520 ABSTEACTS: GEOLOGY 



east. Areally, the igneous rocks are dominant. On the piedmont 

 slopes the hard rock formations are mantled by a sheet of gravels, 

 sand, and stream alluvium 150 feet in maximum thickness and ranging 

 in age from early Quarternary to Recent. 



The principal sedimentary rocks in descending order are about 6000 

 feet in thickness of Mesozoic, red shales, sandstones, and conglomerates, 

 which are chiefly Cretaceous, and mostly Comanche series, resting 

 uncomformably on 4000 feet of Carboniferous and Devonian limestone 

 which in turn rest unconformably on 4000 feet of Cambrian (?) conglom- 

 erate, quartzite, shale, and schist. The most important of these rocks 

 with reference to the mineral deposits are the Paleozoic limestones, 

 which locally are highly altered by contact metamorphism. 



The principal igneous rocks are (1) Tertiary effusives, consisting 

 of beds of tuffs and agglomerates, and flows of rhyolite, andesite, and 

 quartzlatite porphyry, aggregating 2500 feet in thickness; (2) Meso- 

 zoic intrusives, consisting of rhyolite porphyry, aplite, quartz monzonite, 

 and granite, and diabase, gabbro, syenite, and lamprophyric rocks; 

 (3) pre-Cambrian (?) granite (basal). The most abundant and impor- 

 tant of the igneous rocks with reference to the ore deposits are the Meso- 

 zoic acidic intrusives. 



Mineralogically the range is a part of the northwestern continuation 

 in Arizona of the celebrated mining region of Mexico. Seventy-five 

 per cent of the metalliferous deposits occur in the igneous rocks, and 

 25 per cent in the sedimentary rocks, and 54 per cent of the deposits 

 are genetically connected with later intrusives. 



The lower limit of the oxidized zone is irregular and ranges from 

 less than 100 to 300 feet in depth, but sulphides generally begin to appear 

 near the surface. The surface deposits, worked mostly in early days, 

 yielded chiefly rich silver ores; but in depth the deposits change to ores 

 of copper, lead, and zinc, and the metals now produced are gold, silver, 

 copper, lead, zinc, iron, tungsten, and molybdenum. 



The deposits occur chiefly as fissure veins, but also as contact meta- 

 morphic, replacement, and shear zone deposits. These were formed 

 chiefly by ascending thermal solutions that circulated as a close after- 

 effect of the intrusion of the igneous rocks in whichthey occur or with 

 which they are genetically connected. 



The deposits occur in two large and contrasting groups that differ 

 considerably in age and represent two distinct periods of mineraliza- 

 tion. The older and more important group, which besides veins in- 

 cludes important contact metamorphic and later replacement deposits, 



