282  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
guishable  from  certain  beds  in  the  Burke  and  Wallace  formations, 
so  that  it  is  sometimes  difficult  to  identify  the  formation  when  but 
small  areas  are  exposed.  The  formation  is  thickest  and  best  exposed 
in  the  southeastern  part  of  the  district,  particularly  about  3  miles 
northeast  of  Mullan  and  in  the  vicinity  of  St.  Regis  Pass. 
The  Wallace  formation  in  thickness  and  areal  extent  is  second  only 
to  the  Prichard  slate.     It  is  the  most  heterogeneous  of  all  the  for- 
mations distinguished.     The  dominant  rocks  are  thin-bedded,  light- 
green  shales  consisting  chiefly  of  quartz  and  sericite,  associated  with 
impure  limestones,  bluish-gray  argellites,  and  calcareous  qnartzites, 
and   characterized  from  top  to  bottom  by  ripple  marks  and  other 
evidences  of  shallow-water  deposition.     In  addition  to  the  dominant 
constituents,  nearly  all  of  the  beds  contain  more  or  less  calcite,  dolo- 
mite,   and    siderite    (or    other    ferruginous    carbonates),    which    on 
weathering  give  a  yellow  tint  to  the  exposures  of  the  formation.     A 
slaty  structure  is  common,  the  rocks  often  being  highly  fissile,  though 
the  cleavage  never  approaches  in  regularity  that  of  ordinary  clay    I 
slate.     Fresh  surfaces  of  these  green-banded  slates  have  usually  a  I 
peculiar  waxy  luster  that  is  very  characteristic  of  the  Wallace  for-   I 
mation,  as  is  also  the  presence  of  carbonates. 
The  formation  is  well  exposed  at  the  town  of  Wallace,  particularly  I 
at  the  Northern  Pacific  Railway  station.  It  occupies  large  areas  all  I 
along  the  southern  border  of  the  district,  and  is  the  principal  rock 
along  Beaver  Creek.  The  mines  and  prospects  in  the  so-called  "  dry  J 
belt,"  between  Wallace  and  Wardner,  are  mostly  in  the  Wallace  for-  j 
mation. 
The  Striped  Peak  formation  is  the  least  extensive  in  the  district,  j 
The  largest  areas  occur  near  the  peak  whence  the  formation  derives  I 
its  name.  Lithologically  it  is  almost  a  repetition  of  the  St.  Regis  j 
formation,  with  unusually  abundant  ripple  marks. 
Igneous  rocks. — The  most  important  igneous  masses  in  the  district 
are  the    irregular  stock-like   intrustions  of  syenite  outlined   on   the  j 
accompanying  map  (fig.  IT).     The  typical  rock  of  the  larger  areas,  1 
as  determined  by  Mr.   Calkins,   is  a  coarse-grained  syenite   with   a   j 
tendency  toward   porphyritic  development  of  the  dominant  alkali- 
feldspar.     The  other  essential   constituents  are   plagioclase,   amphi- 
bole,  and  pyroxene.     Biotite  is  rare,  and  neither  quartz  nor  nepheline  j 
has  been  detected.     From  this  central  type  is  considerable  variation,  I 
particularly  near  the  contacts  and  in  the  smaller  masses,  which  con-  1 
sist  usually  of  syenite  porphyry.     In  the  largest  mass,  near  Gem,  are 
found    monzonitic   facies   and    a   number   of   interesting   peripheral 
modifications  rich  in  amphibole  and  pyroxene. 
The  larger  syenitic  intrusions  are  surrounded  by  well-marked  zones 
or  aureoles  of  contact  metamorphism.     The  qnartzites  are  altered  to 
