THE   CORDILLERAS.  825 
dition  that  they  presumably  form  the  most  recent  of  the  four  cate- 
gories mentioned. 
These  four  classes  of  rocks  will  now  be  considered  separately: 
Ancient  gneisses  and  schists. — Gneisses  and  schists  of  various  sorts 
and  in  complex  relations,  throughout  similar  to  the  typical  Archean  or 
basal  complex  of  many  other  regions,  are  common  in  Colorado.  The 
gneisses  are  often  of  granitic  composition,  sometimes  more  nearly 
dioritic,  and  dark  amphibole  or  pyroxene  gneisses  and  schists  are  very 
abundant.  In  some  localities  the  dark  basic  rocks  cut  the  more 
siliceous  and  feldspathic  gneisses  in  intricate  fashion,  but  have  been 
greatly  metamorphosed  into  schistose  forms. 
The  origin  of  some  of  the  gneisses  from  igneous  rocks  is  clearly 
shown  in  some  places,  and  it  has  not  been  established  that  any  of 
them  are  metamorphosed  sediments. 
Greenstone  schists  and  allied  rocks. — Ancient  greenstone  schists 
comparable  in  most  respects  with  the  Quinnesec  schist  of  the  Lake 
Superior  region  have  been  found  in  two  districts  of  Colorado.  The 
first  to  be  discovered  was  the  series  of  peculiar  schists  near  Salida 
described  by  Cross.  These  schists  reach  a  thickness  of  nearly  10,000 
feet  and  consist  chiefly  of  schistose  or  gneissoid  rocks  resulting  from 
the  metamorphism  of  basic  igneous  rocks  of  basaltic  or  diabasic 
character.  This  complex  is  not  wholly  basic  and  some  of  the  schists 
were  derived  from  siliceous  lavas  or  intrusives. 
The  second  area  of  basic  schists  is  composed  of  the  Irving  green- 
stone of  the  eastern  Needle  Mountains,  which  is  referred  to  in  another 
place.  These  schists  specially  resemble  the  Lake  Superior  greenstones 
in  petrographic  details  and  in  the  iron  ores  which  were  formerly 
associated  with  them,  as  shown  by  the  pebbles  in  the  succeeding 
sediments. 
It  seems  probable  that  other  areas  of  such  rocks  will  be  found  in 
Colorado,  but  their  direct  correlation  may  be  a  difficult  matter.  The 
basic  rocks  of  the  Encampment  district  in  Wyoming,  described  by 
Spencer,  would  seem  to  belong  to  the  same  general  group  as  those  here 
mentioned. 
Pre-Cambrian  sediments. — The  pre-Cambrian  sediments  of  the 
Needle  Mountains,  Uncompahgre  Canyon,  Rico  Mount  a  in-,  and  Front 
Range  are  preserved  in  such  considerable  thicknesses  as  to  show  that 
once  there  must  have  been  a  very  extensive  development  of  these 
rocks  in  Colorado.  It  must  be  questioned  whether  the  rocks  referred 
to  occasionally  in  the  older  literature  of  Colorado  as  quartzites  or 
sandstones  and  represented  as  in  association  with  gneisses  and  schists 
are  really  of  sedimentary  nature  in  many  cases.  Even  if  they  arc  such, 
it  is  probable  that  they  are  much  older  than  these  distinct  sediments 
which  form  normal  groups  of  conglomeratic,  sandy,  or  shaly  deposits, 
