168  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
I 
which  show  no  faulting  are  similar  to  the  well-known  ''pitches"  of  that  district,  while  th 
concentration  or  "making"  of  the  ore  along  the  top  of  the  shaly  limestone  is  equally  lik 
the  relations  of  the  Wisconsin  ore  to  the  "  oil  rock."  In  Nevada  the  long  period  of  deser 
conditions  has  caused  the  minerals,  which  in  Wisconsin  are  generally  sulphides,  to  b 
largely  altered  to  carbonates  and  oxides.  Similar  ores  are  found  in  Wisconsin,  but  general! 
at  levels  considerably  above  th-  "oil  rock,"  both  stratigraphically  and  topographically 
In  both  cases  the  ore  constitutes  a  run  as  defined  by  Prof.  W.  P.  Jenneya  along  the  intei 
section  of  a  crevice  and  a  favorable  horizon;  here  it  is  rather  just  above  an  unfavorable  < 
impervious  stratum.  The  ore  bodies  are  exceedingly  irregular  in  detail,  following  the  mino 
fractures  and  ramifying  watercourses  through  the  limestone.  In  the  Potosi  mine  smal 
druses  an  inch  or  so  in  diameter  were  noted,  which  were  lined  with  zinc  carbonate,  thougi 
apparently  not  connected  with  any  regular  crack.  Chimneys  of  ore  run  up  irregular! 
along  the  crevices,  but  so  far  no  ore  shoots  running  down  through  the  shale  have  been  found 
Origin. — The  mode  of  occurrence  favors  the  notion  of  a  simple  downward  circulation  c 
ore-bearing  waters  arrested  by  a  nearly  impervious  bed  and  depositing  their  load  of  minera 
along  its  top.  If  the  original  deposition  was  in  the  form  of  sulphides,  as  is  quite  probabk 
the  organic  matter  of  the  shaly  beds  was  probably  important.  The  subsequent  period  o 
oxidation  has,  however,  been  so  long  as  to  obscure  the  records  of  any  early  stages  in  this  or 
body.  If  we  look  back  to  an  earlier  period,  when  rainfall  was  more  abundant  in  this  area 
we  can  readily  conceive  of  the  formation  of  these  ore  bodies  under  present  structural  an. 
topographic  conditions  without  any  demand  on  additional  agencies.  Indeed,  it  is  eve: 
possible  that  in  the  winter  and  rainy  months  the  ore  is  now  being  concentrated  by  a  simpl 
downward  circulation  of  meteoric  waters. 
Whether  such  a  simple  hypothesis  be  in  fact  the  correct  one  in  t  his  case  can  not  be  detei 
mined  without  more  detailed  study.  For  the  present  it  is  sufficient  to  show  that  it  is 
perfectly  possible  explanation  of  the  known  facts.  Near  the  mine,  as  has  been  noted,  i 
an  important  fault  plane  which  presumably  might  have  afforded  openings  for  a  deep  circu 
lation,  though  the  course  of  the  ore  body  is  well  developed  at  right  angles  to  this  plant 
Warm  springs,  as  noted  by  Spurr,&  occur  in  the  mountains  bnl  not  near  the  ore  bodies 
nor  are  ores  known  to  occur  near  the  springs.  One  spring,  a  cold  one,  is  found  not  far  fror 
the  mine.  No  igneous  rocks  are  known  nearer  than  a  dozen  miles  or  more  from  the  min( 
though  in  the  part  of  the  mountains  in  which  they  are  found  there  are  certain  lead  and  cop 
per  deposits  not  visited.  Prof.  E.  M.  Jenneyc  describes  the  lead  deposits  as  similar  i 
character  and  form  to  those  of  Missouri,  and  sees  in  this  similarity  an  additional  reason  fc 
referring  the  Missouri  deposits  to  deep-seated  waters  connected  genetically  with  som 
unknown  igneous  rocks.  To  the  writer  it  would  seem  that  the  analogy  might  quite  a 
well  be  the  other  way  and  that  the  relation  to  igneous  rocks  in  the  Spring  Mountains  wa 
accidental  rather  than  causal. 
It  has  been  customary  in  discussions  of  Western  ore  deposits  to  assign  to  igneous  rock 
an  important  genetic  rdle.  This  has  been  done  for  deposits  essentially  valuable  for  th 
lead,  zinc,  or  copper  content,  as  well  as  for  those  in  which  gold  and  silver  formed  the  mai 
element  of  value.  The  general  validity  of  the  reasoning  as  applied  to  the  precious-met; 
deposits  is  not  here  questioned,  but  it  seems  to  the  writer  that  a  question  may  well  be  raise 
with  regard  to  those  deposits  which  are  mainly  valuable  for  lead  and  zinc  or  in  which  th 
gold  and  silver  values  are  small.  It  is  well  known  that  in  the  Mississippi  Valley  there  ai 
important  deposits  of  lead  and  zinc  ores  found  in  limestone  and  dolomites  and  considere 
by  most  investigators  to  have  no  direct  genetic  relations  to  igneous  rocks.  These  ore  bodk 
are  characterized  by  a  very  simple  association  of  minerals.  They  are  free  from  arseni< 
antimony,  and  the  rarer  complex  sulphides,  and  are  low  in  silver  where  not  wholly  nona: 
gentiferous.  Their  origin  can  not  be  assigned  to  igneous  rocks,  since  there  are  no  sue 
rocks  to  which  to  relate  them  and  adequate  alternative  hypotheses  are  available. 
aJennev,  "W.  P.  Lead  and  zinc  deposits  of  the  Mississippi  Valley:  Trans.  Am.  Inst.  Min.  Eng.,  vc 
22,  p.  L89. 
l»Op.cit.,p.  165. 
o  Personal  communication. 
