532 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
The  raw  materials  occurring  in  the  Valley  of  Virginia  suitable  for 
the  manufacture  of  cement  are  magnesian  limestone,  argillaceous 
limestone,  pure  limestone,  shales,  and  calcareous  marls.     Of  these,  the 
Fig.  23. — Map  of  the  Valley  of  Virginia  from  Woodstock  to  Staunton. 
White  area  includes  Knox  limestone  and  underlying  formations. 
more  important  are  the  argillaceous  and  pure  limestones.  The  mag- 
nesian limestones,  as  mentioned  later,  are  now  being  used  only  in  a 
single  instance  for  the  manufacture  of  a  natural  cement.  The  othei 
materials  cited  may  be  utilized  in  the  production  of  Portland  cement 
