A    NEVADA    ZINC    DEPOSIT.  169 
In  the  West  there  are  considerable  bodies  of  similar  lead  and  zinc  ores  also  Found  in 
limestones,  and  low  in  silver  or  free  from  it.  If  the  explanation  advanced  for  the  Mississippi 
Valley  ores  be  adequate  it  may  well  be  asked  why  a  more  complex  origin  need  be  assumed 
for  these  ore  bodies  in  the  West.  In  a  majority  of  cases  in  the  West  it  is  true  that  gold, 
silver,  arsenic,  antimony,  and  other  metals  characteristic  of  the  ore  bodies  held  to  originate 
in  part,  at  least,  through  igneous  agencies,  occur  with  the  lead  and  zinc,  as  at  Leadville, 
Park  City,  Aspen,  and  elsewhere.  It  may  still  be  asked  why  it  can  not  be  believed  that 
disseminated  lead  and  zinc  in  the  limestone  contributed  in  an  important  degree  to  the  ore 
bodies.  Emmons,  after  an  elaborate  investigation  of  the  country  rocks  at  Leadville,  reached 
the  cautious  conclusion  that,  ''although  the  above  facts  are  not  sufficiently  conclusive  to 
afford  absolute  proof  that  the  metallic  contents  of  the  deposits  were  entirely  derived  from 
the  eruptive  rocks,  they  certainly  show  the  possibility  and  even  probability  that  this  source 
furnished  a  part  at  least  of  the  vein  metals. "«  It  has  been  common  in  subsequent  dis- 
cussions to  consider  the  porphyry  rather  than  the  sedimentary  rocks  as  the  original  home 
of  the  metallic  minerals,  and  to  follow  Emmons  b  in  his  conclusion  that  the  association  of 
the  ores  with  the  limestone  is  due  to  the  chemical  and  physical  characteristics  of  the  rock — 
characters  which  are  believed  to  have  given  it  a  selective  affinity  for  ore  solutions  pene- 
trating the  formations  as  a  whole. 
At  Aspen  there  are  similar  limestones  and  dolomites  associated  with  igneous  rocks,  but 
Spurrc  suggested  only  the  igneous  rocks  as  the  original  source  of  the  metals.  To  the  writer 
it  would  seem  more  probable  that  both  igneous  and  sedimentary  rocks  have  contributed 
their  quota  to  the  ore  bodies. 
The  "  blue  limestone"  of  Leadville  is  a  well-known  metalliferous  horizon  through  several 
Western  States.  As  noted  in  the  description  above,  it  is  in  this  limestone  that  the  Potosi 
ores  occur.  The  zinc  ores  of  the  Magdalena  Mountains  in  New  Mexico  apparently  come 
from  the  same  horizon,  and  in  fact  the  formation  k  quite  as  closely  associated  with  lead 
and  zinc  in  the  West  as  are  the  Cambro-Silurian  rocks  of  the  Mississippi  Valley  and  Appa- 
lachian regions.  Emmons,  in  his  investigations  of  the  Leadville  deposits,  considered  the 
hypothesis  that  the  association  was  primary,  but  concluded  that  the  exceptions  were  too 
numerous  to  warrant  its  adoption.  He  cited  in  particular  the  fact  that  in  the  adjacent 
Ten  Mile  district  the  ores  were  in  the  upper  rather  than  the  lower  Carboniferous.  As  noted, 
he  appealed  to  the  physical  and  chemical  nature  of  the  limestone  to  account  for  the  local- 
ization of  the  ores.  It  is  true,  however,  that  limestones  and  dolomites  of  widely  differing 
characteristics  serve  as  country  rock  to  deposits,  but  that  all  or  nearly  all  of  them  are 
indicative  of  base-level  conditions  on  the  land  at  the  time  of  their  formation — conditions 
under  which  chemical  denudation  and  transportation  are  disproportionately  important. 
Under  such  conditions  the  leaching  of  preexisting  rocks  is  a  most  important  process,  and 
ore  minerals,  as  well  as  others,  may  easily  be  carried  to  the  sea  in  relative  abundance. 
It  would  seem  that  conditions  in  the  sea,  if  limestones  be  formed  or  forming,  should  be  as 
favorable  to  the  deposition  of  the  ore  minerals  as  at  any  later  period  in  which  similar  solu- 
tions may  be  brought  into  contact  with  the  limestone.  The  very  chemical  and  physical 
characteristics  appealed  to  may  well  be  supposed  to  be  operative  in  the  early  as  well  as  the 
later  period,  and  so  the  limestones  as  laid  down  should  be  storehouses  of  disseminated  ore 
minerals.  These  general  and  theoretical  considerations,  coupled  with  the  present  distri- 
bution of  the  ores,  warrant,  it  is  believed,  the  suggestion  that  in  the  Rocky  Mountain  and 
Great  Basin  regions,  as  in  the  Mississippi  Valley,  paleogeography  was  not  without  its  influ- 
ence in  localizing  the  ore  districts.  It  is  not  intended  to  suggest  that  this  is  a  complete 
explanation  of  the  ore  bodies  of  these  districts,  but  merely  to  urge  its  consideration  as  one 
factor  in  a  very  complex  process. 
Geology  and  mining  industry  of  Leadville:  Mon.  U.  S.  Geol.  Survey,  vol.  12,  1886,  p.  582. 
Op.  cit.,  pp.  540  et  seq. 
Geology  of  the  Aspen  mining  district,  Colorado:  Mon.  U.  S.  Geol.  Survey,  vol.  31,  1898,  p.  235. 
