MINERAL    DEPOSITS    OF    NEW    MEXICO.  77 
In  the  section  west  of  Silver  City  the  Ordovician  limestones  are  overlain  by  200  or  300 
feet  of  limestone  which  is  conformable  and  similar  to  those  below,  but  which  contains  a 
fauna  that  is  referred  by  Ulrich  to  the  Silurian.  The  upper  portion  of  this  limestone  is  the 
ore-bearing  horizon  which  has  been  explored  by  the  mines  of  Chloride  Flat.  It  may  be  that 
Silurian  rocks  are  more  widely  distributed  in  this  part  of  New  Mexico  than  is  at  present 
known. 
Black  fissile  shales  of  Devonian  age  overlie  the  Silurian  and  Ordovician  limestones.  In 
general  no  stratigraphic  break  is  observable,  but  Doctor  Gordon  is  of  the  opinion  that 
there  is  an  unconformity  between  the  Devonian  and  the  underlying  rocks  at  Hillsboro. 
About  200  feet  of  these  black  shales  are  succeeded  in  some  places  by  aboat  an  equal 
thickness  of  red  shales,  all  of  Devonian  age. 
Mississippian  (Lower  Carboniferous)  strata  have  been  found  only  in  the  southwestern 
part  of  the  Territory  where  the  older  Paleozoic  formations  are  present.  They  consist  of  some 
350  to  500  feet  of  bluish,  mostly  fine-grained  limestone.  Where  Pennsylvanian  rocks  over- 
lie the  Mississippian  they  also  are  mostly  crystalline  limestones  and  attain  a  thickness  of 
several  hundred  feet.  Where  the  Pennsylvanian  beds  rest  directly  on  the  pre-Cambrian 
rocks,  as  is  the  case  in  the  northern  ranges,  they  consist  at  the  bottom  of  red  conglomeratic 
arkose  or  grits  resembling  granite  in  appearance  and  representing  rapid  erosion  and  depo- 
sition. The  upper  portions  are  of  crystalline  limestone,  with  occasional  thin  beds  of 
shales.  Beds  of  coal  of  promising  quality  and  sufficient  thickness  to  allow  mining  have 
been  developed  to  a  small  extent  in  these  shales  along  upper  Pecos  River.  This  is  one  of 
the  few  known  instances  of  Carboniferous  coal  in  the  West.  In  the  northern  part  of  New 
Mexico  these  strata  are  1,000  feet  thick  at  least  and  probably  considerably  more.  Much 
of  the  "  red  beds"  in  the  southern  and  southeastern  part  of  New  Mexico,  hitherto  described 
as  Permian,  is  probably  of  Pennsylvanian  age;  but  in  the  Guadalupe  Mountains,  according 
to  Girty,  occurs  a  series  of  rocks  which  may  represent  the  Permian,  and  a  few  imperfect 
fossils  collected  at  Lone  Mountain  near  Silver  City  are  said  by  Girty  to  suggest  a  "Permo- 
Carboniferous "  horizon,  although  faulting  and  igneous  intrusions  make  the  stratigraphy 
uncertain. 
The  greater  part  of  the  red  sandstones  and  shales  which  cover  the  Pennsylvanian  lime- 
stone in  the  northern  half  of  the  Territory  have  generally  been  referred  to  the  Jurassic  and 
Triassic  systems,  although  little  or  no  proof  is  known  that  they  belong  there.  The  lower 
strata  are  coarser  and  more  thickly  bedded,  while  the  upper  portion  is  generally  composed 
of  rather  thinly  bedded  fine-grained  sandstones  and  shales.  Some  of  the  beds  are  gypsif- 
erous.  An  interesting  feature  in  regard  to  these  "  red  beds  "  is  the  fact,  evidenced  in  a  num- 
ber of  places,  that  they  were  originally  drab  or  olive-colored  rocks  and  that,  contrary  to  the 
general  belief,  their  red  color  has  been  developed  by  oxidation  since  their  deposition  and 
consolidation.  Yellowish  and  olive  sandstones  overlying  Pennsylvanian  limestones  in  the 
southwestern  part  of  New  Mexico  may  also  belong  here.  Step  faulting  has  been  prominent 
in  most  places  where  these  rocks  were  observed  and  makes  difficult  any  estimate  of  their 
thickness. 
In  the  northern  and  eastern  parts  of  New  Mexico  Cretaceous  beds  have  long  been  known. 
Sandstones  and  shales,  belonging,  respectively,  to  the  Dakota  and  Colorado  formations, 
are  overlain  by  the  coal  measures  which  are  supposed  to  be  of  Laramie  or  Fox  Hills  age.  In 
the  southwestern  part  of  the  Territory,  Cretaceous  rocks  have  not  heretofore  been  known, 
but  in  dark  shales  some  hundreds  of  feet  thick  occurring  just  west  of  Silver  City,  fossils 
have  been  found  which  have  been  identified  by  Stanton  as  within  the  Benton  group  of  the 
Cretaceous.  Similar  beds  were  found  farther  to  the  northwest,  and  all  of  them  are  probably 
of  the  same  horizon  as  those  described  from  the  Clifton  quadrangle,  Arizona. a 
Partially  consolidated  gravels,  sands,  and  marls  occur  as  wash  plains  and  immense  allu- 
vial fans  at  the  foot  of  the  higher  elevations.  Some  Tertiary  fossils  have  been  found  in* 
these  beds  in  the  vicinity  of  Santa  Fe,  and  most  of  such  deposits  are  accordingly  referred 
to  the  Tertiary  system,  while  some  probably  belong  to  the  Quaternary. 
a  Lindgren,  W.,  Geologic  Atlas  U.  S.,  folio  129,  U.  S.  Geol.  Survey,  1905,  p.  5. 
