GEOLOGICAL SKETCH OF THE REGION OF TUCSON. 53 
We can not doubt that the enormous deposits around the Santa Rita 
are of remote antiquity, probably pre-Tertiary, antedating the continen- 
tal uplift at the close of the Miocene. They may be known as the Santa 
Rita tufas. 
THE SIERRITAS AND THE TUCSON MOUNTAINS. 
The mountains forming the southwestern limit of the valley and on 
the right bank of the Santa Cruz River are largely granitic, with strata 
of subcarboniferous limestone and shales partly metamorphosed and 
copper-bearing. The Twin Buttes copper mine is found here, and other 
mines at Mineral Hill, and at the Azurite Camp, all in association with 
porphyritic dikes and garnetiferous veins, the result of alteration of the 
limestone. 
The Sierritas give place farther north to the Tucson Mountains, com- 
posed in great part, opposite Tucson, of volcanic tufas and agglomerates, 
forming hills of very picturesque and uneven outline, as may be seen by 
reference to plate 51, made from a photograph taken a mile or two south 
and west of the Desert Laboratory. These tufas are regularly stratified 
and are upraised. They are probably pre-Tertiary or Cretaceous in age, 
the equivalents in this respect of the stratified tufas of the Santa Rita 
Mountains on the opposite side of the valley. Similar tufas are found as 
far south as the Cerro Colorado and beyond toward Nogales and Sonora, 
Mexico. 
The prolongation northward of similar rhyolitic tufas is found beyond 
Tucson, especially in the foothills bordering the Santa Cruz, where there 
are extensive outcrops of stratified tuffs, rhyolites, and andesitic rocks 
which are uplifted at various angles. 
At the Pictured Rocks the stratification is very distinct, with ripple- 
marks upon the surfaces of the slabs of rock, which have the composition 
of porphyritic andesite. These rocks are near the Old Yuma Mine, 
opened upon a quartz-vein bearing lead-ore and gold. 
The western portion of the Tucson Mountains is made up in part of 
ancient sediments of Paleozoic age—limestones, sandstones, and shales. 
Blue limestone, probably lower Carboniferous, much traversed by flint, 
crops out in Snyder’s Mountain, and is quarried and burned for lime. 
There are plutonic rocks in great variety in the form of dikes. 
A fine-grained gray andesite in the hills west of Tucson affords an 
excellent building stone, used especially for rubble walls. 
The extensive rhyolitic intrusions north and south and on both sides 
of the Santa Cruz Valley command attention as marking a long line of 
disruption and faulting of the crust, accompanied, no doubt, by crustal 
movements at different periods, parallel to the axes of the uplift of the 
Paleozoic strata. The more distinctly characterized and more fusible 
volcanic rocks, less viscid than the older rhyolites were, are of a later age 
