4(U  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull.  225. 
of  these  minerals  are  the  feldspars,  augite,  and  hornblende,  and  nephe- 
lite  and  sodalite  occur  in  smaller  amounts. 
Upon  the  decomposition  of  the  rocks  the  more  soluble  elements  of 
these  minerals  are  mostly  removed  by  water,  leaving  the  alumina  and 
silica,  with  small  proportions  of  lime,  magnesia,  iron,  and  the  alkalies. 
These  are  washed  over  and  over  and  deposited  in  favorable  places  by  j 
water,  much  in  the  same  manner  as  are  river  silts,  and  such  deposits, 
whether  fluviatile,  lacustrine,  or  marine,  are  called  clays.  This  man-; 
ner  of  decomposition  explains  the  banding  and  the  presence  of  grains 
of  sand  and  minerals  in  the  clays.  Clays  so  deposited  are  called, 
sedimentary  clays,  and  this  name  may  be  applied  to  all  the  clays  mined; 
in  the  upper  Ohio  Valley. 
Clay  which  is  formed  by  the  disintegration  of  rock,  but  which, 
instead  of  being  washed  away  by  water,  has  remained  near  the  parent, 
roek,  is  called  residual  clay.  Cla}^  of  this  kind  is  found  in  the  uplands,); 
back  from  the  rivers. 
Physical  properties. — The  indispensable  properties  of  clay  are  (1)' 
plasticity  when  wet,  (2)  permanence  when  burnt,  (3)  refractoriness. 
To  be  commercially  valuable  a  clay  need  not  possess  all  these  proper-, 
ties — as,  for  example,  flint  clays — but  the  manufacturer  must,  by! 
mixture  or  otherwise,  obtain  these  three  properties  in  his  clay  before 
he  can  manipulate  it. 
By  these  three  properties  clays  have  been  classed  into  flint,  non-j 
plastic,  or  china  cla}^s;  plastic,  ball,  or  pottery  clays;  brick  cla}7s,  and 
refractory  cla3rs. 
Plasticity  seems  to  depend  on  some  relation  of  the  clay  to  watei 
which  is  not  yet  well  understood.  No  clay  is  plastic  without  water,] 
and  flint  clays  can  not  be  made  plastic  by  any  amount  of  grinding  with] 
water. a 
Plasticity,  however,  does  not  depend  upon  the  degree  of  consolida- 
tion of  the  clay,  for  shales  and  slates,  which  are  consolidated  clay, 
may  be  made  plastic  by  grinding  with  water,  while  flint  clay,  which  h 
also  consolidated,  can  not,  as  just  stated,  be  made  plastic.  Manv 
plastic  cla}^s,  besides  shales  and  slate,  may  at  times  be  very  hard,  as 
the  Lower  Kittanning  clay,  and  yet  retain  their  plasticity.  The  clays' 
of  Beaver  County  are  more  or  less  consolidated,  but  perhaps  those 
least  so  are  the  terrace  cla}^s  of  New  Brighton. 
Permanency  when  burnt  does  not  necessarily  depend  on  fusibility 
for  in  crude  ware,  such  as  flower  pots,  the  clay  is  not  vitrified,  but  ii 
high-grade  ware  vitrification  is  necessary.  Fusibility  of  clays  depend; 
on  the  amounts  of  accessories — lime,  magnesia,  alkalies,  and  iron- 
present;  and  from  this  point  of  view,  since  fusibility  is  usualh 
necessary,  the  fluxes  should  not  perhaps  be  called  accessories.  The] 
have  been  called  detrimentals,  because  the  presence  of  large  amount] 
«Ohio  Survey  vol.  7,  Orton. 
