sruiiR.]  DEVELOPMENTS    AT    TONOPAH,   NEV.  147 
ascending  waters  had  to  force  themselves  up  along  irregular  courses. 
This  mineralization  is  probably  the  same  in  time,  nature,  and  origin 
as  that  at  Gold  Mountain,  which  is  1  miles  south  of  Tonopah,"  and 
may  very  likely  be  the  same  as  that  in  the  newly  discovered  camp  of 
Goldfield,  which  lies  about  23 J  miles  south  of  Tonopah. 
PROGRESS  OF  KNOWLEDGE  OF  UNDERGROUND   FORMATIONS. 
It  was  explained  last  year  that  the  geology  at  Tonopah  was  so 
complex  that  no  accurate  detailed  knowledge  sufficient  to  guide  min- 
ing operations  with  certainty  could  be  obtained  far  from  the  regions 
which  have  been  explored  underground  by  mine  workings,  and  it  was 
pointed  out  that  with  the  increase  in  number  of  such  workings  the 
chances  for  successful  mining  in  the  immediately  contiguous  regions 
could  be  progressively  more  closely  estimated.     It  has  indeed  turned 
out  to  be  true  that  the  developments  of  the  past  year  have  had  this 
effect  in  adding  to  our  knowledge  of  underground  geology  and  in 
increasing  our  basis  for  future  estimates.     The  actually  productive 
area  has  been  slightly  but  not  greatly  enlarged,  and  within  this  area, 
is  previously  mentioned,  most  of  the  mine  workings  have  encoun- 
;ered  serious  drawbacks  to  successful  mining  in  the  shape  of  large 
md  irregular  bodies  of  the  later  and  intrusive  Tonopah  rhyolite- 
lacite.     These  bodies  do  not  come  to  the  surface  over  most  of  the 
Tea,  and  their  presence  could  not  have  been   accurately   foretold, 
Ithough   the   probability   that    such   intrusions   would   be   encouh- 
ered  anywhere  and  everywhere  was  pointed  out  by  the  writer  in  his 
revious  report.     The  complexity  of  the  intrusions  was  not  exag- 
erated  in  the  ideal  cross  section  previously  published.'' 
Besides  the  intrusions  of  later  eruptives,  cutting  off  the  earlier- 
rmed  andesitic  veins,  the  complicated   faulting,  characteristic  of 
e  district,  which,  in  the  early  history  of  its  development,  made  the 
raction  vein  practically  impossible  to  follow  and  to  exploit  eco- 
mically,  has  been  found  to  manifest  its  effects  in  the  other  veins, 
tailing  more  or  less  serious  hindrance  to  mining  operations.     The 
izpah  vein  is  not  only  broken  in  detail  by  minor  faults,  but  is  cut 
to  the  east,  the  west,  and  at  the  bottom  by  faults  of  greater  mag- 
jtude.     Within  the  important  and  rich  veins  of  the  Montana  Tono- 
h  complicated  faulting  has  operated  so  as  to  displace  the  ore  bodies 
a  very  puzzling  way.     These  faults  have  different  systems  and 
ii  tjgir  effect  is  too  complicated  for  explanation  within  a  short  space, 
t  they   are  explained  more  at  length   in   the  final   report.     The 
nopah   Extension   vein,   up   to  the  time   of  examination   by   the 
ter,  had  continued  to  be  unusually  free  from  faulting. 
"Bull.  U.  S.  Geol.  Survey  No.  213,  p.  87. 
"Bull.  U.  S.  Geol.  Survey  No.  225,  p.  10S. 
