298  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
is  mixed  in  proper  proportion  with  the  Mancos  shale  of  the  quarry. 
The  color  of  this  clay  is  dark  blue.  It  is  hard  and  is  quite  free  from 
physical  impurities.     The  bricks  produced  are  white  to  buff  in  color. 
Between  the  longitudes  of  Durango  and  Mancos  the  Mancos  shale 
and  some  of  the  fire-clay  beds  in  the  coal-bearing  Mesaverde  format  ion 
occur  in  continuous  outcrop.  The  Mancos  also  forms  the  basal  part 
of  Mesa  Verde,  a  wide  mesa  lying  just  south  of  Mancos.  Another 
shale  bed,  the  Lewis,  having  a  total  thickness  of  about  1,600  feet  and 
differing  little  in  character  from  the  Mancos,  outcrops  between  the 
Mesaverde  and  Laramie  formations  in  Rages  Basin,  southwest  of 
Durango.  Its  exposure  is  continuous  westward  as  far  as  La  Plata 
River  in  the  vicinity  of  Fort  Lewis.  Nowhere  in  this  district,  so  Par 
as  known,  has  any  part  of  this  shale  been  worked  for  the  manufacture 
of  clay  products;  hence  a  practical  test  of  its  adaptability  to  such 
purposes  is  as  yel  entirely  lacking.  Judging  from  its  physical  charac- 
teristics, however,  this  shale  bed  doubtless  is,  in  part  at  least,  well 
adapted  to  the  manufacture  of  ordinary  brick. 
With  the  exception  of  the  San  Juan  and  Gallup  districts,  yet  to  be 
discussed,  this  dist  rid  is  t  be  only  one  in  t  he  field  having  at  present  the 
transportation  facilities  and  markets  necessary  for  any  great  develop- 
ment of  the  clay  industry,  and  even  this  district  is  greatly  handicapped 
by  high  freight  rates  and  a  consequent  limited  market.  The  market 
for  the  clay  products  of  Durango  outside  of  the  local  demand  extends 
to  Chama,  X.  Mex.,  on  the  east ,  and  to  Telluride  and  Silverton,  Colo., 
on  the  north. 
Vitrified  brick  has  had.  as  yet,  no  market  in  this  district;  hence 
no  attempt  has  been  made  to  utilize  the  shales  for  the  production  of 
brick  of  this  class. 
At  Rock  wood,  on  the  Denver  and  Rio  Grande  Railroad,  about  16 
miles  north  of  Durango.  limestone  is  quarried  with  which  the  shale  of 
the  Durango  Prosed  Brick  Company's  quarry  might  he  mixed  in  the 
manufacture  of  Portland  cement.  An  analysis  of  this  limestone,  made 
by  I\  TI.  Bates,  at  the  structural  material  laboratory  of  the  Tinted 
States  Geological  Survey,  follow-: 
Analysis  of  limestone  from  the  Rockwood  quarry. 
Silica  (Si02) 0.24 
Alumina  (A1203) L3 
Ferric  oxide  I  Fe2<  >3) 08 
Manganese  oxide  (MnO) .05 
Lime  (CaO) 55.87 
Magnesia  ( MgO ) 19 
Sulphuric  anhydride  (S03) .10 
Potash  (K20) 08 
Water  at  100°  C 07 
Ignition  loss  (includes  carbon  dioxide) 4.;.  17 
100.  28 
