46 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



the southwest slope of Brougher Mountain, also, a coarse agglomerate was 

 observed in one place; and it is very likely that this and other of the mountains 

 of this group have had a similar history. 



Formation of the present Brougher dacite. After these explosions a column 

 of lava rose and filled the vent. It is not likely that this lava ever overflowed, 

 for no traces of flows have been found, and from such important vents the lava, 

 if poured out, would be sufficient in quantity not to have been wholly swept 

 awav bv erosion. 



Scale of feet 

 50 100 



Fio. 6. Horizontal plan showing eddying in the cooling lava of a volcanic (dacite) neck; plotting of strong flow*truc. 

 ture on top of eastern shoulder of Golden Mountain; attitude of flow planes nearly vertical, usually dipping 70 

 to 90 north, sometimes dipping south. 



/'low structure and other phenomena. The flow structure in these necks was 

 carefully observed, and the conclusion was reached that only at the contact do 

 the flow lines indicate the direction of the original flow. Away from the contact 

 the lines follow all imaginable curves (fig. 5). It is plain that after the rapid 

 cooling of the glassy lava near the contacts the liquid material standing in the 

 neck circulated and eddied extensively before cooling. There may b seen in 



