94  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
The  economic  interest  in  this  faulting  lies  largely  in  the  circum- 
stance that  the  veins  in  the  early  andesite  have  been  cut  and  displaced 
thereby. 
Recent  erosion.— To  complete  the  geological  history,  we  have  to  con- 
ceive of  the  considerable  period  of  erosion  which  stripped  from  the 
surface  a  great  thickness  of  volcanic  material,  leaving  the  resistant 
volcanic  necks  standing  out  as  hills,  and  laying  bare  the  present  sur- 
face. Certain  blocks  of  limited  extent  had  been  raised  by  the  fault- 
ing above  the  level  of  the  rest,  and  here  the  surface  of  erosion  reached 
the  early  andesite  and  uncovered  the  rich  veins  contained  in  it;  and 
it  is  the  discovery  of  these  veins  which  has  made  the  subject  of  the 
geology  of  the  still  covered  region  of  such  lively  interest  to  miners. 
Sequence  of  formations  and  events. — The  following,  then,  is  the 
sequence  of  events  as  deciphered  for  the  vicinity  of  Tonopah : 
Sequence  of  formations  and  events  in  the  vicinity  of  Tonoxjah. 
Early  andesite. 
Fracturing. 
Vein  formation.     (Primary  minerals,  quartz,  valencianite,  stephanite,  pyrite, 
chalcopyrite?)     Values  good;  gold  and  silver,  silver  predominant. 
Erosion. 
Later  andesite. 
Probable  erosion. 
Dacite. 
Dacite  breccia. 
Rhyolite  breccias,  flows,  and  dikes,  intermingled  with  slightly  stratified  or  inter- 
stratified  pumiceous  or  tuffaceous  fragmental  material. 
Vein  formation.     (Primary  minerals,  quartz,  pyrite.)     Values  relatively  low; 
gold  and  silver,  gold  apt  to  predominate. 
Erosion. 
Tuffs,  with  an  occasional  thin  rhyolite  flow. 
Elevation  of  tuffs. 
Tilting. 
Basalt. 
Chief  faulting.     (Affects  everything  preceding.) 
Rhyolite  (white)  intrusion  (probably  Ararat,  Oddie,  Rushton  hills). 
Vein   formation.     Primary  minerals,   quartz,    chalcedony,   calcite,    siderite, 
pyrite.)     Values  low;  gold  and  silver,  gold  apt  to  predominate. 
Erosion. 
Dacite  intrusion  (Butler,  Brougher,  Golden,  Siebert  mountains). 
Mineralization    (chalcedony,   manganese).     Values    slight    to    insignificant. 
Mud  veins. 
Erosion. 
Glassy  rhyolite  flow  (slopes  of  Oddie  and  Brougher). 
DESCRIPTION  OF  THE  ROCKS   OF  THE  REGION. 
Distribidio7i  and  characteristics  of  the  early  andesite.— This  was 
originally  a  hornblende-andesite,  containing  probably  some  biotite 
and  pyroxene.  In  the  immediate  vicinity  of  Tonopah  it  outcrops 
only,  so  far  as  observed,  on  Mizpah  Hill  and  Gold  Hill,  but  it  has  been 
