GEOLOGICAL SKETCH OF THE REGION OF TUCSON. ee: 
ates, and porphyries. These formations of volcanic origin appear to con- 
stitute the bulk of the main range south of Greaterville and to extend to 
the very summit of the highest peak. 
The detrital bordering slopes so characteristic of the mountain ranges 
of the Southwest are well developed on both sides of the Santa Rita, not 
alone toward the Santa Cruz on the west, but on the eastern side north and 
south of Greaterville, where there are extensive detrital deposits of great 
thickness and regularity of slope. This formerly continuous slope is now 
cut through by several channels due to modern drainage, and these 
trenching channels, or washes, of reassorted gravels and coarse detritus 
of the ancient slopes are found to be gold-bearing and have been largely 
worked. 
On the western side of the Santa Ritas, south of Box Canyon, there 
are several deeply cut valleys. A wide and deep valley between Old 
Baldy and Mount Hopkins is drained by Madera Creek, which rises in 
Santa Cruz County. 
The celebrated large ring meteorite known as the Irwin-Ainsa meteor- 
ite, now in the National Museum at Washington, was found at the mouth 
of this canyon. 
Still farther south Montosa Creek, nearly opposite Tubac, cuts through 
the most western of the rock exposures at the head of the slope and 
reveals strata of compact blue limestone trending northwest and south- 
east and dipping westerly about 45°. It contains obscure brachiopod 
fossils. It is underlaid conformably by a thick series of red shales which 
in turn rest upon a pegmatoid granular granite with very little or no mica. 
A diagrammatic sketch section across the cropping shows the relations of 
the rocks. 
There are intrusions of porphyry in dikes which have changed portions 
of the blue limestone to white and the red shales in great part to green 
epidote associated with beds and seams of speculariron. These shales 
are estimated to have a thickness of over 500 feet. They become more 
sandy and granular, like sandstones, near the granite. They may be 
known by the name of the locality, as the Montosa shales. The pegma- 
titic granite, against which these formations lie, contains in places pyritous 
impregnations without distinct veins. These sulphides on decomposing 
give rusty outcrops and gold is liberated in fine grains. 
Montosa Creek brings down from the high ridges a great variety of 
crystalline rocks, in boulders, such as granite, syenite, gabbro, and feld- 
spar-porphyry. At the south end of the higher parts of the range there 
is an extensive development of intrusive diorite with many mineral veins 
and mines, known since the occupation of the country as the Saléro. 
This diorite is partly overlaid on the east and south by the extensively 
developed stratified tufas of the Santa Rita series. They consist of gran- 
ular mixtures, agglomerations, and breccias of rhyolitic volcanic porphy- 
ries, pumice, and volcanic ash. 
