42  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
The  Archean  rocks,  ancient  looking  and  much  metamorphosed,  are  separated  from  tl 
later  and  fresher-looking  Algonkian  series  by  a  great  unconformity. 
Younger  igneous  rocks  have  cut  the  sediments  of  the  district  as  basic  dikes  and  sills,  ar 
vast  chambers  have  been  sloped  out  and  occupied  by  intrusions  of  granite. 
In  the  structure  of  the  district  many  large  normal  faults  were  noted.  A  few  great  fol< 
were  observed,  but  in  general  monoclinal  blocks  prevail. 
Routes 
Faults 
I  Upthrown  S'de  of  faults 
J—  Downtr,rown  vde  of  faults 
I         Direction  o-f  d.p  of  fault- 
'  planes(ior  ei*/ays  hnotvn) 
SCALE    OF    WaCS 
Fig.  2.— Outline  map  of  northern  Idaho  and  northwestern  Montana. 
STRATIGRAPHY. 
ARCHEAN. 
The  Archean  rocks  are  well  shown  on  the  west  shore  of  Cceur  d'Alene  Lake.  Here  sedi- 
ments, now  in  the  form  of  highly  crystallized  schists,  had  been  penetrated  by  numberless 
dikes  and  intrusions  of  granite,  later  changed  to  typical  gneiss.  These  intrusions  probabl) 
took  place  early  in  the  geological  history  of  the  formation  and  long  before  extensive  meta- 
morphism  crystallized  the  whole  into  its  present  form. 
ALGONKIAN    AND   PALEOZOIC. 
Cceur  d'Alene  section. — A  good  idea  of  the  Algonkian  formations  of  the  district  can  be 
given  by  briefly  describing  the  Coeur  d'Alene  series  and  using  this  as  a  basis  of  comparisor 
for  the  other  Algonkian  rocks. 
