\)<2  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
Subsequent  to  the  eruption  of  this  rock  fractures  were  formed  in 
it,  and  these  became  the  channels  for  circulating  solutions,  probably 
ascending  hot  waters,  closely  connected  with  the  volcanism.  The 
waters  were  remarkable  for  intensity  of  action.  The  rocks  which 
they  traversed  have  been  chemically  transformed,  the  chief  process 
being  silicification,  so  that  they  are  now  found  more  or  less  completely 
altered  to  quartz  and  sericite  (fine  muscovite).  Along  fracture  zones 
the  rock  lias  been  replaced  almost  entirely  by  quartz  carrying  silver 
and  gold,  and  these  constitute  the  veins  by  which  Tonopah  is  known. 
Some  parts  of  the  homblende-andesite,  on  the  other  hand,  show 
chiefly  calcite  and  chlorite,  with  pyrite,  as  alteration  products;  but 
such  phases  have  not  yet  proved  to  lie  immediately  adjacent  to 
important  veins,  and,  as  a  rule,  do  not  seem  to  be  indicative  of  the 
proximity  of  the  best  class  of  ore. 
Later  andesite  eruption. — There  is  evidence  that  these  events  were 
followed  by  a  considerable  period  of  erosion,  long  enough  to  bring  the 
veins  to  the  surface.  Then  another  eruption  of  andesite  occurred. 
This  later  andesite,  which  was  abundant  in  amount,  differed  from 
the  earlier  andesite  in  containing,  as  dark  minerals,  pyroxene  and 
biotite  rather  than  hornblende,  and  it  also  differed  in  some  structural 
and  textural  points.  It  is  probable  that  this  andesite  came  up  through 
the  hornblende-andesite  in  places,  and  also  flowed  over  it,  completely 
covering  it  and  its  contained  veins. 
Rhyolite  and  dacite  eruptions. — Following  probably  another  period 
of  erosion  and  volcanic  rest,  eruption  was  vigorously  renewed  and 
there  were  outbreaks  at  many  closely  adjacent  points.  The  lavas 
were  rhyolites  and  siliceous  dacites,  closely  related  to  one  another. 
Some  vents  emitted  one,  some  the  other  rock.  This  period  of  volcan- 
ism was  long  and  a  number  of  different  lava  flows  of  dacite  and  rhyo- 
lite alternated  or  mingled.  The  eruptions  were  often  explosive,  as  is 
shown  by  the  considerable  quantity  of  pumiceous  and  fragmental 
material  in  the  volcanic  breccias  of  this  period. 
Lake  sediments. — During  this  volcanic  epoch  a  large  fresh- water 
lake  formed,  of  unknown,  but  probably  great  extent.     In  it  wer 
deposited  white,  finely  stratified  ash  and  tuff,  and  some   beds  are  | 
entirely  made  up  of  late  Tertiary  infusoria.     In  places  the  sediments 
of  this  lake,  exposed  near  Tonopah,  are  several  hundred  feet  thick. 
It  is  probable  that  this  lake  was  partly  filled  by  sediments  and  then 
drained  by  an  uplift.  Near  the  top  of  Siebert  Mountain  (a  white  tuff 
mountain  southwest  of  Mount  Brougher)  there  is  what  appears  to  be 
a  river  channel  belonging  to  this  post-elevation  epoch,  containing 
rounded  waterworn  pebbles  of  rocks  such  as  now  outcrop  to  the  north 
and  east  of  Tonopah,  and  fragments  of  silicified  wood,  showing  the 
presence  of  vegetation  at  that  time.  This  uplift  may  be  ascribed  to 
volcanism,  for  it  was  soon  followed  by  renewed  outbursts  of  lava. 
Eruption  of  basalt  and  intrusion  of  later  dacites  and  rhyolites. — I 
Above  the  tuffs  on  Siebert  Mountain  is  a  thin  sheet  of  slaggy  basaltic 
•e  I 
