CLAY    DEPOSITS    IN    NEW    MEXICO    AND    COLOKADO.  297 
The  plastic  clays  include  alluvial  beds,  adobe  clays,  certain  uncon- 
solidated Tertiary  shales,  and  residual  deposits  derived  from  the 
weathering  of  clay  shales. 
The  fire  clays  include  deposits  of  a  semirefractory  nature,  the  beds 
of  which  now  mined  are  interbedded  with  the  sandstones,  shales,  and 
coals  of  the  Mesaverde  formation.  Some  of  these  are  directly  asso- 
ciated with  the  coal  beds  and  are  mined  in  connection  with  them. 
COMMERCIAL  DEVELOPMENT. 
Durango- Mancos  district. — The  present  development  of  the  clay 
industry  in  the  Durango-Mancos  district,  so  far  as  known,  is  confined 
to  the  vicinity  of  Durango,  Colo.  The  Durango  Pressed  Brick  Com- 
pany began  operations  as  early  as  1898,  and  in  .1902  two  plants  were 
manufacturing  bricks  near  Durango,  but  the  company  named  is  the 
only  one  now  operating  in  the  district.  The  plant  is  located  on  the 
Rio  Grande  Southern  Railway  1  mile  west  of  Durango.  The  shale 
used  in  making  building  brick  is  procured  from  the  upper  part  of  the 
Mancos,  about  100  feet  stratigraphically  below  the  lowest  coal  bed  of 
the  Mesaverde  formation.  It  is  dark  gray  or  black  in  color,  hard,  and 
at  some  places  contains  a  little  lime  as  a  physical  impurity.  This  lime 
is,  however,  generally  present  in  concretionary  form,  and  can  be  easily 
removed  while  the  shale  is  being  quarried.  It  is  reported  that  near- 
by beds  contain  a  higher  percentage  of  lime  than  the  bed  quarried  at 
this  brick  plant.  The  following  analysis  of  the  shale  was  made  by 
P.  H.  Bates  at  the  structural  material  laboratory  of  the  United 
States  Geological  Survey  at  St.  Louis. 
Analysis  of  shale  from  quarry  of  Durango  Pressed  Uriel:  <  ompany. 
Silica  (Si02) 48.  87 
Alumina  (A1203) 12.26 
Ferric  oxide  (Fe203) I.  44 
Manganese  oxide  ( MnO ) 07 
Lime  (CaO) ,.10.06 
Magnesia  (MgO ) 3.  82 
'    Sulphuric  anhydride  (S03) 45 
Alkalies..  iNa*° 55 
\K20 1 .  76 
Water  at  100°  C 1.  27 
Ignition  loss,  including  carbon  dioxide  (Co2) 16.  39 
98.  94 
The  color  of  the  burned  bricks  is  red.  A  dry-press  Boyd  machine 
is  used  in  their  manufacture.  It  is  reported  that  the  bricks  shrink 
very  little  after  sun  drying. 
The  Durango  Pressed  Brick  Company  produces  each  year  a  small 
output  of  a  semirefractory  brick,  which  is  generally  used  for  boiler 
linings.  In  the  manufacture  of  this  class  of  brick  the  clay  from  a  bed 
2  to  5  feet  in  thickness,  lying  just  below  the  coal  bed  above  mentioned, 
