NICKEL    DEPOSITS    OF    NICKEL    MOUNTAIN,  OREGON.  125 
ably  deposited  at  about  the  same  time.  Here  and  there  these  cement- 
ing materials  do  not  fill  all  the  spaces  between  the  fragments.  In 
such  places  the  cavities  are  lined  with  a  thin  film  of  silica  having  a 
mammillated  surface.  This  thin  film  appears  to  have  been  deposited 
later  than  the  general  mass  of  cementing  quartz  and  nickel  silicate. 
The  distinctly  conglomeratic  ore  differs  from  the  brecciated  ore  in 
that  the  constituents  are  rounded  rather  than  angular.  This  is  par- 
ticularly well  shown  by  the  nickel  silicate  itself,  which  consists  of 
rounded  concretions  varying  from  the  size  of  a  pin's  head  to  that  of  a 
walnut.  When  broken  open,  these  are  usually  found  to  consist  of 
homogeneous,  apple-green,  amorphous-looking  nickel  silicate,  which 
on  close  inspection  is  seen  to  be  penetrated  by  minute  films  of  white 
silica;  but  in  some  of  the  ore  the  nickel  silicate  forms  only  a  shell  on 
the  outside  of  the  pebble,  the  inside  consisting  of  decomposed  sei  pen- 
tine  or  of  brecciated  ore,  in  which  the  small  fragments  of  nickel  sili- 
cate, iron  oxide,  and  serpentine  are  plainly  seen.  Many  specimens  of 
the  ore,  both  brecciated  and  conglomeratic,  show  slickensided  sur- 
faces, indicating  movement  subsequent  to  the  formation  of  the  ores. 
The  ore  found  beneath  the  flat-lying  deposits  occui  3  as  small  veins 
and  minute  veinlets  in  the  peridotite,  which  contains  innumerable 
fractures.  These  veins  and  veinlets  run  in  various  directions,  forming 
an  irregular  network,  but  in  the  main  they  appear  to  be  related  to 
zones  of  fracture  and  brecciation  that  have  a  general  northeast- 
southwest  direction.  These  zones  are  of  considerable  width,  but  the 
individual  fractures  are  narrow,  the  largest  vein  observed  being  not 
more  than  6  inches  wide  and  most  of  them  less  than  1  inch.  The 
vein  filling  consists  of  nickel  silicate  and  silica,  but  iron  oxides  are  also 
present,  and  in  some  places  the  material  is  of  the  nature  of  a  cemented 
breccia.  Between  the  veinlets  in  the  fractured  zones  the  material  is 
chiefly  hydrated  iron  oxide  in  which  rounded,  bowlderlike  masses  of 
fairly  fresh  peridotite  occur.  In  places  the  fractures  are  still  unfilled, 
the  walls  being  of  peridotite  but  little  altered. 
Fault  planes  with  slickensided  surfaces  were  observed  at  several 
places  within  the  nickel  area.  The  relations  of  many  of  these  fault 
planes  to  the  ore  bodies  indicate  that  movements  have  taken  place 
subsequent  to  the  forming  of  the  general  mass  of  the  ore. 
ORIGIN   OF   THE   ORES. 
The  field  evidence  strongly  suggests  that  the  nickel  silicates  are  a 
decomposition  product  of  the  peridotite  with  which  the  ores  are  so 
intimately  associated.  The  evidence  derived  from  chemical  analyses 
supports  this  view. 
The  nickel,  which  is  by  analysis  found  in  small  quantities  in  the 
fresh  peridotite,  appears  to  be  associated  with  the  constituent  olivine 
Bull.  315—07 9 
