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1923] NATURAL SCIENCES OF PHILADELPHIA 49 
Oasis, where the National Government, a Presbyterian mission, 
and the traders have a beautiful village. All west of the Tucson 
Range to the western limit of Pima county belongs to the Papago 
Indians, a tribe rich in horses and cattle, and in good health. It is 
the desert proper, but well covered with mesquite, paloverde and 
the giant cactus. Columns of the latter run up fifty to sixty feet 
occasionally, and the ribs are largely used by Indians and Mexicans 
in fencing and house building. The forests of giant cacti are 
sometimes thirty miles wide. The desert trees made good fire- 
wood and fencing but do not grow larger in this region than apple 
trees of the east. Little real timber is seen west of Kitt’s Peak. 
Low parts of the desert are selected for farming, and the Indians 
have many acres of corn and wheat under cultivation. If the rains 
come they get a fair crop. There were not as many species of 
native plants westward of Tucson as eastward, but there were 
changes and new societies, hour after hour. The organ cactus 
(Cereus thurberi), here near its northern limit, we saw for the first 
time, also the crucifix tree and another species of the paloverde 
group. The organ cactus grows only fifteen or twenty feet high 
and branches freely, or rather, the stems spread at the surface of 
the soil. It is only about eight inches in diameter, but has the 
same rib construction as the giant cactus. 
The hills and low mountains of the region west of the Tucson 
range, as well as a large part of the Tucson range itself, are capped 
with eruptive rock, ‘‘malpais”’ and basalt. This gray volcanic 
rock is oxidized by the weather, polished by the sand blast, thus 
the hills take on a dark bronze color. Among the broken fragments 
or blocks of this flow a prehistoric people left trenches and for- 
tresses, plainly constructed for defensive purposes. Thus trench 
warfare is not as new as some may believe, for the Indiars now 
living here do not know who built these fortifications. The graves 
of this people were also on the hillsides, built above-ground of lava 
blocks and timbered, and in this climate much of the wood is still 
sound. On top, the level spaces had squares and circles cleared in 
the lava for dwellings. In picture-writings their stories are re- 
corded upon the larger voleanic blocks and cliffs. 
Sonorellas in every hill was the rule until near the Big Ajo range. 
Then a light-colored powder appeared on the lava, and a week passed 
without finding a shell, except a few of the small fry found in drift. 
One hundred and one miles west of Tucson the fine desert Son- 
