466  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
ticity,  but  this  relation  is  not  always  obvious.  A  high  tensile  strength 
is  desirable,  since  it  aids  the  ware  to  resist  cracking  in  drying,  and  also 
permits  it  to  withstand  handling  before  burning. 
Tensile  strength  of  different  kinds  of  clay. a 
Pounds  per  square  inch. 
Kaolins 5  to    10 
I  Wick  clays -  -     GO  to  100 
Pottery  clays 150  to  200 
Some  very  plastic  clays 200  to  400 
Chemical  properties. — The  chemical  analyses  of  clays  given  on  page 
471  show  the  chief  constituents  to  be  alumina,  silica,  lime,  magnesia, 
potash,  soda,  iron,  and  water.  These  components  stand  in  causal 
relation  to  some  of  the  physical  properties  of  clay  just  referred  to, 
but  the  relations  can  not  always  be  made  out  from  the  chemical 
composition  alone.  Plasticity,  for  example,  runs  through  a  wide 
range  in  chemical  composition.  The  practical  clay  worker,  therefore, 
usually  considers  a  physical  test  far  more  valuable  than  a  chemical 
analysis,  and  with  good  reason,  for  there  is  really  no  way  to  find  out 
what  a  clay  can  be  made  to  do  but  to  try  it.  Clays  of  the  same 
chemical  composition  yield  diverse  products  on  firing,  and  hence 
their  physical  characters  are  generally  of  greater  importance  in 
determining  their  value. 
GEOLOGIC    OCCURRENCE. 
The  clays  of  this  region  may  be  classified  geologically  into  Carbon- 
iferous clays,  those  found  in  consolidated  rocks  of  Carboniferous  age; 
and  Quaternary  clays,  those  found  in  the  unconsolidated  Glacial  and 
river  deposits. 
CARBONIFEROUS    CLAYS. 
The  rocks  of  Beaver  County  belong  in  the  middle  of  the  Carbonif- 
erous system,  which  occurs  over  a  great  part  of  the  Appalachian  region 
of  the  United  States.  These  rocks,  which  produce  most  of  the  clay 
used  in  western  Pennsylvania,  have,  along  Ohio  River,  been  divided 
into  Lower  Barren  Measures,  or  Conemaugh  formation;  Lower 
Productive  Measures,  or  Allegheny  formation;  and  Pottsville  con- 
glomerate. These  beds  rise  gently  toward  the  northwest  in  low 
northeast-southwest  rolls. 
Pottsville  conglomerate. — This  is  made  up  of  hard  sandstones  and 
conglomerates  and  is  seen  in  the  bed  of  Ohio  River  only  near  Smiths 
Ferry  at  very  low  water;  elsewhere  on  the  Ohio  it  is  capped  by 
river  deposits.  It  is  seen  in  the  bed  of  Beaver  River  opposite 
Beaver  Falls  and  New  Brighton;  thence  it  rises  northward  and  is  well 
«  Prof.  Paper  U.  S.  Geol.  Survey  No.  11,  p.  21. 
