104  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
the  rock  is  comparatively  fresh  or  much  decomposed  apparently 
makes  little  difference  in  the  abundance  of  the  pyrite.  In  most  cases 
the  pyrite  is  found  in  tiny  seams  or  gashes  in  the  more  or  less  altered 
rock,  a  habit  indicating  that  it  originated  subsequent  to  the  other 
minerals.  Frequently,  however,  the  pyrite  occurs  in  such  a  way  as  to 
seem  a  primary  constituent  and  one  of  the  first.  For  example,  it 
occurs  as  inclusions  in  feldspar  phenocrysts,  also  as  isolated  particles 
in  fresh  rock.  On  the  whole,  the  pyrite  here  appears  indigenous  to 
the  granite,  and  its  occurrence  as  secondary  streaks  seems  to  be  due 
to  rearrangement.  The  easy  solubility  of  iron  sulphide  and  the 
insolubility  of  magnetite  explains  why  original  pyrite  should  have 
been  rearranged  while  the  magnetite  was  not.  Pyrite,  like  magnet- 
ite, is  decidedly  more  abundant  in  certain  pegmatites  near  Empire 
and  in  the  pegmatitic  portions  of  the  granite  than  in  the  finer-grained 
granite  mass. 
PegmatAtes. — Besides  the  old  pegmatite  spoken  of  as  intercalate^ 
between  the  lamina?  of  the  gneiss,  there  is  a  series  of  several  other  peg 
matites.  It  is  impossible  to  always  classify  these  pegmatites  as  t( 
age  on  account  of  their  similarity.  Nevertheless,  it  is  not  difficult  t( 
ascertain  that  there  have  been  intrusions  at  several  different  periods 
for  pegmatites  have  been  observed  which  are  contemporaneous  asso 
ciates  of  several  of  the  granites  of  the  region,  and  even  of  the  dark 
basic,  diorite-like  rocks. 
The  pegmatites  contain  varying  proportions  of  one  or  more  of  th 
following  minerals :  Quartz,  feldspar,  biotite,  and  muscovite.  Mag 
netite  and  pyrite  are  common  accessory  minerals.  Occasionally  th 
pegmatite  consists  almost  entirely  of  white  or  red  feldspar,  and  a 
other  times  it  assumes  the  phase  of  veins  composed  essentially  of  pur 
quartz.  The  pegmatites  are  usually  rather  siliceous,  but  magnetite  i 
frequently  found  in  them  in  large  quantities.  The  granites  and  othe 
igneous  rocks  often  contain  magnetite  as  an  accessory  constituent,  bn 
the  magnetite  is  vastly  more  abundant  in  the  pegmatite  than  in  tl 
other  rocks.  Places  were  observed  where  the  segregated  magnetit 
in  the  pegmatite  or  in  the  quartz  veins  of  pegmatitic  origin  w£ 
nearly  equal  in  volume  to  the  rest  of  the  rock  mass,  and  in  consequenc 
almost  formed  an  iron  ore.  Pyrite  is  also  an  original  constituent  ( 
the  pegmatites  at  times. 
Porphyries. — Dikes  of  porphyry  are  found  scattered  throughoi 
the  portion  of  the  mineral  belt  studied.     The  porphyries  are,  so  fa 
of  unknown  age,  but  are  certainly  younger  that  the  gneisses,  granite  i 
pegmatites,  and  diorites,   as  they  are   found  intersecting  and   aL 
including  fragments  of  all  of  these  formations. 
The  porphyries  consist  of  a  great  many  varieties,  or  at  least  phas 
of  several  different  forms.  These  rocks  may  be  divided  into  tv 
main  groups — (1)  felsitic  porphyries  and   (k2)  granitic  porphyries. 
