122  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
member  persisting  for  any  considerable  distance.  These  rocks  belong- 
to  the  series  generally  known  as  the  "  Red  Beds,"  and  to  the  lower 
division  of  that  series,  the  Cutler  formation,  which  is  very  probably 
of  Permian  age.  The  intruding  bodies  are  diorite-  or  monzonite-por- 
phyries — light  grayish-green  rocks  with  white  feldspars  and  dark 
hornblende  phenocrysts.  The  porphyry  sheets  are  usually  about  50 
feet  thick,  though  they  are  often  thinner.  While  they  generally  fol- 
low the  bedding  planes  of  the  rocks  which  they  intrude,  frequently 
they  are  crosscutting  and  often  irregular.  The  porphyry  dikes  are 
generally  about  25  feet  thick  and  have  the  same  lithologic  appearance 
as  the  sheets,  from  which  they  can  not  be  distinguished  except  upon 
structural  evidence.  The  basic  dikes,  which  cut  the  green  porphyries, 
are  very  dark  and  of  a  denser  texture.  These  are  only  a  few  feet 
wide. 
The  age  of  the  green  porphyries  is  not  definitely  known.  They  are 
later  than  the  "  Red  Beds,"  which  they  intrude,  and  from  their  simi- 
larity to  intrusions  of  like  character  in  neighboring  districts  they  are 
assumed  to  be  Tertiary. 
Fissures. — Both  the  sedimentary  and  igneous  rocks  are  jointed  anc 
fissured  in  several  directions.  There  is  a  well-defined  set  of  fissure; 
running  a  few  degrees  south  of  east.  This  set  of  fissures  is  notabj 
in  the  Durango  Girl  and  Neglected  mines,  and  is  mentioned  by  Mr 
Purington  as  occurring  also  in  the  southern  part  of  the  La  Plat* 
Mountains.  Mr.  Purington  believes  that  the  east-west  fissures  are  o 
earlier  age  than  the  other  fissures  and  that  they  are  also  older  than 
some  of  the  intrusive  rocks. 
Metamorphism — The  sandstones,  which  are  usually  red  and  friable 
are  considerably  metamorphosed  in  the  vicinity  of  the  ore  deposit* 
where  they  have  been  indurated  by  the  deposition  of  silica,  and  thei 
color  has  been  changed  to  light  gray.     It  appears  that  the  iron  oxide 
to  which  their  usual  red  color  is  due,  was  either  removed  or  altered 
The  shales,  too,  are  more  or  less  silicified,  and  their  color  is  similarl 
changed,  so  that  it  is  sometimes  difficult  to  distinguish  shale  froi 
sandstone.     The  fractures  of  the  sediments  where  they  are  brecciate 
are  sharp  and  clear  cut,  suggesting  that  their  induration  partly  pr< 
ceded  the  movement  which  crushed  them.     Since  their   induratio 
appears  to  be  (\\w  to  the  action  of  circulating  water  rather  than  to  coi  ■ 
tact  metamorphism  induced  by  the  heat  of  the  porphyry,  it  won] 
seem  that  there  was  some  fracturing  before  and  some  after  the  met; 
morphism  of  the  sediments.     The  fracturing  and  the  metamorphis 
were  probably  concomitant. 
