24(>  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         r bull.  260. 
which  it  is  composed.    These  consist  of  an  upper  scries  of  red  quartz-  I 
ites,  estimated  at  oxer  a  thousand  feet  in  thickness,  beneath  which 
are  several  hundred  feet  of  white  quartzites,  the  whole  underlain  by 
limestones.     No  monzonite  outcrops  were  observed  along  the  imme-   1 
diate  western  face  of  the  massif,  but  the  alteration  "of  the  blue  lime-   I 
stones  into  white  marble   which  extends  in   irregular  tongues   and    ! 
clouds  through  the  general  belt  of  the  unaltered  rock  at  the  base  of 
the  cliff  suggests  that  this  rock  probably  occurs  at  no  great  depth 
below  the  surface. 
No  fossils  have  yet  been  found  in  the  sedimentary  rocks  of  the  re-   j 
gion,  and  their  age  is  purely  a  matter  of  surmise.     The  heavy  quartz-  I 
ites  in  this  portion  of  the  Great  Basin  have  hitherto  proved  to  be  of 
Cambrian  age,  but  limestones  underlying  Cambrian  quartzites  have  I 
not  yet  been  observed  so  far  east  as  this,  though  they  are  known  to 
occur  near  the  east  base  of  the  Sierra  Nevada.     The  monzonites  are   j 
evidently  later  than  all  the  sedimentary  rocks;  the  andesites  are  pre- 
sumably of  still  more  recent  age,  and,  from  analogy  with  other  re-  I 
gions,  probably  Tertiary.     The  faulting  must  necessarily  have  taken  I 
place  at  a  still  later  period,  since  it  cuts  all  these  rocks. 
The  deposits. — The  deposits  of  the  Cactus  mine  are  opened  in  the  I 
bed  of  Copper  Gulch  just  above  the  point  where  it  bends  from  its  j 
northwest  trend  to  take  a  more  direct  westward  course  down  the  val-  1 
lev  slopes.  It  occurs  entirely  within  the  monzonite  body,  which  here  1 
occupies  the  bed  of  the  gulch  and  the  hills  immediately  south  and  west  I 
of  it.  The  ore  occurs  within  a  zone  of  fracture  and  brecciation  of  I 
irregular  width  which  is  somewhat  wavy  and  variable,  both  in  strike  J 
and  in  dip,  but  in  general  follows  the  bed  of  the  gulch  below  the  1 
forks,  having  an  average  direction  of  about  N.  30°  W.  In  dip  it  is 
nearly  vertical,  the  little  departure  from  the  perpendicular  that 
could  be  detected  being  to  the  northeast. 
The  ore  is  mainly  pyrite,  generally  rather  coarsely  crystalline,  with 
a  little  chalcopyrite,  and  contains  practically  no  lead  or  zinc.  It 
carries  a  little  silver  and  a  fraction  of  an  ounce  of  gold,  the  precious 
metals,  it  is  claimed,  being  sufficient  in  amount  to  repay  the  cost  of 
mining  and  concentration.  No  other  metallic  minerals  were  observed 
by  the  writer  except  a  small  amount  of  sooty  sulphide  of  copper  in  the 
secondarily  enriched  zone,  where  the  ore  in  considerable  stretches  is 
said  to  run  up  to  7  per  cent  of  copper  and  in  spots  to  contain  20  to 
100  ounces  of  silver  to  the  ton.  At  the  outcrop  there  is  little  evidence 
of  the  existence  of  ore  beyond  an  occasional  green  stain  on  the  granite, 
and  yet,  in  the  winze  at  the  forks  of  the  gulch,  sulphide  showing  little 
or  no  oxidation  is  found  within  a  few  feet  of  the  surface.  This  is  a 
little   remarkable   in   this  arid   climate,  where   oxidation    is   apt    to 
