846  PRE-CAMBPJAN   GEOLOGY   OF    NORTH    AMERICA. 
miles,  but  on  the  west  their  extent  is  not  known,  since  they  are  over- 
lapped by  younger  formations.  The  rocks  within  the  sedimentary 
belt  strike  in  general  nearly  east- west,  and  they  seem  at  first  sight  to 
have  an  enormous  thickness,  since  they  dip  almost  invariably  toward 
the  south.  An  examination  shows  the  sediments  to  be  in  an  east- west 
synclinorium,  with  axial  planes  of  both  major  and  minor  folds  dip- 
ping to  the  south.    Strike  faults  and  transverse  faults  are  common. 
A  complex  of  igneous  rocks  comprising  granite,  quartz,  diorite,  and 
gabbro  is  found  both  north  and  south  of  the  synclinorium,  and  the 
gabbro  occurs  also  within  the  synclinorium.  The  relations  of  the 
granite  and  quartz  diorite  to  the  sediments  are  not  definitely  known, 
but  their  distribution  is  such  as  to  suggest  that  they  are  intrusive  into 
the  hornblende  schists  at  the  base  of  the  sedimentary  series,  and  that 
with  the  hornblende  schists  they  form  the  basement  upon  which  the 
sedimentary  rocks  were  deposited.  The  gabbro  is  intrusive  into  the 
sediments. 
Darton,127  in  1905,  describes  and  maps  the  geology  of  the  Sun- 
dance quadrangle,  in  Wyoming  and  South  Dakota.  Mica  schists,  be- 
lieved to  be  of  sedimentary  origin,  assigned  to  the  Algonkian,  similar 
in  most  respects  to  those  which  outcrop  in  the  central  part  of  the 
Black  Hills,  appear  in  several  irregular  areas  in  the  Nigger  Hill  up- 
lift. A  portion  of  them  appear  to  be  in  place,  upturned  in  the  center 
of  the  uplift,  but  other  portions  are  included  in  the  younger  igneous 
rocks  which  apparently  completely  surround  them.  These  schists  are 
penetrated  by  intrusive  rocks  of  various  kinds,  including  granites, 
amphibolites  of  Algonkian  age,  and  other  rocks.  In  the  Bear  Lodge 
uplift  large  masses  of  probable  Algonkian  granites  are  inclosed  in  the 
trachytes. 
Smith  (W.  S.  Tangier),128  in  1905,  describes  Algonkian  granite 
found  on  the  Bear  Lodge  Mountains  in  the  southwestern  portion  of 
the  Aladdin  quadrangle,  mapped  by  Darton  and  O'Harra.  The 
granites  occur  as  large  fragments  in  later  igneous  rocks. 
Darton,77  in  1905,  in  connection  with  a  discussion  of  the  artesian 
wells  of  the  central  Great  Plains,  maps  and  describes  the  pre-Cam- 
brian  rocks  of  eastern  Wyoming.  This  is  an  excellent  summary  of 
the  present  state  of  knowledge  of  these  rocks,  especially  in  regard  to 
distribution. 
Darton,129  in  1906,  maps  and  describes  the  geology  of  the  Bighorn 
Mountains,  lying  principally  in  the  Bald  Mountain,  Dayton,  Cloud 
Peak,  and  Fort  McKinney  quadrangles,  in  Wyoming.  Pre-Cambrian 
granite  forms  the  core  of  the  mountains.  The  granite  outcrops  in 
the  west-central  and  southern  parts  of  the  Dayton  quadrangle,  in 
isolated  areas  in  the  Bald  Mountain  quadrangle,  in  the  eastern  part 
of  the  Cloud  Peak  quadrangle,  in  the  western  part  of  the  Fort  Mc- 
Kinney quadrangle,  and  in  isolated  areas  at  both  the  north  and  south 
