146 GEOLOGY- OF TONOPAH MINING DISTRICT, NEVADA. 



The deformation displayed in this section of the vein is analogous to the 

 monoclinal folding of strata, in which the fold passes into a fault if the deforma- 

 tion be carried farther than the stretching strength of the rocks. Since all the 

 veins in the Tonopah district have normally decided dips, ranging from vertical to 

 about 30, it may be believed that the flatter-dipping portions of the Fraction 

 vein as seen in the cross section have been deformed, and that the steeper portions 

 represent more nearly the original attitude. It appears then that the vein has 

 been deformed by movements acting in a nearly horizontal plane. These 

 movements have shoved the vein and the inclosing country rock to the north on 

 the upper side, and being distributed have caused rolls or folds which in places 

 break and form faults. 



These west-northwest tangential faults are, however, not persistent^ parallel 

 to the veins, but may trend across them at a slight angle. The result is seen in 

 the western part of the workings on the 237- and 300-foot levels, as shown in 

 the figures. On the 300-foot level the west-northwest fault cuts out the vein 

 gradually. The vein runs parallel with the fault for some distance, appearing 

 and reappearing as lenses of quartz along the fault zone, until it entirely 

 disappears. 



CAUSE OP FAULTING. 



To explain this singularly intense, complicated, and peculiar faulting there 

 must be found a cause competent to thrust the blocks on the northwest side 

 of the northeast faults to the north in a nearly horizontal direction, and. 

 to shove the upper layers of rock and vein past the lower layers in a nearly 

 horizontal direction also. The volcanic neck of Brougher Mountain, whose 

 edge is only about 1,400 feet southwest of the Fraction No. 1 workings, has been 

 thrust up after the other rocks were erupted and the mineral-bearing veins 

 formed. Its smallest diameter in a north-south direction is about 1,200 feet, and 

 the examination of its contact zone shows that it probably extends downward in 

 much the same form as it appears at the surface, as a solid column of lava. 

 The intrusion of this column was probably competent to produce this complicated 

 faulting, and to exert the violent horizontal pressure indicated in the Fraction 

 workings. It has been determined independently that the faults of the region, 

 as a whole, came into existence at about the period of the intrusion of the 

 dacitic necks, of which Brougher Mountain is one. The conclusion arrived at 

 therefore falls in line with the general facts. 



COMPOSITION OF VEIN. 



A small quantity of rich ore was taken from the upper levels. This ore 

 showed ruby silver and argentite and in one case native silver, all in leaves or 

 films on cracks or crevices, evidently secondary. The rich quartz itself, as in 



