238  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
characterized  in  places  by  limestone  concretions,  some  of  which  are 
\  ery  large.  A  sample  was  taken  along  the  railroad  track  1  mile  above 
Spencer  siding,  where  the  shale  is  greenish  gray  in  color,  fine  grained, 
and  without  gypsum;  It  is  soft  and  could  be  readily  mined  by  steam 
shovel.     The  following  analysis  is  by  Phillips  and  Bates: 
Analysis  of  Pierrt  shale. 
Silica  (Si02) (30.  66 
Alumina  (A1203) 22.  L3 
Ferric  oxide  I  Fe2<  >3) 1-1 
Manganese  oxide  i  MnO) 44 
Lime  (CaO) L.59 
Magnesia  i  Mg( )] 1.  54 
Sulphuric  anhydride  (S03   43 
Alkalies..  |Na*° 53' 
\  k_,0 2.  16 
[gnitionloss 9.28 
99.  97 
The  shale  has  high  silica  and  rather  low  magnesia  and  alkalies. 
Its  silica  content  is  2.6  times  that  of  the  alumina  and  ferric  iron, 
the  iron  content  being  so  small  that  the  rock  would  form  with  lime- 
stone an  infusible  mixture. 
GYPSUM. 
Two  or  three  per  cent  of  gypsum  is  usually  added  to  the  clinkei 
of  Portland  cement  to  retard  setting.  Gypsum  occurs  in  consider- 
able amount  in  the  *'  Red  Beds"  (Spearfish  formation)  exposed  aloi 
Stockade  Beaver  ('reek. 
ECONOMIC    CONDITIONS. 
Some  of  the  rocks  just  described  are  excellent  raw  material  for 
Portland  cement;  and  the  supply  at  the  points  where  the  samples 
were  taken  is  adequate  to  run  a  Portland  cement  plant  for  a  long 
time.  Further,  if  the  present  outcrop  of  any  of  these  limestones  or 
shales  were  exhausted,  the  same  rock  could  be  found  beneath  a  thin 
covering  of  soil  over  large  areas  in  the  immediate  vicinity.  The 
average  sample  collected  represents  thicknesses  of  20  to  150  feet. 
The  analyses  represent  not  alone  the  shale  or  limestone  at  the  point 
where  samples  were  taken,  but  give  a  fair  idea  of  the  shale  or  lime- 
stone along  the  same  strike  for  a  long  distance. 
The  best  combination  for  a  Portland  cement  mixture  is  about  90 
per  cent  of  the  natural  cement  rock  and  10  per  cent  of  the  Minne- 
kahta  limestone.  The  highly  clayey  limestone  or  natural  cement  I 
inck  occurs  along  the  railroad,  and  the  limestone  could  be  obtained 
by  running  a  spur  to  a  point  on  Stockade  Beaver  Creek  1  mile  above 
the  L  A  K  ranch.     This  branch  road,  which  would  not  be  more  than 
