COAL  BETWEEN   GALLUP  AND   SAN    MATEO,    N.    MEX.  365 
TOPOGRAPHY  AND    GEOLOGY. 
The  topographic  features  are  comparatively  simple,  and  the  same 
may  be  said  of  the  geology.  The  boundary  of  the  Cretaceous  system 
is  expressed  in  a  general  way  in  the  topography,  which  is  shown  on  the 
Fort  Wingate  and  Mount  Taylor  topographic  sheets  of  the  Geological 
Survey.  On  the  Fort  Wingate  sheet  a  narrow  ridge  is  represented  as 
running  in  a  north-south  direction  about  2f  miles  east  of  Gallup. 
The  bold  portion  of  this  ridge  or  hogback  marks  the  boundary  of  the 
Cretaceous  rocks,  which  dip  steeply  westward.  This  hogback  stands 
in  bold  relief  for  a  few  miles  northward  from  the  Atchison,  Topeka 
and  Santa  Fe  Railway.  Beyond  this  stretch  it  swings  to  the  east  and 
is  continuous  with  an  escarpment  trending  in  a  southeasterly  direction 
by  way  of  Hosta  Butte  to  and  beyond  the  vicinity  of  San  Mateo,  in 
the  Mount  Taylor  quadrangle. 
The  relation  of  the  coal-bearing  formations  of  this  region  to  those 
in  the  northern  part  of  the  San  Juan  River  region  was  not  deter- 
mined in  the  work  of  1905.  Schrader  classified  the  rocks  in  three 
divisions — upper  Montana,  lower  Montana,  and  Colorado. 
Regarding  the  Hosta  Butte  district,  Schrader  a  says: 
The  upper  coal  group  [upper  Montana]  was  observed  only  m  the  northwestern  part 
of  the  district,  in  the  southeast  corner  of  the  Navajo  Indian  Reservation,  where  it 
has  an  extent  of  about  30  miles  in  an  east- west  direction.  *  *  *  The  middle  and 
lower  coal  groups  enter  the  Hosta  Butte  district  from  the  direction  of  Mount  Taylor, 
and  after  extending  northwestward  across  the  district,  curve  southward  and  form  the 
hogback,  which  crosses  the  railroad  3  miles  east  of  Gallup.  Their  geologic  age  is  not 
definitely  determined,  but  from  the  evidence  of  a  few  fossils  collected  in  each  of  them 
it  is  probable  that  the  middle  group  represents  the  lower  part  of  the  Montana  formation 
and  that  the  lower  group  represents  a  part  of  the  Colorado  formation. 
Schrader  shows  the  three  divisions  on  his  map,  stating  in  connection 
with  it  that  the  relation  of  the  upper  and  lower  Montana  to  the 
Mesaverde  was  at  that  time  unknown.  However,  in  Shaler's  recon- 
naissance in  1906,6  the  true  Mesaverde  was  traced  southward  from  its 
type  locality  in  the  Mesa  Verde,  on  the  Rio  Mancos  in  Colorado,  and 
found  to  be  continuous,  though  possibly  not  identical  in  entirety  with 
the  coal  groups  at  Gallup.  As  a  result,  Shaler  mapped  the  coal- 
bearing  strata  on  the  south  side  of  the  field  on  a  new  basis,  substi- 
tuting Mesaverde  for  the  upper  and  lower  Montana  of  Schrader. 
The  Colorado  formation  of  Schrader,  subjacent  to  the  Mesaverde, 
consisting  of  clay  and  arenaceous  shale,  and  bearing  coal  beds,  was 
mapped  by  Shaler,  on  the  basis  of  fossil  evidence,  as  coal-bearing 
Mancos.  The  lower  portion  of  the  Mancos  is  Colorado  and  the  upper 
portion  Montana.0     Consequently  Shalerd  shows  only  two  groups  of 
a  Op.  Cit.,  p.  252. 
b  Shaler,  M.  K.,  A  reconnaissance  survey  of  the  western  part  of  the  Durango-Gallup  coal  field  of 
Colorado  and  New  Mexico:  Bull.  U.  S.  Geol.  Survey  No.  316,  1907,  pp.  376-426. 
c  Cross,  Whitman,  La  Plata  folio  (No.  60),  Geologic  Atlas  U.  S.,  U.  S.  Geol.  Survey,  1899. 
d  Op.  cit.,  PI.  XXII. 
