102 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



FLOW BRECCIATION NEAR CONTACT. 



Near the contact in many places the rhyolite is peculiarly brecciated, showing 

 great blocks jumbled together, with, however, rhyolitic matrix between. The dim 

 outline of these blocks and the rhyolitic matrix show that .they were formed when 

 the lava was in the process of cooling and only partly rigid. This brecciation 

 decreases away from the contact, but in places occupies a zone upwards of 100 

 feet wide. The breccia indicates that the plug was forced upward while cooling. 



FISSURE VEINS IN THE RHYOLITE PLUG. 



Many sharp fractures cut the rhyolite, increasing in number as the contact 

 is reached. These are chiefly parallel to the contact. They have been filled with 



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FIG. 14. Cross section of outcropping fissure vein In Ararat rhyolite neek near margin. Heptile claim, north of the 

 Boston Tonopah shaft. 1, Dark-brown calcite and siderite, mixed; 2, white calcite, beautifully banded; 3, quartz 

 mixed with calcite; 4, white rhyolite (wall rook). 



material as described below, and constitute veins that are locally as much as 20 feet 

 thick, but are exceedingly irregular and nonpersistent. These veins conspicuously 

 follow the contact and are coterminous with it; they do not extend into the older 

 intruded rocks, but often run back into the rhyolite. A prominent line of veins, 

 as shown in PI. XIV, extends due north across the top of the hill, from the vicinity 

 of the Wingfield tunnel. These are fine examples of veins which have filled open 

 fissures. 



