Arizona republican., 1916-12-15
GEOLOGICAL RECONNAISSANCE
 0F THE JEROME DISTRICT

quartz, porphyries, rhyolite porphyries and trachytes, found their way. Sketch No. fD These also followed general northwest and southeast direction.
 The folding of the original lava beds is now only evidenced by the miscro- scope. but it Is this original folding that no doubt determined the trend of the later shearing. intrusions and schistosity of both the schists and diOrtes
 IL was probably at the time of the intrusion of the diorite that mineraliza- ton took place. The siiiceous dikes are younger ard frequently cut the ore bodies.
 The schist reached the next accur- ately determinable geological period of time in this folded altered condition with diorite and more siliceous (acid) dikes cutting it and with the primary suiphide ores already in place. Sketch 4) In parentheses il is well to state here that an acid dike Is merely one in which the silica contents are higher than in basic dike The period of time during which all this was taking place has been called the pre- -Cambrian period by the government geologists and they have determined the Yavapai schists themselves to belong to a DOT tion of this pre- Cambrian time known as the AigOnkian period.
 We arrive at the Cambrian age then with the schist in the condition just described and further relieved by ero- sion or otherwise of whatever sedi- ments may have covered it. At this time it's surface had not only been re- lieved of such sediments, but it had been worn very nearly flat, and at the beginning of the Cambrian age was un- der water Such was the condition at the beginning of the Cambrian period
 Sandy sediments were deposited upon the schist to a thickness varying up to a maximum of 90 feet and these now form the coarse red sandstone which lies everywhere next to the schist and diorite. This sandstone Is of Cambrian age and bears the name of Tonto Sandstone. On top of this and lying flat with it-great beds of limestone were deposited during two subsequent geological ages-the De- yonian age and the CarbouiferOus These limestones in places reach a maximum thicknes of $50 feet and in addition some 300 feet of sandstone was deposited on top of the Carboniferous limestone. This sandstone is also of the Carboniferous age. Sketch No. 5.)
 At this time. that is the end ofsthe Carboniferous period. the country south N lifted out of water. which movement bent the beds of limestone and sand stone leaving them wrtn slight dip to the north, very noticeable at First View and from Russell. For a long time-for several long ages-ttey re- mained in this position subject to all the destructive eroding action of wind. rain and frost These agents carved out valley and hills: near Jerome wearing through the upper sandstones to the limestones which were likewise sculptured into hill and dale. tSketch No. 6)
 In rather recent times. as geological count goes, that is du2ing the Tertiary period. great eruptions of lava took place. probably from the direction of what is now Flagstarr. These lava flows covered the whole country about Jerome filling the valleys and finally covering the hills just described. tSketch No. 7) The lavas came from many different eruptions and show many beds and varieties Of composition They reach in places TOO feet in thick ness
 After the lavas, that Is younger than they, great faulting took place in q northwest southeast direction Sketch No. s. This faulting was the last geological event of importance and the country thus faulted and modified by erosion presents the surface with which the modern miner Is now con- cerned. Sketch No a.,
 A more detailed study can now be considered of the rocks and their structural relations.
 Ore Deposits
 As the ore deposits belong to the pre- -Cambrian period their description is now in order.
 All the large ore bodies occur typi- caliy as enormous pipes or chimneys of lenticulsr cross section measured in hundreds of feet. Their form can best be vizualized by comparing it to q flattened cigarette standing almost vertically. The cross section of the ore body is always longer in the direction of the schstosity of the schist or dio- rite in which 1t is found and it's dip also follows this schistosity Cracks in the nature of veins often extend rom the pipes in d northwest and southeast direction.
 The primary ores were chalcopyrite, borntte and pyrite in a siliceous gangue with such q preponderance of the form CT two minerals as to make these ore bodies probably the highest grade large copper sulphlde mines of the world.
 Oxidation in the upper levels has ex- tended to depth of from 150 to 400 feet and here has produced zone of enormously enriched ore The enrch- ment is due to secondary chalcocte or copper glance. This zone of high en- richment reaches in places a thickness or two hundred feet Below if the