46 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
bounded by the high and rugged range of the Catalina Mountains and 
their continuation southward known as the Rincons, on the east and 
south by the Whetstones and the Santa Ritas, on the southwest by the 
Sierritas, and west by the Tucson Mountains, of which Tumamoc Hill 
is an outlying spur. The Tortolita Mountains, a detached group of 
mountains lying west of the Santa Catalina Mountains, may be regarded 
as forming a part of the northern and western boundary of the valley. 
The region so apparently inclosed by mountains has an approximate 
breadth of 18 to 30 miles; a length of 40 miles; in the aggregate an area of 
Over I,000 square miles. The elevation is generally from 2,400 feet above 
tide at Tucson to 3,500 feet at the upper margin of the detrital slopes 
bordering the higher ridges. 
From this upper margin, the ground descends toward the Santa 
Cruz River, and in the middle and lower portions constitutes what is 
known as the ‘‘mesa,” apparently a great plain but in reality a continuous 
slope, modified locally by the erosion due to the rains and rivers. In 
traveling over the lower portion of the extended mesa, the horizon line 
appears perfectly level, like the horizon line at sea. . 
There is a great variety in the age and the composition of the rocky 
ridges, which, rising to a height of 5,000 feet or more above the general 
level of the mesa region, give a wide range of climatic conditions and of 
vegetation. In the season of winter rains in the valley, the summits 
receive a coating of snow which by gradual melting maintains a supply 
for springs and rivulets until the season of summer rains. 
The principal streams of the region, in addition to the Santa Cruz, 
are the Rillito, at the foot of the Catalinas, north of Tucson; the Pantano 
Wash, rising in the Whetstone Mountains east of the Santa Ritas, 
receiving several accessions, including Rincon Creek and an underground 
flow of Davidson’s Canyon. 
The principal canyons of the south slope of the Catalinas are the Pima, 
Ventana, Sabino, Bear Canyon, Soldier’s Canyon, and Agua Caliente. 
The Sabino with Bear Canyon drains an extended portion of the pine- 
forest region of the summit and has running water throughout the year, 
which it is proposed to utilize for power and water for the city of Tucson. 
The Ventana Canyon is so named from the peculiar window-like 
opening through the rocks at the crest of the mountain. The opening 
is visible from Tucson. 
THE SANTA CATALINA RANGE. 
The Santa Catalina Mountain range is one of the most prominent 
and picturesque of the central mountain system of Arizona. It presents 
a bold rocky front towards Tucson, and rises to an altitude of 9,125 feet 
in Mount Lemon and to 9,225 feet in Mount Rice. Its general trend is 
northwest and southeast. Considered together with the Graham Moun- 
