jansome.]  COEUR    d'aLENE    DISTRICT,  IDAHO.  283 
hornfels.  The  impure  quartzites  and  argillites  are  recrystallized  as 
aggregates  of  andalusite,  garnet,  sillimanite,  biotite,  muscovite  quartz, 
and  feldspar,  and  in  the  calcareous  Wallace  beds  amphibole  and 
pyroxene  are  developed. 
The  dike  rocks  of  the  region,  which  seem  to  have  no  direct  connec- 
tion with  the  syenitic  intrusions,  are  not  of  great  structural  or  eco- 
nomic importance.  They  have  not  yet  been  carefully  studied,  but 
will  be  described  by  Mr.  Calkins  in  the  final  report  on  the  district. 
Structural  features  of  the  district. — The  sedimentary  rocks  of  the  I 
Coeur  d'Alene  district  have  been  complexly  folded,  the  folds  in  several 
instances  being  overturned,  so  that  the  older  formations  overlie  the  \ 
younger.  They  have  also  been  extensively  faulted  and  so  strongly 
compressed  as  to  develop  slaty  cleavage  in  all  but  the  massive  quartz-  i 
tes.  While  the  prevalent  strike  of  the  beds  is  northwesterly,  it  is 
)lain  that  no  simple  compression  along  northeast-southwest  lines 
■an  satisfactorily  account  for  the  often  highly  complex  character  of 
he  folds  and  the  numerous  and  important  deviations  from  the  domi- 
nant northwesterly  strike.  In  the  greater  part  of  that  portion  of  the 
listrict  lying  south  of  the  South  Fork  the  general  strike  is  about 
ivest-northwest,  and  the  axis  of  the  major  folds  are  traceable  for 
'onsiderable  distances.  In  the  vicinity  of  Kellogg  Peak  and  Ward- 
ler,  however,  the  strike  becomes  more  northerly  and  the  folds  more 
rregular.  In  the  region  lying  north  of  the  South  Fork  and  west 
>f  Beaver  and  Xinemile  creeks  the  essential  structure  is  that  of  an 
mticline,  with  a  steep  pitch  to  the  north.  The  oldest  formation 
exposed  in  this  anticline  is  the  Prichard  slate,  which  .occurs  along  the 
South  Fork  from  Osburn  westward,  and  in  consequence  of  this 
structure  forms  a  semicircular  area  north  of  the  river. 
From  Mullan  northward  past  Murray  is  a  broad  belt  of  close  and 
complex  folding  in  which  north-south  strikes  prevail.  This  struc- 
ure  passes  into  a  zone  of  more  open  folds  to  the  east. 
The  region  contains  a  number  of  important  faults,  of  which  the 
jnajoritv  strike  nearly  west-northwest.  Both  normal  and  reversed 
faults  occur,  the  dip  of  the  reversed  faults  being  usually  steep. 
1  ne  west-northwest  faults  are  particularly  abundant  in  the  southern  . 
part  of  the  district,  where  seven  important  dislocations  of  this  group 
lave  been  mapped  and  studied  by  Mr.  Calkins.  These  have  throws 
If  from  1,000  to  at  least  4,000  feet  and  observed  lengths  up  to  18 
biles.  The  most  prominent  fault  of  this  group  extends  from  a 
point  east  of  Mullan,  past  Wallace1,  through  Osburn,  and  for  an 
Jnknown  distance  west  of  Wardner.  The  fault  is  normal,  with 
lownthrow  to  the  south,  and  the  dislocation  brings  the  Wallace  for- 
mation against  the  Prichard  slate  near  Osburn — a  relation  that 
mplies  a  throw  of  at  least  4,000  feet. 
