The Santa Rita Mountains, in southern Arizona, begin about 10 miles north of Nogales and extend northward about 40 miles, curving slightly to the east and terminating in Empire Mountain. Davidson Canyon partly separates Empire Mountain from the main range. The highest peak, Old Baldy, has an elevation of 9,432 feet, or about a mile above the surrounding country, and therefore is a conspicuous feature.
The Santa Rita Mountains consist of thick masses of plutonic and volcanic rocks, which during late Mesozoic and early Tertiary time penetrated the strata. These igneous rocks constitute the highland, and the slopes and bordering plains are mantled by detritus, largely derived from the igneous core of the range.
The pressure of the intrusive masses arched up large areas of the Cretaceous rocks, especially in the northern part of the range, and at some places the Paleozoic rocks were brought to the . . .