fANHisE.]  THE    CORDILLERAS.  315 
mountain  is  composed  of  a  similar  massive  red  granite,  upon  the  slopes 
yi  which  rests  the  Carboniferous  formation,  for  the  most  part  limestone, 
in  many  places  nearly  vertical  yet  but  slightly  metamorphosed. 
Stevenson,55  in  1879,  describes  a  continuous  Archean  area  on  the 
western  side  of  the  district  running  from  Spanish  peaks  south.  It  forms 
the  axis  of  the  Culebra  range,  continues  through  the  Taos  and  the  Mora 
ranges,  and  passes  into  the  Cimarron  range.  The  Santa  Fe  and  United 
States  anticlines  show  Archean  rocks  which  are  separated  from  the 
main  area.  The  rocks  show  great  uniformity  in  character,  including 
gneissoid  granite,  gneiss,  and  mica-schist  as  the  j>redominant  types. 
Stevenson,56  in  1881,  gives  a  systematic  account  of  the  Archean 
rocks  of  southern  Colorado  and  northern  New  Mexico.  Four  areas  are 
seen  within  the  district.  The  most  western  marks  the  course  of  the 
Santa  Fe  axis;  the  second,  that  of  the  Culebra-Mora  axis,  and  the 
third  and  fourth  that  of  the  Cimarron  axis.  The  rocks  in  the  Santa  Fe 
axis  are  gneiss,  mica-schist,  which  resembles  sandstones,  and  granite. 
South  of  the  Santa  Fe  road  are  frequent  exposures  of  an  exceedingly 
aoarse  granite,  which  resembles  a  metamorphosed  conglomerate,  the 
pebbles  being  thoroughly  distinct.  With  this  are  many  beds  of  almost 
black  gneiss,  holding  beds  of  snow-white  quartz.  The  Culebra-Mora 
axis  varies  in  width  from  5  to  25  miles.  It  includes  granite,  gneissoid 
granite,  micaceous  and  hornblendic  schists,  and  quartzites.  Compact 
gneiss,  quartzite-like  in  character,  is  found  in  the  main  canyon  of  Cos- 
tilla creek.  Bands  of  quartzites  are  found  on  Comanche  creek,  the 
north  fork  of  Moreno  creek,  in  Costilla  creek  range  on  Coyote  creek, 
and  in  the  vicinity  of  Santo  Nino  on  the  Cebolla  creek.  These  are 
sometimes  found  in  gneissoid-granite  and  sometimes  in  mica-schist.  The 
granite  below  the  junction  of  the  forks  of  Moreno  creek  is  very  coarse 
and  resembles  conglomerate.  The  rocks  of  the  Cimarron  axis  include 
mica- schist,  coarse  granite,  and  gneiss  sometimes  resembling  quartzite. 
rhe  dips  of  the  Archean  rocks  are  much  contused  and  the  distortion  at 
most  localities  is  so  great  that  neither  the  succession  of  the  strata  nor 
the  general  structure  could  be  made  out  during  the  brief  examinations. 
Positive  proof  of  nonconformability  to  the  overlying  Carboniferous  is  not 
easily  obtained,  the  main  obstacle  in  the  way  of  making  the  determina- 
tion being  the  character  of  the  rock.  Usually  the  disturbance  near 
the  junction  of  the  two  series  is  very  violent  and  the  rate  of  dip  changes 
greatly  within  a  short  distance,  sometimes  becoming  even  reversed. 
But  distinct  nonconformability  may  be  asserted  as  existing  in  the  vicin- 
ity of  Costilla  peak,  where  the  enormously  thick  Carboniferous  series 
terminates  abruptly  against  the  Archean  core  of  the  Cimarron  axis. 
No  absolute  evidence  exists  to  settle  the  age  of  these  rocks.  Litholog- 
ically,  they  bear  a  close  resemblance  to  the  Lauren tian  series  of  the  east, 
and  at  more  northern  exposures  within  the  Rocky  mountain  region  they 
have  been  referred  by  all  observers  to  that  age.  The  coarse  gneissoid 
and  often  conglomerate  granite  immediately  underlying  the  Carbonifer- 
ous at  many  localities  may  possibly  be  of  somewhat  later  origin. 
