leuBB.]  ORE    DEPOSITS    OF    TONOPAH,    NEVADA.  109 
regarded  with  great  favor,  hut  the  fact  that  the  veins  are  in  this  posi- 
tion has  no  connection  with  the  mountain;  they  are  there  rather  in 
spite  of  the  mountain  than  by  virtue  of  it,  and  it  would  certainly  have 
been  much  better  for  mining  operations  if  the  rhyolite  had  not  been 
erupted  and  the  mountain  formed. 
Most  of  the  dacite  breccias  and  some  of  the  brecciated  rlr^olite  are 
in  the  nature  of  flows,  and  may  be  sunk  through  by  shafts  to  lower 
formations,  such  as  the  later  andesite  or  the  earty  andesite.  Much  of 
the  fine-grained,  brecciated  rhyolite,  however,  is  intrusive,  so  that,  as 
a  rule,  this  rock  also  is  not  a  good  place  to  sink,  except  with  caution. 
The  tuff  formation,  of  course,  is  always  sedimentary  and  can  be  sunk 
through  to  reach  the  underlying  formation,  which  may  be  any  one  of 
the  older  rocks. 
To  sum  up,  then,  the  later  andesite  is  the  most  favorable  of  the 
capping  rocks;  the  pumiceous  dacite  breccia  with  mterstratified, 
coarse,  tuffaceous  rocks  is  next,  while  the  brecciated  rhyolite  should 
be  prospected  very  cautiously  to  see  whether  it  is  intrusive.  The  tuff 
may  be  pierced  to  discover  the  underlying  rock  where  the  geological 
relations  do  not  make  this  plain,  while  intrusive  dacite  and  rhyolite 
necks  should  be  approached  very  cautiously  indeed. 
It  must  be  understood  that  these  characterizations  appty  only  to  the 
immediate  vicinity  of  Tonopah,  or  the  Tonopah  district  proper.  On 
i  account  of  the  sudden  rise  and  fame  of  the  camp  the  name  Tonopah 
has  spread  far  beyond  its  limits,  and  has  been  applied  to  mines  lying 
in  quite  other  districts,  sometimes  50  or  60  miles  away.  Here,  and 
also  in  neighboring  districts  which  have  not  borrowed  the  name  Tono- 
pah, the  geological  conditions  are  different.  In  the  Gold  Mountain 
district,  for  example,  about  4  miles  south  of  Tonopah,  the  veins  lie 
in  rhyolites  and  tuffs  and  evidentty  belong  to  the  rhyolitic  period;  yet 
I  they  carry  in  some  cases  considerable  values  in  gold  and  silver. 
The  position  of  the  early  andesite  away  from  the  outcrops  can  be 
foretold  to  only  a  limited  and  uncertain  extent,  for  in  dealing  with  a 
complex  of  volcanic  flows  and  dikes  like  that  in  Tonopah  there  is  no 
rule  which  governs  their  distribution  and  by  which  their  position  in 
an  unknown  locality  can  always  be  defined  with  certainty.     There- 
i  fore,  the  prospecting  should  be  done  cautiously  and  the  district  should 
be  developed  from  the  known  region  as  a  center;  thus  little  by  little 
la  greater  knowledge  of  the  geological  detail  will    be   gained.     On 
[account  of  the  lack  of  certainty  of  results  in  this  work,  in  many 
leases  it  would  probably  be  more  economical,  both  of  time  and  of 
fc  money,  to   explore  with  a  diamond   drill   rather   than  with  shafts. 
■[Certainly  the  sinking  of  a  two-  or  three-compartment   shaft  in  an 
I  unproved  portion  of  the  district  is  often  unwise. 
The  early  andesite  once  located,  the  question  of  finding  the  veins 
■remains,  and  after  that  the  question  of  finding  pay  ore  within  the 
■kerns.     The  geological  evidence  goes  to  show  that  the  mineralizing 
