THE    CORDILLERAS.  847 
ends  of  the  area  treated,  those  in  the  southwestern  part  forming  the 
core  of  the  Bridger  Range.  It  consists  of  gray  and  red  granite, 
mostly  massive,  cut  by  basic  dikes.  Middle  Cambrian  rocks  rest 
unconformably  upon  the  granite. 
Ball,130  in  1007,  describes  the  geology  of  the  Hartville  district  of 
Wyoming.  Steeply  dipping  pre-Cambrian  rocks  are  unconformably 
overlain  by  flat  lying  or  gently  dipping  Carboniferous  and  Mesozoic 
rocks.  The  oldest  rocks  are  an  interbedded  series  of  siliceous  dolo- 
mitic  limestones  and  muscovitic  and  biotitic  schists  of  sedimentary 
origin.  Interbedded  Avith  these  are  some  quartzose  beds  and  jaspers, 
while  a  second  series  of  similar  beds  appears,  on  general  structural 
grounds,  to  lie  unconformably  on  the  interbedded  limestones  and 
schists.  The  next  youngest  formation  consists  of  diorites  and  gabbros 
and  porphyries  of  similar  chemical  composition,  which  in  instances 
are  mashed  into  chlorite  and  hornblende  schist  and  into  hornblende 
gneiss.  An  intrusion  of  granite  (similar  to  Black  Hills  granite) 
followed,  accompanied  by  aplite  and  pegmatite  dikes.  The  pink  or 
gray  coarse-grained  granite  is  sometimes  massive  and  again  is  gneis- 
sic.  Younger  than  the  granite  are  dikes  of  diabase,  a  greenish  gray 
rock  with  small  feldspar  phenocrysts. 
Prior  to  the  deposition  of  the  Carboniferous  series  the  pre-Cam- 
brian rocks  were  folded  into  a  great  syncline  opening  out  to  the  east. 
The  resultant  mountains  and  valleys  were  then  worn  down  to  «  level 
surface  or  plain  by  streams  and  the  weather,  and  on  the  sharply 
upturned  pre-Cambrian  rocks  the  Carboniferous  and  Mesozoic  rocks 
were  deposited. 
Ball,130  in  1907,  describes  the  pre-Cambrian  complex  near  the  large 
dike  of  iron  ore  at  Iron  Mountain,  in  the  east  central  part  of  Albany 
County,  Wyo.,  as  consisting  of  three  granular  igneous  rocks — an 
anorthosite,  the  iron  ore,  and  a  granite.  The  anorthosite  is  the  oldest 
of  these  and  is  cut  by  dikes  and  lenticular  masses  of  iron  ore  and 
granite.  The  relative  age  of  the  iron  ore  and  granite  was  not  cer- 
tainly determined,  since  the  exposures  are  poor  when1  the  two  rocks 
are  close  to  each  other.  All  evidence,  however,  points  to  the  iron 
ore  being  the  older. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
Granitic  and  gneissic  rocks  of  pre-Cambrian  age  form  the  core  of 
the  Cordilleran  range,  running  southeast-northwest  through  Wyo- 
ming,  including  the  Park  or  Sierra  Madre  Mountains  and  the  Medi- 
cine Bow,  Laramie,  Sweetwater,  Wind  River,  Wyoming,  Bridger, 
Gros  Ventre.  Teton,  AJbsaroka,  and  Bighorn  ranges. 
Some  of  the  granites  and  gneisses  constitute  the  base  upon  which 
the  pre-Cambrian  sedimentary  rocks  lie,  and  should  be  assigned  t<> 
