A    NEVADA    ZINC    DEPOSIT.  107 
acidic  porphyry  rock.  The  Cambrian  rocks  are  found  in  the  northern  part  of  the  range  and 
include  a  blue-gray  crystalline  limestone,  greenish-yellow  shales,  and  thin  bands  of  dark 
sandstone.  A  quartzite  associated  with  the  limestone  is  probably  also  of  Cambrian  age. 
The  Carboniferous  rocks  include  Mississippian  and  Pennsylvanian  limestones  and  a  con- 
siderable thickness  of  red  shale  and  sandstones.  The  Mesozoic  rocks  are  represented  by 
conglomerates,  sandstones,  and  shales  and  carry  Jurassic  fossils. 
Structurally  the  range  is  somewhat  peculiar,  being  apparently  anticlinal  in  a  north- 
south  section  and  showing  a  wrinkled  synclinorium  in  an  east-west  section.  The  main 
mass  of  the  mountains  consists  of  Carboniferous  limestones.  The  Cambrian  rocks  have 
been  brought  up  and  the  Mesozoic  let  down,  with  reference  to  the  Carboniferous,  by  heavy 
thrust  faulting. 
No  very  careful  or  detailed  study  of  these  or  any  of  the  surrounding  mountains  has  yet 
been  made  and  very  little  is  really  known  of  the  geology.  In  the  present  instance  no  attempt 
was  made  to  study  the  geology  of  the  district,  attention  being  concentrated  upon  the 
ore  body  and  the  mine. 
Ore  deposit. — The  Potosi  mine  is  located  near  the  mountain  marked  Olcott  Peak  on 
Spurr's  map.  The  limestone  here  is  predominantly  fine  grained,  breaking  with  a  clean 
conchoidal  fracture  and  being  usually  dark  colored  and  occasionally  very  bituminous. 
Certain  of  the  beds  are  cherty,  but  the  chert  is  unequally  distributed,  much  of  the  lime- 
stone being  apparently  free  from  it.  The  beds  in  which  the  ore  occurs  are  not  cherty. 
Fossils  occur  rather  abundantly,  though  none  were  found  immediately  in  connection  with 
the  ore  bodies.  The  forms  collected  were  examined  by  G.  H.  Girty  and  found  to  be  com- 
mon Carboniferous  species, such  as  especially  characterize  the  "blue  limestone"  at  Lead- 
ville.  The  rocks  have  been  folded  sharply  and  faulted.  The  ore  is  found  in  certain  little- 
disturbed  and  nearly  horizontal  beds  having  a  slight  dip  to  the  east.  The  adits  by  which 
the  mine  is  opened  are  in  a  perpendicular  cliff  facing  west,  at  the  head  of  a  deep  arroyo. 
This  cliff  seems  to  mark  a  fault  scarp  revealed  by  erosion.  The  fault  plane,  which  is 
apparently  vertical  and  runs  approximately  north  and  south,  separates  the  little-disturbed 
beds  on  the  east  from  the  crumpled  and  folded  limestones  which  form  spurs  on  either  side 
of  the  arroyo  and  stretch  off  to  the  west. 
The  adits,  of  which  there  are  two,  are  about  800  feet  above  the  plain,  or  at  an  altitude 
of  approximately  (3,500  feet.  Above  these  the  hill  rises  first  abruptly  and  then  more 
gently  at  least  300  feet.  The  main  ore-bearing  bed  is  flat  lying,  is  about  5  feet  thick,  and 
lies  immediately  above  a  body  of  thin-bedded  shaly  limestone  about  20  feet  thick.  This 
shaly  limestone  is  dark  colored,  fine  grained,  and  hard.  It  is  made  up  of  individual  beds 
about  2  inches  thick  separated  by  clay  partings  one-fourth  to  one-half  inch  thick. 
At  right  angles  to  the  cliff — that  is,  running  east — the  limestone  is  cut  by  a  series  of 
narrow  crevices,  in  the  main  pitching  south.  These  show  no  faulting  and  are  better 
developed  in  the  limestone  above  the  shaly  bed  than  in  it  or  below.  A  prospect  drift, 
however,  run  in  a  hundred  feet  or  so  at  the  base  of  the  shaly  limestone  follows  one  of  these 
crevices.  Similar  crevices  showing  some  mineralization  are  said  to  be  traceable  as  much 
as  1,000  feet  along  their  strike,  though  offsetting  cross  faults  occur.  There  was  not  time 
to  verify  these  statements. 
The  ore  consists  mainly  of  carbonate  of  zinc.  Carbonate  of  lead,  some  sulphate,  and 
galena  also  occur.  There  are  occasional  copper  stains.  No  pyrites  was  seen,  but  iron  oxide 
stains  are  common.  The  mine  is  dry  and  oxidation  has  gone  far.  The  galena  is  only 
slightly  argentiferous.  A  random  sample  examined  in  the  Survey  laboratory  by  George 
Steiger  showed  practically  no  silver  present.  It  is  said  that  some  ore  shipments  have  run 
as  high  as  12  ounces  in  silver  to  the  ton,  but  ordinarily  neither  gold  nor  silver  is  determined. 
The  main  shipments  are  zinc  carbonate  running  40  to  45  per  cent  of  zinc.  A  mixed  grade 
running  35  per  cent  of  zinc  and  20  per  cent  of  lead  is  also  occasionally  sold.  There  is  no 
known  gold  value. 
In  general  appearance  and  in  mode  of  occurrence  the  ore  bodies  are  curiously  like  some 
which  have  been  worked  in  the  upper  Mississippi  Valley  region.     The  long-pitching  crevices 
