136 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 19D2. [bull. 213. 
At the Coronado mine and other places in that vicinity small dikes 
of diabase occur. 
Southwest of Morenci a sedimentary series has been found which 
appears to unconformably cover the Paleozoic rocks. At one place 
fossils, indicating a Cretaceous age, were obtained. These rocks, 
however, are only of secondary importance as far as the ore deposits 
are concerned. 
GEOLOGICAL STRUCTURE. 
The geological structure of the pre-Tertiary rocks is rendered very 
complicated by extensive faulting. In few places does this faulting 
affect the covering basalt and rhyolite, from which it is to be con- 
cluded that the main epoch of disturbance antedates the volcanic 
eruptions of the Tertiary period. The Paleozoic era in this region was 
evidently one of quiescence and deposition, and it is believed that 
undisturbed deposition continued through the larger part of the Cre- 
taceous period. The intrusion of porphyry took place during the 
late Cretaceous or the earliest Tertiary, for we find bodies of that 
rock intruded into Cretaceous sediments as well as into older rocks. 
In many places this intrusion was accompanied by very great dis- 
turbance, causing a fracturing and shattering of the sedimentary series 
into which it was intruded. The important ore deposits were formed 
during and a short time after this intrusion of porphyry. Alteration, 
gradually changing and often enriching these ore deposits, has, how- 
ever, continued from their deposition to the present time. 
The deposition of the ores was followed by very extensive fractur- 
ing and faulting, affecting, as already mentioned, all of the rocks in 
the district except the younger lavas. 
From the form of the remaining patches of quartzite it would seem 
as if the surface of the granite and the whole overlying series had 
been buckled, perhaps elevated in dome-like shape, and then frac- 
tured extensively. The geological map will show the complicated 
nature of this faulting. The main faults extend in an east-west or 
northeast-southwest direction. Faults having a throw of over 1,000 
feet are common, and in the Paleozoic series, where conditions are 
favorable for deciphering the structure, as for instance near Morenci, J 
the complication is particularly apparent. Among more important 
faults may be mentioned that at the Coronado mine, where the south 
side is dropped 1,000 feet, and that cutting across Chase Creek east 
of Morenci, where again the Paleozoic series has been dropped 1,500 
feet or more. 
An extensive erosion, resulting in very irregular surface forms, fol- 
lowed these disturbances. Then, probably in the latter part of the Ter- 
tiary period, the whole region was flooded by rhyolites and basalts. 
Following this, probably in the early part of the Pleistocene, the level 
of the Gila River became grnatly raised by accumulations of detrital 
