82  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
Economically  they  arc  not  very  important  and  have  contributed  but  little  to  the  metal 
production  of  the  Territory,  though  some  low-grade  bodies  may  be  developed  into  impor- 
tant mines.     Placer  deposits  are  connected  with  some  of  them,  but  t  hey  are  rarely  rich. 
POST-CRETACEOUS  DEP<  >SITS. 
Almost  the  entire  metal  production  is  obtained  from  deposits  of  post-Carboniferous  agel 
li  is  possible  that  some  of  these  are  late  Mesozoic,  but  all  of  them  concerning  which  we  have 
definite  knowledge  are  later  than  the  Benton  (Cretaceous)  and  range  in  age  almost  up  to 
the  present  time.  Wit  hout  fear  of  serious  error  they  may  be  referred  to  as  post-Cretaceous. 
One  of  the  most  positive  results  of  this  reconnaissance  consists  in  the  accumulated  evidence 
of  their  geographical  and  genetical  connection  with  the  great  igneous  activity  which  mani- 
fested itself  in  this  Territory  from  the  earlier  part  of  the  Tertiary  up  to  a  recent  date. 
The  deposits  arc  divided  into  two  distinct  series.     The  first  and  earlier  is  directly  con- 
nected with  great  intrusive  bodies  of  acidic  porphyries  and  granitic  rocks,  and  consists  of  ji 
contact-metamorphic  deposits,  fissure  veins,  and  replacement  deposits  in  limestone.     The 
second  and  later  is  connected  with  the  Hoods  of  lavas  which  overwhelmed  many  areas  of  this 
Territory,  and  consists  of  fissure  veins,  probably  also  replacement  deposits  in  limestone. 
EARLIER   SERIES. 
The  deposits  connected  with  the  intrusive  rocks  have  thus  been  separated  from  those 
which  are  in  some  way  allied  to  the  surface  flows.  It  is  recognized  that  in  some  districts, 
like  southwestern  Colorado,  erosion  has  exposed  the  plutonic  rocks  which  were  intruded  at 
lower  levels  at  the  time  of  the  effusion  of  the  lavas  at  the  surface.  In  New  Mexico  there  is 
no  evidence  of  such  deep  trenching  of  the  Tertiary  lavas. 
The  intrusive  stocks  of  porphyries  and  granites  are  distinctly  older  than  the  surface  (lows 
and  their  irruption  took  place  when  probably  the  whole  of  New  Mexico  was  covered  by 
thick  horizontal  Cretaceous  sediments,  forming  a  pliable  am!  plastic  sheet,  underneath  which 
the  conditions  were  present  for  the  formation  of  laccoliths  and  stocks.  Whether  the  magma 
broke  through  this  covering  or  not  is  not  known.  A  long-continued  erosion  in  many  places 
exposed  these  int  rushes  and  after  this  had  been  done  renewed  igneous  activity  followed  and 
surface  lavas — andesites,  rhyolites,  and  basalts — covered  large  areas.  The  writers  would 
refer  the  earlier  epoch  of  intrusion  to  the  early  Tertiary  and  the  latter  epoch  of  effusion  to 
the  middle  or  laic  Tertiary. 
A  review  of  the  deposits  of  early  Tertiary  age  involves  a  mention  of  all  the  important 
im  rusive  stocks  of  the  Territory. 
To  begin  from  the  north,  Colfax  County  contains  a  stock  of  syenitic  or  granite  porphyry, 
intruded  in  upper  Cretaceous  and  earlier  terrancs.  Contact-metamorphic  deposits  mark  its 
periphery  at  certain  places,  probably  where  the  Colorado  shales  are  cspecialby  calcareous. 
Garnet,  epidote,  hematite,  and  magnetite  are  the  gangue  minerals  and  are  accompanied  by 
small  amounts  of  chalcopyrite  and  in  some  cases  by  good  values  in  gold.  The  same  phe- 
nomena are  repeated  in  the  Cimarroncito  district  a  few  miles  to  the  south,  where  copper  is  the 
predominant  metal.  The  igneous  rock, as  well  as  the  Cretaceous  rocks  adjoining,  are  cut  by 
a  great  number  of  small  auriferous  veins,  which,  though  not  often  workable,  have  enriched 
with  placer  gold  practically  all  the  branches  of  the  Cimarron  which  head  in  this  area. 
At  Cerrillos,  in  Santa  Fe  County,  similar  intrusive  rocks  form  a  smaller  stock  in  upper 
Cretaceous  or  Laramie  rocks,  the  sandstone  and  shale  of  which  have  proved  less  suitable  for 
contact  metamorphism.  The  stock  is  traversed  by  a  number  of  northeast  veins  which 
carry  zinc  blende  and  argentiferous  galena  and  in  places  extend  for  some  little  distant 
out  into  the  sedimentary  rocks. 
The  Ortiz  and  San  Pedro  mountains  are  small  isolated  groups  a  short  distance  south  of 
Cerrillos  and  also  in  Santa  Fe  County.  Both  form  laccolithic  intrusions  in  Cretaceous  and 
older  rocks.  Along  the  contact  of  the  porphyry  with  the  shales  baking  and  contact  metal 
morphism  is  observed.  Along  the  south  contact  of  the  Ortiz  Mountains,  where  calcareous 
Cretaceous  strata  adjoin  the  porphyry,  low-grade  contact-metamorphic   deposits  occur, 
