770  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
But  distinct  unconformity  may  be  asserted  as  existing  in  the  vicinity 
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  Laurentian  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  granite,  or  granite  conglomerate,  immediately  under- 
lying the  Carboniferous  at  many  localities  may  possibly  be  of  some- 
what later  origin. 
Keyes,12  in  1905,  discusses  the  fundamental  complex  beyond  the 
southern  end  of  the  Rocky  Mountains  in  New  Mexico.  Most  of  the 
extensive  formations  composed  of  granites,  schists,  and  gneisses,  which 
form  the  axial  foundations  of  so  many  of  the  mountain  ranges  of 
the  region,  are  now  believed  to  be  of  much  later  geological  age  than 
is  generally  understood  to  be  covered  by  the  title  Azoic  or  Archean. 
Criteria  are  discussed  for  the  separation  of  the  Archean  and  Pro- 
terozoic  rocks,  and  emphasis  is  placed  on  the  sedimentary  origin  of 
the  Proterozoic  as  a  criterion  for  its  separation  from  the  Archean. 
Brief  descriptions  are  made  of  the  crystalline  rocks  of  the  Culebra, 
Taos,  Cimarron,  Las  Vegas,  Mora,  Santa  Fe,  Placer  (Ortiz),  Naci- 
miento,  and  Jemez  mountains,  the  Zuni  dome,  the  Sierra  Oscura,  the 
San  Andreas,  Organ,  Caballos,  and  Santa  Rita  mountains  and  the 
Mogollon  uplift. 
Lindgren  and  Graton,13  in  1906,  as  a  result  of  a  reconnaissance 
of  the  mineral  deposits  of  New  Mexico,  state  that  the  northern  half 
of  the  central  mountain  belt — the  Mora,  Taos,  and  Santa  Fe  ranges — 
is  composed  largely  of  ancient  crystalline  rocks,  which  can  without 
hesitation  be  assigned  to  pre-Cambrian  time.  The  cores  of  most  of 
the  ranges  which  constitute  the  southern  part  of  this  belt,  such  as 
the  Sandia,  Magdalena,  Caballos,  and  Franklin  mountains,  are  like- 
wise composed  of  these  rocks,  as  are  also  some  of  the  ridges  in  the 
desert-range  province,  for  example,  the  Burro  Mountains.  Red 
and  gray  gneisses,  which  represent  sheared  granites,  are  most  abun- 
dant, although  many  of  these  rocks  are  only  a  little  foliated,  and 
much  massive  granite  is  present.  Dikes  and  other  intrusive  bodies 
of  diorite,  gabbro,  diabase,  etc.,  have  in  most  cases  suffered  sufficient 
shearing  and  mineral  alteration  to  be  converted  into  dark  schistose 
rocks,  which  may  be  grouped  together  as  amphibolite.  Extreme  folia- 
tion of  the  granitic  gneisses  frequently  results  in  the  production  of 
quartz-sericite  schists,  and  less  commonly  quartz-biotite  schists. 
Some  sedimentary  rocks,  such  as  quartz-pebble  conglomerates,  sand- 
stones, shales,  and  limestones,  also  enter  into  the  pre-Cambrian  com- 
plex and  have  been  metamorphosed  more  or  less  completely  into 
schists.  Dikes  of  pegmatite  and  aplite  cut  many  of  these  foliated 
rocks,  but  are  themselves  pre-Cambrian. 
