502  CONTBIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.  [bull.  260. 
This  point  has  been  emphasized  because  many  writers  on  the  sub- 
ject have  either  explicitly  stated  or  implied  that  it  is  the  magnesian 
carbonate  of  the  Rosendale,  Akron,  Louisville,  Utica,  and  Milwaukee 
rocks  that  causes  them  to  yield  a  natural  cement  on  burning. 
The  manufacturing  methods  at  a  natural-cement  plant  are  of  the 
simplest  kind,  including  merely  the  burning  of  the  cement  rock  and 
the  pulverizing  of  the  product. 
The  burning  is  carried  on  in  vertical  kilns,  closely  resembling  lime 
kilns  in  shape,  size,  etc.  The  limestone  and  fuel  are  usually  fed  into 
the  kiln  in  alternate  la3^ers,  though  at  a  few  plants  more  advanced 
types  of  kilns  are  in  use.  The  burned  product  is  crushed  and  then 
reduced  to  powder,  commonly  in  buhrstone  mills.  Recently  advances 
have  been  made  in  crushing  practice,  and  several  plants  now  reduce 
their  product  in  tube  mills. 
Since,  within  very  wide  limits  of  composition,  any  clayey  limestone 
will  give  a  natural  cement  on  burning,  it  can  readily  be  seen  that  sat- 
isfactory natural-cement  materials  must  be  widely  distributed  and 
of  common  occurrence.  Hardly  a  State  is  entirely  without  limestones 
sufficiently  clayey  to  be  available  for  natural-cement  manufacture. 
The  sudden  rise  of  the  American  Portland  cement  industry,  how- 
ever, has  acted  to  prevent  any  great  expansion  of  the  natural-cement 
industry.  It  would  be  difficult  to  place  a  new  natural  cement  on  tin1 
market  in  the  face  of  competition  from  both  Portland  cement  and  from 
the  older  and  well-established  brands  of  natural  cement.  Such  new 
natural -cement  plants  as  have  been  started  within  recent  years  have 
mostly  been  located  in  old  natural-cement  districts,  where  the  accu- 
mulated reputation  of  the  district  would  help  to  introduce  the  new 
brand.  The  only  exceptions  to  this  rule,  indeed,  were  the  Pembina 
plant  in  North  Dakota,  the  Rossville  plant  in  Georgia,  and  a  plant  in 
the  State  of  Washington.  Of  these  the  Pembina  plant  was  established 
with  the  intention  of  making  Portland  cement,  but  the  raw  mate- 
rials soon  proved  to  be  unsuitable,  and  the  plant  was  converted.  The, 
plant  in  Washington  is  located  in  an  area  where  any  kind  of  cement 
is  readily  salable.  The  Rossville  plant  was  built  by  an  Akron,  N.  Y., 
cement  manufacturer  to  utilize  a  peculiarly  satisfactory  natural! 
cement  rock. 
