242 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
GEOLOGY. 
In  a  broad  way  the  Durango-Gallup  coal  field  is  a  huge  synclinal  basin,  with  a  long 
from  north  to  south  of  130  miles  and  a  breadth  from  easl  lowest  of  about  100  miles.  Tl 
coal-bearing  rocks  form  the  rim  of  this  bas!n  and  dip  toward  the  center  at  angles  vary! 
from  a  small  amount  up  to  70°  or  80°.  The  area  oft  lie  basin  is  approximately  12,000  squa 
miles,  and  probably  it  is  all  underlain  by  coal-bearing  rocks,  but  in  the  center  of  the  bas 
these  are  deeply  covered  by  younger  Tertiary  rocks,  except  where  their  regular  dips  a 
disturbed  by  local  folds  or  where  they  are  exposed  by  the  erosion  of  the  deeper  vallej 
Although  the  area  of  i  he  Geld  is  thus  put  at  12,000  square  miles,  it  must  be  understood  t li. 
in  only  a  small  part  of  this  area  is  the  eoal  near  enough  to  the  surface  to  be  workable. 
The  coal-bearing  rocks  consist  of  shales,  sandstones,  clays,  and  a  few  limestone  lentils  ar 
concretions,  and  they  comprise  five  groups  or  formations  of  Upper  Cretaceous  age.  Who 
upturned,  even  at  gent  le  angles,  as  between  Monero  and  Dulce,  near  Elvado,  at  La  Plata,  ai 
at  Gallup,  they  form  hogback  ridges,  frequently  rising  several  hundred  feei  above  the  valla 
which  occupy  the  intervening  areas  of  softer  shale.  The  generalized  section  of  the  c<9 
bearing  rocks  is  as  follows: 
G(  ru  i  alia  d  si  ction  of  coal  and  lign  iti  bearing  rocks  of  the  Durango-Gallup  field. 
NORTHERN    PART. 
Formation. 
Character. 
Thickness 
Feeti 
700-1,01 
2,000  '-',51 
400-1,01 
1 ,  200 
Friable  gray  sandstone  wit  li  sandy  shales,  containing  va  ria- 
ble  beds  of  black  lignite  (in  all  of  the  area  mapped  except- 
ing the  bell  southward  from  Montezuma  ('reek,  north  of 
Juanita,  Colo.)  and  massive  friable  gray  sandstone  at 
Sandj  greenish  and  drab  shales,  with  sandy  clays,  contain- 
ing occasional  thin  layers  of  impure  limestone  and  concre- 
tionary masses  ai  differenl  horizons. 
shales  witn  interbedded  whitish  sandstone  and  variable 
beds  of  coal  (workable  in  the  Durango,  Monero,  and  Naci- 
miento  dist  rid  -  '  w  it  h  massive  gray  sandstone  at  base. 
Sofl   dark  gray  or  drab  to  almost   black  clay  shales',  with 
thin  lenses  and  concretions  of  limestone.    It  includes  near 
the  base  an  impure  limestone  bed  3  feet  thick,  which  out- 
crops between  Horse  Lake  and  the  west  fronl  of  Jemez 
peak.  New  Mexico,  and  which  is  the  representative  of  the 
i .  reenhorn  limestone. 
Lew  is  shale"  |  Montana) .... 
Mesa\  erde  a  \  Montana  i 
Ma  qcos    sha  te  a  i  lower  pa  n 
of  Montana  and  upper  pari 
oi  I  olorado  I. 
SOUTHERN    PART. 
shale  and  sandstone  of  Mon- 
tana  age,  embracing  the 
upper  coal  group. 
Shale  and  sandstone  of  Mon- 
tana  age,  containing  the 
middle    coal    group    near 
top. 
Colorado    shale,    including 
lower  coal  group  near  top. 
Gray  clay  shales,  brown  lignitic  shales, variable  sandstone 
beds  and  lignites. 
Massive  gray  sandstone,  buff  to  drab  clay  shales  and  lig- 
nites, massive  argillaceous  sandstones,  drab  clay  shales, 
with  interbedded  argillaceous  sandstones,  and  massive 
gray  sandstone  at  base. 
Dark-gray  to  drab  fossiliferous  sandy  shales,  containing 
lignite  beds  of  economic  \  alue  only  eastward  from  M.i  n 
anna  bake. 
2,000  3m 
500-1 
500  1,01 
a  Names  of  formations  used  by  Dr.  Whitman  Cross  in  La  Plata  folio  (No.  60),  Geologic  A  t  las  ('.  S. 
COAL. 
For  convenience  of  description  the  territory  examined  is  divided  into  eleven  district 
which  are  located  around  t  he  tin i  of  t  he  basin,  as  shown  on  the  accompanying  map  (PI.  Vll 
Iti  the  main  these  are  not  separate  basins,  but  are  areas  in  which  the  occurrence  of  the  coal 
fairly  uniform  and  which  are  generally  accessible  from  ecu  am  lines  of  approach. 
