848  PRE-CAMBRIAN    GEOLOGY    OF    NOBTH    AMEBICA. 
the  Archean.  Such  older  gneisses  have  been  discriminated  from  later 
intrusives  only  in  limited  areas. 
Pre-Cambrian  sedimentary  rocks,  including  quartzite,  conglom- 
erate, slate,  marble,  schist,  and  gneiss,  cut  by  a  variety  of  intrusives, 
occur  in  occasional  outcrops  between  the  Black  Hills  of  South  Dakota 
and  the  Hartville  district,  on  both  sides  of  the  Laramie  Mountains, 
in  the  Hartville  district,  in  Medicine  Bow  Mountains,  in  the  Sierra 
Madre  or  Park  Mountains,  in  the  Seminoe  Mountains,  in  the  Sweet- 
water mining  district  of  the  Wind  River  Range,  and  on  Mount  Sheri- 
dan in  the  southern  part  of  Yellowstone  Park.  They  are  mapped  in 
most  detail  in  the  Hartville  and  Grand  Encampment  areas.  (See 
Ball's  and  Spencer's  accounts.) 
The  reference  of  these  rocks  to  the  pre-Cambrian  is  based  on  the 
fact  that,  except  in  a  few  localities  where  there  is  known  to  be  over- 
lap, they  are  overlain  by  deposits  of  middle  Cambrian  age,  with  an 
intervening  unconformity  that  indicates  a  very  long  period  of  time. 
The  hornblende  schists  derived  from  surface  volcanics  in  the  En- 
campment district  are  also  provisionally  referred  to  the  Algonkian. 
They  are  believed  by  Spencer  to  be  the  basement  upon  which  the  sedi- 
mentary rocks  of  this  district  were  deposited,  but  they  are  so  corre- 
lated because  of  their  similarity  to  the  Irving  greenstone  of  Colorado, 
which  has  been  provisionally  called  Algonkian.  They  are  similar 
also  to  greenstones  and  green  schists  in  both  the  Archean  and  the 
Algonkian  of  the  Lake  Superior  region. 
The  difficulty  of  determining  the  relations  of  the  granites  and 
gneisses  to  the  pre-Cambrian  sediments  is  illustrated  by  Van  Hise's 
description  of  contacts  in  the  Medicine  Bow  Mountains.  The  Algon- 
kian elastics  of  Medicine  and  Mill  peaks  of  the  Medicine  Bow  Moun- 
tains consist  of  slates  and  slate  conglomerates  bearing  granite  pebbles, 
above  which  are  thick  layers  of  quartzite,  and  above  these  are  cherts 
and  cherty  limestones.  These  rocks  appear  to  be  in  isoclinal  folds 
overturned  toward  the  west.  The  conglomerates  are  much  mashed 
and  the  quartzites  approach  quartz  schists.  On  the  east  side  of  the 
mountain  one  finds,  in  passing  from  the  Archean  toward  the  Algon- 
kian, that  the  granite  becomes  less  plentiful  and  the  gneiss  more  lam- 
inated, grading  into  banded  gneiss,  which  appears  to  change  by 
imperceptible  stages  into  fine-grained  green  schist,  and  finally  into 
black  slate.  On  the  west  side,  at  the  base  of  the  sedimentary  series, 
are  found  the  slate  conglomerates  which  bear  granite  pebbles.  It  is 
believed  that  the  facts  are  best  explained  by  regarding  the  Algonkian 
clastic  rocks  of  the  Medicine  Bow  Range  as  unconformable  upon  the 
Archean.  The  apparent  gradations  are  on  the  east  side  of  a  fold  with 
an  eastward-dipping  axial  plane.  Therefore  the  junction  between  the 
two  series  is  here  a  horizon  of  great  slipping,  and  one  where  the 
