44 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



BROfC.HER DACITE. 



Location. This rock forms most of the important hills, the others being 

 composed mainly of rhyolite of the same age and origin as the dacite. The dacite 

 hills are Butler, Brougher, Siebert, and Golden (Pis. V and VI). 



Volcanic necks. These eminences represent the necks of former volcanoes or 



.the columns of lava which rose from the abyssal regions to the surface. The 



Brougher Mountain neck (as mapped) is roughly circular, though slightly 



elongated. Butler Mountain is ellip- 

 tical, Siebert Mountain is irregular, 

 while Golden Mountain is elongated 

 and irregular. Butler, Brougher, 

 and Golden are all elongated in an 

 east- west direction. 



Contact phenomena. The proof of 

 their origin is found, chiefly in their 

 contact phenomena. The contacts 

 are usually marked by a belt of 

 dacite, which appears in the hand 

 specimen as a black glass and which 

 is shown by the microscope to be a 

 glassy phase of the dacite. This 

 band is generally several feet thick, 

 and locally as much as a hundred 

 feet. Powerful flow lines, parallel 

 to the contact, are usually observed, 

 and not infrequently the actual con- 

 tact with the intruded rock can be 

 seen. The contacts are typically 

 vertical, but they are by no means 

 regular. They frequently dip out 

 from the mountains, and perhaps 

 more frequently into them, and are 

 often wavy (figs. 4 and 5). The earlier andesite, the Fraction dacite breccia, 

 the Tonopah rhyolite-dacite, the Siebert tuffs, and the basalt are at various 

 places intruded along the contact of these dacite necks, and thus the age of the 

 dacite is established. The intruded rocks are usually hardened and silicified 

 near the contact, and contraction cracks in them are coated with chalcedony. 



Dike* from main masse*. The contacts are irregular in detailed horizontal 

 plan, and tongues are frequently sent out into the intruded mass. Along faults 



5 10 



Scale 

 20 



40 feet 



Flo. 4. Vertical sketch section showing contact of intrusive 

 dacite with tuff, southwest base of Butler Mountain, a, Da- 

 cite; 6, dacite glass, contact face; e, tuff, broken at contact. 



