76  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
THE  ROCKS. 
The  geological  features  of  New  Mexico  are  extremely  varied.  Beginning  with  the  old 
pre-Cambrian  basement,  which  here,  as  to  the  north,  constitutes  the  cordilleran  protaxis  of 
the  continent,  rocks  of  almost  every  period  up  to  the  most  recent  deposits  are  known  some- 
where in  the  Territory.  The  three  great  divisions  of  rocks — igneous,  sedimentary,  and 
metamorphic — are  abundantly  represented. 
PRE-CAMBRIAN    COMPLEX. 
The  northern  half  of  the  central  mountain  belt  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  likewise  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  abundant,  although  many  of  these 
rocks  are  only  a  little  foliated,  and  much  massive  granite  is  present.  Dikes  and  other  intru- 
sive bodies  of  dioritc,  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  foliation  of  the  granitic  gneisses  frequently  results  in 
the  production  of  quartz-sericitc  schists,  and  less  commonly  quartz-biotite  schists.  Some 
sedimentary  rocks,  such  as  quartz-pebble  conglomerates,  sandstones,  shales,  and  limestones, 
also  enter  into  the  pre-Cambrian  complex  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. 
SEDIMENTARY    ROCKS. 
On  the  basement  of  old  crystalline  rocks  thick  sediments  of  Paleozoic,  Mesozoic,  and 
Cenozoic  age  have  been  deposited  and  have  suffered  little  or  nothing  from  the  effects  of 
regional  metamorphism.  In  the  north  half  of  the  Territory,  where  the  ancient  crystalline 
rocks  are  overlain  by  sediments,  the  Carboniferous  strata  rest  unconformably  upon  the 
pre-Cambrian.  The  most  widely  distributed  sedimentary  formations  are,  therefore,  Car- 
boniferous or  younger,  and  the  older  Paleozoic  strata  have  generally  been  considered 
absent,  although  Cambrian  a  and  Silurian  b  have  been  reported.  Representatives  of  each 
age  from  Cambrian  to  Tertiary,  inclusive,  are  now  known  to  be  present^  and  in  the  south- 
western part  of  New  Mexico  Cambrian,  Ordovician,  Silurian,  and  Devonian  strata  separate 
the  Carboniferous  beds  from  the  pre-Cambrian.  In  general  these  strata  succeed  each  other 
without  apparent  unconformity  from  the  beginning  of  deposition  of  sediments  to  the  period 
of  eruption  at  the  close  of  the  Cretaceous  system.  The  geological  section  is  by  no  means 
complete,  however,  and  in  some  cases  actual  unconformities  have  been  observed.  These 
will  be  mentioned  specifically  under  structure. 
Quartzites,  often  of  dark  color  because  of  iron  stain,  containing  thin  bands  of  sandy  shale 
and  siliceous  limestone,  are  found  resting  upon  the  pre-Cambrian  gneisses  in  numerous 
places  in  the  southwestern  part  of  the  Territory.  This  formation  ranges  in  thickness  at 
various  places  from  about  100  feet  to  over  1,000.  In  a  shaly  layer  from  the  Caballos  Moun- 
tains Doctor  Gordon  has  found  Lingulella  shells,  which  unquestionably  define  this  quart  zite 
horizon  as  Cambrian. 
Overlying  the  Cambrian  quartzite  without  break  are  600  to  1,000  feet  of  limestone,  very 
cherty  in  the  lower  half  and  often  much  silicified.  Fossil  fragments  suggest  that  the  lower 
part  may  be  Cambro-Ordovician,  while  good  specimens  from  higher  horizons  are  certainly 
Ordovician.  This  formation  has  been  found  in  the  Caballos  and  Florida  mountains,  .it 
Kingston,  Hillsboro,  Cooks  Peak,  Silver  City,  and  probably  occurs  in  the  Georgetown 
mining  district. 
a  Yung  and  McCaffery,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  33,  1003,  p.  354. 
'-(lurk,  Ellis,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  24,  1804,  pp.  138-107. 
cCf.  Gordon  and  Graton,  Am.  Jour.  Sci.  May,  1000. 
