GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 49 
Devonian horizon above the basal quartzites, which are probably Cam- 
brian in age. 
The plutonic rocks are represented by intrusive dikes traversing the 
older crystallines. There is good reason to believe upon stratigraphical 
and lithological evidence that the Carboniferous and the Jura-Trias 
series and later Secondary rocks are developed in the region between Vail’s 
Station and the summit of the railway pass leading over to the valley of 
the San Pedro. 
THE VALLEY OF THE SAN PEDRO RIVER. 
The geographic and geologic relations of the valley of the San Pedro 
to the Tucson region are such as to require more than a passing allusion. 
This valley east of the Santa Catalina Mountains was once the bed of a 
lake-like or estuarine sheet of water, described in 1902 and named Lake 
Quiburis.* 
The San Pedro River, anciently the Quiburis, though in times of 
drought a small and insignificant stream, drains a considerable area, and 
is bordered throughout its course by mountain ranges forming a valley 
from 10 to 20 miles in width and nearly 150 miles in length. The chief 
ranges on the right bank, or eastern side, are the Mule Mountains (Tomb- 
stone), the Dragoons, and the Galiuros; and on the west, the Huachucas, 
the Whetstones, Rincons, and Santa Catalinas. The valley is in general 
parallel with that of the Santa Cruz, the next great valley to the westward, 
and with the Sulphur Springs Valley eastward. This river has cut its 
way through extensive horizontal beds of unconsolidated light red clays 
and sediments of great thickness, often terraced by the river erosion and 
extending high up on the sides of the bordering mountains. One of the 
best cross-sections is found on the line of the Southern Pacific Railway, 
which crosses the valley nearly at right angles to its course at Benson. 
Benson, in the bottom of the valley, has an altitude of 3,576 feet above 
the sea. The river is about 50 feet lower. The lacustrine clays rise 
from this point on each side to the height of about 3,000 feet. The 
exact limit of clay deposition is not easily determined. It appears most 
probable that the height of the water was about 4,000 feet above the tide. 
Wells bored in the valley pass through similar sediments for 500 feet with- 
out reaching bed-rock. 


*Lake Quiburis, an Ancient Pliocene Lake in Arizona. University of Arizona 
Monthly, vol. 1v, No. 4, February, 1902. A paper read by invitation at the meet- 
ing of the Cordilleran Section of the Geological Society of America at Berkeley, 
California, January, 1902. 
‘‘Rio Quiburis, now more generally known as the San Pedro, was explored in 
1697 by a party of 20 men and a sergeant, commanded by Cristobal Martin Bernal, 
who was joined at Quiburis Rancheria by another party under Kino. The united 
force, with 30 Indians, marched down the river to the Gila, thence to Casa Grande 
and returned up the Santa Cruz.’’ (Bancroft’s Works, vol. xvil, pp. 355-359). 
