824  PRE-CAMBRIAN   GEOLOGY  OF   NORTH   AMERICA. 
SUMMARY  OF  PRESENT  KNOWLEDGE. 
COLORADO,  GENERAL. 
The  following  general  summary  is  furnished  by  Whitman  Cross: 
Introductory  note. — In  using  the  term  pre- Cambrian  it  is  to  be 
noted  that  the  only  Cambrian  sediments  known  in  Colorado  belong 
to  the  upper  Cambrian  or  Saratogan  series  and  are  very  thin  in  all 
described  localities.  It  is,  therefore,  not  impossible  that  some  of  the 
massive,  iinmetamorphosed  granites  and  other  igneous  rocks  may  be 
of  early  Cambrian  age. 
General  state  mod. — The  pre-Cambrian  rocks  form  a  large  part  of 
the  Front  and  Park  ranges  and  of  the  Wet,  Sangre  de  Cristo, 
Sa  watch,  and  Needle  (Quartzite)  mountains,  and  they  further  ap- 
pear in  the  canyon  of  the  Gunnison  River  and  locally  beneath  the 
sediments  of  the  Uncompahgre  Plateau.  Their  distribution  is,  in 
the  main,  quite  correctly  shown  by  the  Hayden  map  of  the  State. 
It  was  the  view  of  the  Hayden  geologists,  as  of  most  of  their  con- 
temporaries, that  these  ancient  granites,  gneisses,  and  schists  repre- 
sented highly  metamorphosed  sediments;  the  more  massive  and 
coarsely  crystalline  the  granite  the  more  extensive  the  metamorphism 
was  believed  to  be.  This  idea  was  applied  by  Peale,  for  example,  to 
the  granite  of  Pikes  Peak,  by  Marvine  to  granites  and  gneisses  of 
the  northern  Front  Range,  and  by  Endlich  to  the  granular  rocks  of 
the  Needle  Mountains.  This  conception  was  a  common  one  at  the 
time  of  the  Hayden  Survey,  but  it  has  been  shown  to  be  erroneous, 
so  far  as  the  massive  granites  and  many  of  the  gneisses,  at  least,  are 
concerned. 
Within  the  last  twenty-five  years  the  pre-Cambrian  complex  has 
been  studied  with  some  care  by  various  investigators,  but  in  a  few 
localities  only.  The  numerous  routes  of  travel  now  crossing  the 
mountains  have,  however,  permitted  the  observation  that  the  results 
of  special  studies  apply  in  general  to  the  whole  complex.  The  broad 
conclusion  can,  therefore,  be  drawn  with  safety  that  the  ancient  rocks 
mapped  by  the  Hayden  and  contemporary  geologists  under  the  one 
rubric  "  Metamorphic  "  fall  into  four  main  categories : 
1.  A  complex  of  gneisses  and  schists,  the  oldest  rocks  of  the 
mountains. 
2.  A  complex  of  basic  igneous  rocks,  now  for  the  most  part  changed 
to  greenstone  schists  and  gneisses. 
3.  A  series  of  quartzites,  slates,  schists,  and  conglomerates,  all  of 
undoubted  sedimentary  origin. 
4.  Massive  igneous  rocks,  chiefly  granitic,  occurring  in  large  and 
small  bodies,  cutting  the  gneisses  and  sediments  in  some  places  and 
elsewhere  showing  by  their  comparatively   iinmetamorphosed   con- 
