to very loamy sand. The coarser soils are 
Practically without vegetation and are of little interest in this 
