208  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART   I. 
The  anorthosite  is  a  bluish  to  medium-gray  holocrystalline  rock  of 
medium  to  coarse  grain.     It  is  composed  almost  wholly  of  gray  feld- 
spar, which  in  some  places  shows  a  feeble  play  of  blue  colors.     Large 
flat  crystals  (phenocrysts)  of  gray  feldspar,  from  2  to  3  inches  long^ 
and  one-quarter  to  one-half  inch  thick,  occur  in  some  portions  of  the 
rock.     Irregular  masses  of  the  rock  are  made  up  wholly  of  a  granular 
aggregate  of  these  large  crystals.     Immediately  east  of  the  iron  ore 
dike,  for  a  distance  of  6  feet  from  the  ore,  the  anorthosite  contains . 
crystals  of  magnetite  (probably  titaniferous)  from  one-eighth  to  3; 
inches  in  diameter.     In  other  parts  of  the  mass,  also,  magnetite  is> 
abundant.     Biotite  and  greenish  black  pyroxene  are  likewise  visible 
to  the  naked  eye  in  some  portions  of  the  anorthosite.     As  seen  under 
the  microscope  the    anorthosite  is  a  fine  to  coarse-grained  evenly 
granular  igneous  rock,  composed  predominantly  of  labradorite  (plagio-- 
clase  feldspar).     Biotite  is  everywhere  present,  while  titaniferous  iron  i 
ore,  olivine,  a  monoclinic  pyroxene,  occur  in  some  specimens,  and  at 
single  thin  section  contains  apatite  and  a  little  quartz.     The  secondary' 
minerals  include  chlorite,  serpentine,  calcite,  and  sericite. 
The  mass  of  iron  ore  is  an  igneous  dike  1  j  miles  long  and  40  feet  to 
300  feet  wide,  the  greatest  observed  width  being  at  the  point  where'1 
Chugwater  Creek  cuts  through  the  mass.     The  dike  trends  east  of 
north;  most  of  it  lies  north  of  the  creek.     It  widens  and  contract 
rather  abruptly  throughout  its  course.     Toward  the  north  it  gradually 
narrows  and  finally  disappears  while  300  Peel   south  of  Chugwatei 
Creek  it  narrows  slightly  and  then  abruptly  ends.     At  several  places 
it  is  almost  cut  in  two  by  wedgelike  masses  of  granite,  but  throughout' 
practically  its  whole  length  it  is  bordered  by  anorthosite.     The  eon- 
tact  between  the  anorthosite  and  the  ore,  where  exposed,  is  sharp,, 
neither  rock  having  undergone  important  gradational  changes.     A\ 
second  dike  of  iron  ore  is  exposed  300  feet  downstream  (east),  on  the1 
south  side  of  Chugwater  Creek.     This  dike  varies  in  width  from  6  tc 
20  feet  and  is  clearly  intrusive  in  the  anorthosite,  with  which  it  has- 
sharp  contacts.     About  one-eighth  of  a  mile  southeast  of  the  south i 
end  of  the  main  mass  is  a  third  dike  of  iron  ore  in  anorthosite.     The-' 
trend  of  this  dike,  which  is  300  feet  long  and  from  10  to  30  feet  wide, 
is  approximately  parallel  to  that  of  the  main  mass.     Several  small  I 
magnetite  dikes  that  are  from  10  to  50  feet  long  and  have  maximum' 
widths  of  3  feet  lie  east  of  this  mass,  in  parallel  alignment. 
The  iron  ore  is  a  black,  granular,  holocrystalline  igneous  rock,  with 
constituent  grains  varying  from  one-eighth  to  one-half  inch  in  diame- 
ter. It  has  a  metallic  or  submetallic  luster.  Changes  in  granularity 
occur  in  irregular  masses  or  along  well-defined  parallel  planes.  In 
consequence  of  this  distribution  the  rock  has  at  some  places  an  orig- 
inal gneissic  structure.     The  greater  portion  of  the  iron  is  free  from 
