emmons.]  NEGLECTED    MINE,   COLORADO.  123 
ORE    DEPOSITS. 
Forms  of  the  ore  deposits, — The  ore  deposits  fall  into  two  groups : 
(1)  those  which  occur  as  a  filling  between  two  walls  of  different  rock, 
ind  (2)  those  which  are  situated  along  fractured  zones  at  or  near  the 
contact  between  the  sedimentary  rocks  and  the  porphyry,  where  im- 
pregnation and  partial  replacement  has  occurred.  The  best  example 
>f  the  first  class  is  the  Cumberland  mine,  just  northwest  of  Cumberland 
Mountain,  where,  according  to  Mr.  Purington,  there  is  a  well-defined 
vein  several  feet  wide,  consisting  mainly  of  white  quartz.  The  de- 
posits of  the  Durango  Girl  and  Neglected  mine  belong  chiefly  to  the 
second  class.  There  are  gradations  between  these  two  classes  of 
deposits,  and  sharp  differentiation  is  not  always  possible.  Even  in 
:he  two  mines  last  mentioned  small  fissures  occur  in  the  shattered 
ione,  and  these  are  filled  with  white  siliceous  minerals,  mainly  quartz, 
in  which  the  precious  minerajs  may  occur. 
Minerals  of  the  ore  deposits. — The  metalliferous  minerals  of  the  La 
Plata  district  are  the  tellurides  of  gold  and  silver  (sylvanite,  petzite, 
md  probably  calaverite),  native  gold,  native  quicksilver,  amalgam, 
freibergite,  tennantite,  stephanite  (and  other  sulphantimonides  and 
sulpharsenides  of  silver),  pyrite,  marcasite,  chalcopyrite,  galena, 
cine  blende,  realgar,  hematite,  and  magnetites  Dr.  W.  F.  Hillebrand, 
who  examined  the  ores  from  the  Durango  Girl  mine,  found  also  a 
nercury  sulphide,  probably  cinnabar.  Though  there  is  a  considerable 
variety  of  gangue  minerals  in  the  deposits  of  the  western  part  of  the 
[ja  Plata  Mountains,  in  the  properties  examined  in  the  Oro  Fino  dis- 
rict  the  gangue  appears  to  be  composed  chiefly  of  quartz,  kaolinite, 
hydrous  silica,  and  sericite. 
General  character  of  the  ore  deposits. — All  of  the  workings  in  the 
)ro  Fino  district  are,  so  far  as  known,  in  or  near  the  green  porphyry 
ntrusives.  In  some  instances  dark  basic  dikes  cut  the  porphyries 
[ear  the  ore  deposits,  but  in  many  cases  these  are  absent,  or  at  least  are 
lot  exposed.  The  structural  features  which,  from  an  economic  stand- 
loint,  are  the  most  important  appear  to  be  the  fracture  zones,  which 
k  the  cases  observed  run  nearly  east  and  west.  It  is  along  these  that 
be  ore  bodies  lie.  The  ore  may  be  developed  in  the  porphyry,  at  its 
ontact,  or  in  the  sedimentary  rocks  near  the  porphyry.  Often  it 
ppears  to  be  a  replacement  of  the  porphyry,  which,  in  the  vicinity  of 
he  fractured  zone,  is  altered  and  decomposed.  Both  porphyry  and 
pdiments  are  sometimes  impregnated  with  pyrite  for  a  short  distance 
jkvay  from  the  broken  zone,  and  through  them  are  little  stringers  of 
kite  quartz,  which  carry  flakes  and  specks  of  telluride. 
In  cases  where  the  sedimentary  rocks  are  impregnated  there  does 
1    a  This  list  is  a  revision  of  that  given  by  Mr.  C.  W.  Purington  in  folio  GO,  page  13. 
