NORTHERN  IDAHO  AND  NORTHWESTERN  MONTANA. 
Tabular  section  of  the  Cmur  d'Alene  series. 
43 
No 
Name. 
Striped  Peak  formation 
Wallace  formation 
St.  Regis  formation 
Revett  quartzite 
Burke  formation 
Prichard  slate. 
Description. 
Sandstones,  siliceous,  generally  flaggy  to  shaly;  colors 
mostly  green  and  purple;  characterized  by  shallow- 
water  features,  as  ripple  marks,  sun  cracks,  etc. 
Thin-bedded  sandy  shales,  underlain  by  rapidly  alter- 
nating thin  beds  of  argillite,  calcareous  sandstone, 
impure  limestone,  and  indurated  calcareous  shale; 
these  underlain  in  turn  by  green  siliceous  argillites. 
Shallow-water  features  throughout.  Slaty  cleavage 
common. 
Sandstones,  generally  flaggy  or  shaly;  usually  fine 
grained  and  much  indurated;  colors  mostly  "green 
and  purple;  characterized  by  shallow-water  features. 
White  quartzites,  generally  rather  thick  bedded ;  inter- 
stratified  with  subordinate  quantities  of  micaceous 
sandstone. 
Gray,  flaggy,  fine-grained  sandstones  and  shales,  with 
interbedded  purple  quartzitic  sandstone  (the  pro- 
portion varies  widely  in  different  parts  of  the  dis- 
trict) and  white  quartzite.  The  formation  charac- 
terized throughout  by  shallow-water  features. 
Mostly  blue-black,  blue-gray  to  light-gray  slates,  gen- 
erally distinctly  banded.  Considerable  interbedded 
gray  sandstone.  Upper  portion  characterized  by 
rapid  alternations  of  argillaceous  and  arenaceous 
layers,  and  by  shallow-water  features.  Base  not  ex- 
posed. 
Approxi- 
mate 
thickness. 
Feet. 
1,000+ 
4,000 
1,000 
1,200 
2,000 
,  000+ 
17,200 
Modifications  ofCveur  d'Alene  section. — Most  of  the  beds  forming  this  section  vary  greatly 
toward  the  north  and  east;  some  of  them  entirely  disappear,  others  grow  much  thinner, 
and  the  remaining  ones  get  somewhat  thicker.  Moreover,  new  rocks  make  their  appear- 
ance both  above  and  below  the  Cceur  d'Alene  series. 
The  rock  which  gradually  takes  the  place  of  the  Prichard  slate  toward  the  north  is  a  gray, 
siliceous,  fine-grained  quartzite  which  weathers  brownish  gray.  It  is  well  shown  north  of 
Sylvanite  and  has  been  named  Creston  quartzite  by  Daly  of  the  Canadian  Survey.  Toward 
the  east,  however,  the  Pritchard  slate  was  observed  to  outcrop  throughout  the  30  miles 
between  Plains  and  Dixon. 
The  Burke  formation  is  essentially  the  same  in  composition  throughout  the  district  under 
survey.  The  Revett  quartzite  was  not  observed  north  of  the  Kootenai  nor  east  of  Missoula 
River.  The  St.  Regis  red  and  green  beds  were  not  well  defined  north  of  the  Cabinet  Moun- 
tains. Toward  the  east  they  were  traced  to  Iron  Mountain,  Montana.  The  Wallace  forma- 
tion is  much  thicker  toward  the  Cabinet  Mountains.  Here  too,  as  well  as  toward  the  east, 
it  contains  a  higher  percentage  of  lime.  What  appeared  to  be  Striped  Peak  sediments  were 
found  at  intervals  toward  the  north,  but  could  not  be  definitely  correlated. 
Limestone  beds  of  considerable  extent  occur  around  the  south  end  of  Lake  Pend  Oreille. 
They  are  thought  to  have  been  deposited  later  than  the  Coeur  d'Alene  rocks,  but  were  not 
definitely  correlated,  as  all  their  observed  boundaries  were  faults  or  igneous  rocks. 
Above  the  Coeur  dAlene  series  a  great  thickness  of  green  and  gray  argillaceous  shales, 
overlain  by  limy  brownish-yellow  weathered  argillite,  has  been  developed.  This  is  well 
shown  in  the  Cabinet  Mountains  north  of  Bull  Lake. 
Other  limestones  occur  between  Libby  and  Jennings  and  on  Swamp  Creek  southeast  of 
Libby.  These  latter  beds  are  quite  pure  and  are  associated  with  red  and  green  shales. 
Near  Lothrop,  on  Missoula  River, 'limestone  beds  were  also  found,  containing  traces  of  what 
may  be  crustacean  remains.  More  detailed  study  may  prove  these  limestones  to  be  of 
Cambrian  or  Ordovician  age. 
