CLAY    DEPOSITS    IN    NEW    MEXICO    AND    COLORADO.  301 
The  Carboniferous  (Aubrey)  limestone,  which  occurs  near  the  rail- 
road some  distance  east  of  Gallup,  and  also  in  the  vicinity  of  Fort 
Wingate  and  Nutria  southeast  of  Gallup,  appears  to  be  a  limestone  of 
very  pure  grade  and  might  be  used  in  connection  with  this  shale  in 
Portland  cement  manufacture.  The  abundance  of  coal  at  Gallup  and 
in  its  vicinity,  and  the  fact  that  no  Portland  cement  plants  are  located 
near  enough  to  this  point  to  compete  seriously  with  a  plant  established 
here  are  factors  that  might  insure  the  success  of  such  a  plant.0 
Other  areas  in  the  field. — The  only  areas  within  the  field  that  could 
possibly,  under  present  conditions  of  transportation,  produce  clay  of 
any  kind  on  a  commercial  basis  have  been  discussed  above.  This 
limitation,  however,  is  entirely  dependent  on  present  transportation 
facilities  and  market  conditions  and  not  on  the  absence  of  good  clay 
and  shale  deposits. 
South  of  Jewett  the  Mancos  shale  outcrops  over  a  wide  area  west  of 
the  " Hogback"  to  the  latitude  of  Crozier,  where  it  disappears,  dip- 
ping steeply  eastward  under  flat-lying  Tertiary  sediments  and  lavas. 
At  the  southern  limit  of  the  Choiskai  Mountains,  east  of  Fort  Defiance, 
Ariz.,  this  formation  reappears,  dipping  steeply  in  the  same  direction. 
From  this  point  it  trends  southward,  being  crossed  by  the  Santa  Fe 
Railroad  at  Manuelito.  From  here  the  Mancos  shale,  flattening 
rapidly,  courses  southeastward,  passing  about  10  miles  east  of  Zuni 
village,  thence  southward  to  the  latitude  of  Ojo  Caliente,  where  its 
outcrop  swings  abruptly  eastward  for  a  few  miles.  From  this  point 
it  courses  northward,  the  rocks  dipping  steeply  to  the  east  through 
Raman  and  Nutria  and  on  beyond  the  horizon  in  the  " Hogback" 
east  of  Gallup.  Northeast  of  Gallup  the  strike,  including  that  of  the 
Mancos  shale,  again  changes,  in  this  case  to  the  east,  the  shale  occur- 
ring in  continuous  outcrop  a  few  miles  north  of  the  Santa  Fe  Railroad 
until  it  passes  beyond  the  limits  of  the  area  under  discussion. 
Throughout  its  outcrop  the  shale  at  this  horizon  is  characteristically 
the  same,  varying  only  in  its  thickness,  which  ranges  from  750  feet 
to  more  than  2,000  feet. 
South  of  Jewett  the  Lewis  shale  covers  a  rather  limited  area,  finally 
occupying  a  thickness  of  only  150  feet  on  Rio  Chaco  east  of  Crozier. 
Its  apparent  quality,  so  far  as  concerns  its  uses  considered  in  this 
paper,  is  the  same  throughout  its  occurrence. 
Within  the  area  outlined  by  the  outcrop  of  the  Mancos  shale,  out- 
side of  the  three  districts  already  discussed,  many  beds  of  shale  and 
fire  clay  were  noted  at  numerous  places.  These  beds  are  of  variable 
thickness  and  quality,  but  doubtless  many  of  them  have  properties 
that  fit  them  for  use  in  the  manufacture  of  clay  products. 
Within  the  limits  of  the  field  under  discussion  many  clays  of  the 
varieties  classed  as  plastic  clays  were  noted.     Among  these  is  what  is 
a  Eckel,  E.  a,  Bull.  U.  S.  Gcol.  Survey  No.  243,  PI.  I. 
Bull.  315—07 20 
