THE    CORDILLERAS.  845 
quartzites,  all  cut  by  dikes  of  eruptive  rocks.  They  occur  in  granite 
basins  in  unconformity  with  the  Archean,  and  form  important  bands 
in  numerous  localities.  The  strike  of  the  series  varies  from  north 
to  northeast,  and  the  dip  of  the  strata  is  seldom  less  than  65°  to  75°. 
The  thickness  of  the  entire  series  has  not  been  absolutely  measured. 
but  including  the  eruptive  band,  which  does  not  form  an  important 
part,  the  maximum  thickness  in  Wyoming  is  about  20,000  feet.  Typ- 
ical areas  have  been  found  in  the  Black  Hills  in  Wyoming,  and 
occasional  outcrops  from  that  place  to  the  Ilartville  Hills — one  ex- 
posure being  east  of  Lusk,  another  at  Rawhide  Butte,  and  a  large 
one  in  Whalen  Canyon.  Algonkian  rocks  also  occur  at  Halleck  Can- 
yon, Plumbago  Canyon,  in  the  Medicine  Bow  Mountains,  nearly  all 
of  the  Sierra  Madre,  in  the  Seminoe  Mountains,  and  in  the  Sweet- 
water mining  district  of  the  Wind  River  Range.  None  of  these  lo- 
calities have  been  examined  in  detail,  but  it  is  thought  that  these  rocks 
were  at  one  time  sedimentary  and  that  they  have  been  changed  by 
metamorphism  to  schists.  Tn  the  Sweetwater  district*the  rocks  are 
chiefly  schists,  but  there  are  many  dikes  of  eruptive  rock  which  fol- 
low the  strike  of  the  formation. 
Smith  (W.  S.  Tangier)  ,1L>r'  in  1903,  describes  the  geology  of  the 
Hartville  quadrangle,  in  Wyoming.  The  Whalen  group,  assigned  to 
the  Algonkian,  consists  of  gneisses,  schists,  quartzites,  and  limestones, 
all  very  schistose,  the  schistosity  standing  nearly  vertical.  These  rocks 
occur  principally  in  the  northeastern  part  of  the  quadrangle.  Quartz- 
ites and  micaceous  schists  form  the  greater  part  of  the  exposed  rocks 
of  the  Whalen  group,  and  in  places  they  grade  into  each  other,  so  that 
no  definite  separation  can  be  made.  Some  of  the  quartzites  are  more 
or  less  calcareous.  Iron  ore  occurs  within  and  near  the  contact  of  the 
limestones  and  schists  of  the  Whalen  group  on  the  west  side  of 
Whalen  Canyon.  Information  at  hand  is  not  sufficient  for  a  deter- 
mination whether  there  are  several  ore-bearing  horizons  or  a  single 
horizon  repeated  by  folding.    Ore  is  being  mined  at  Sunrise. 
Spencer,126  in  1904,  discusses  the  geology  of  the  Encampment  dis- 
trict of  Wyoming.  Pre-Cambrian  rocks  form  the  main  mas>  of  the 
Sierra  Madre,  with  Mesozoic  beds  dipping  away  from  them.  They 
comprise  sedimentary  and  igneous  rock's.  The  sedimentary  rocks  are, 
from  the  base  up:  Hornblende  schists,  derived  from  surface  volcanic 
rock's,  interbedded  with  thin  but  persistent  beds  of  sandy  shale  and 
impure  limestone,  limestone,  quartzite.  slate,  and  conglomerate.  In 
the  Encampment  area  the  quartzite  and  slate  formation  i-  more  in 
evidence  than  any  other  of  the  bedded  rocks,  but  all  occur  in  a  lim- 
ited area  having  the  form  of  a  narrow  triangle,  with  its  apex  on  the 
Encampment  River  about  .">  miles  south  of  Encampment,  and  its 
base,  about  7  miles  wide,  in  the  foothills  on  the  wesl  side  of  the  range. 
The  belt  of  quartzites  and  associated  strata   i^  exposed   for  about  20 
