THE   CORDILLERAS.  811 
Pegmatites  of  different  ages  are  abundantly  represented.  These 
are  intimately  associated  with,  and  probably  genetically  connected 
with,  most  of  the  various  igneous  injections  enumerated.  The  most 
abundant  of  these  is  younger  than  all  of  the  above-described  forma- 
tions, and  is  associated  closely  with  and  grades  into  granitic  and 
aplitic  facies.  Rare  minerals  are  not  common  in  the  pegmatites, 
although  tourmaline,  apatite,  and  garnet  occur.  Magnetite  is  also 
frequent  in  the  pegmatite.  While  usually  associated  in  the  pegmatite 
with  biotite  and  less  often  with  muscovite,  magnetite  may  be  the  only 
femic  mineral.  It  sometimes  occurs  in  quartz  veins  of  pegmatitic 
origin.  In  places  magnetite  forms  masses  4  inches  in  diameter  and 
may  constitute  one-third  of  the  pegmatite. 
At  three  places  in  the  quadrangle  residuals  of  a  well-sorted  quartz- 
itic  sandstone  were  seen.  A  lithologically  similar  sandstone  occurs 
in  place  south  of  the  quadrangle,  on  the  North  Fork  of  the  South 
Platte,  above  Shawnee.  Beyond  the  fact  that  this  sandstone  con- 
tains pebbles  of  the  coarse  gray  granite,  its  age  is  not  known. 
The  gneisses  and  plutonics  are  cut  by  dike  rocks,  which  are  pre- 
sumably post-Cambrian. 
Fenneman,76  in  1905,  describes  briefly  the  distribution  and  struc- 
ture of  Algonkian  quartzite  of  South  Boulder  Canyon,  in  the  Front 
Range  of  Colorado.  Van  Hise  is  followed  in  regard  to  relations  to 
the  Archean. 
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  Colorado. 
Lindgrex  and  Ransome,78  in  1906,  map  and  describe  the  geology 
of  the  Cripple  Creek  district  of  Colorado.  They  show  a  number  of 
modifications  of  the  earlier  map,  and  further  subdivide  the  pre- 
Cambrian  rocks  of  the  district.  The  most  ancient  rocks  in  the  dis- 
trict are  fibrolitic  muscovite  schists  and  fine-grained  granitic  gneisses, 
the  former  being  possibly  derived  from  sediments  and  the  latter  prob- 
ably derived  from  granite.  The  gneisses  are  typically  exposed  in  the 
streets  of  Cripple  Creek  and  at  the  terminal  station  of  the  Colorado 
Springs  and  Cripple  Creek  District  Railway.  The  schists  may  be 
well  seen  in  Poverty  Gulch  between  the  Abe  Lincoln  mine  and  the 
railway  trestle  bridge,  and  near  the  station  of  the  Florence  and  Crip- 
ple Creek  Railroad  in  Cripple  Creek.  This  gneiss  was  not  separately 
shown  on  the  older  map,  being  included  partly  with  the  granites, 
which  were  mapped  as  a  unit,  and  partly  with  the  schist.  Intrusive 
into  the  gneisses  and  schists  is  the  Pikes  Peak  granite,  prevailing 
over  a  large  area  in  the  district,  especially  in  the  vicinity  of  Squaw 
Mountain  and  Victor.  A  still  later  intrusive  i<  the  Cripple  Creek 
granite,  which  occupies  a  considerable  area  extending  westward  from 
Anaconda  beyond  the  bounds  of  the  area  studied,  and  well  exposed 
