WHITE    CLAYS    OF    SOUTH    MOUNTAIN,  PENNSYLVANIA.         323 
Cambrian  altered  basic  and  acidic  lavas  are  exposed.  The  Cumber- 
land Valley  is  composed  of  Cambro-Ordovician  limestones,  which  in 
the  eastern  part  of  the  valley  are  interstratified  with  shales  and  sili- 
ceous beds,  and  in  the  western  part  are  overlain  by  Utica  and  Eden 
shales.  All  of  the  rocks  are  closely  folded,  at  many  places  overturned, 
and  here  and  there  faulted;  and  a  southeast  cleavage  is  strongly 
developed  in  the  shales  and  softer  sandstone  layers  of  the  mountain 
and  in  the  limestone  adjacent  to  the  mountain. 
MODE    OF    OCCURRENCE    OF    THE    CLAY. 
From  the  plain  of  the  Cumberland  Valley  the  mountain  rises 
abruptly  in  a  continuous  straight  ridge  that  is  broken  here  and  there 
by  water  gaps.  The  clay  deposits  occur  at  the  northwestern  foot  of 
this  outer  ridge.  In  the  vicinity  of  Mount  Holly  Springs  they  are 
also  found  on  opposite  sides  of  an  interior  longitudinal  valley — that 
of  Mountain  Creek.  The  clay  is  associated  with  beds  of  sand  and 
colored  clays  and  with  the  wash  from  the  mountain  slope  above.  It 
is  nearly  everywhere  accompanied  by  secondary  deposits  of  iron  and 
manganese  ores.  The  clay  was  first  exposed  to  view  in  mining  the 
great  iron  deposits  of  this  region  twenty  or  more  years  ago,  but  its 
value  has  only  recently  become  known. 
One  is  inclined  to  conclude  at  first  sight  that  the  clay  is  a  trans- 
ported surficial  deposit;  that  it  was  derived  from  the  decomposition 
of  argillaceous  limestone  and  accumulated  at  the  base  of  the  slope  by 
creep  and  stream  action,  and  that  the  iron,  having  been  leached  out 
in  the  process  of  transportation,  was  deposited  in  the  adjacent  wash. 
Careful  study,  however,  shows  that  most  of  the  accumulations  have 
not  originated  in  this  way,  although,  no  doubt,  the  material  in  some 
places  has  moved  down  the  steep  slopes  on  which  it  originally  lay  and 
is  more  or  less  confused  with  surface  wash. 
The  fact  that  on  the  surface  the  white  clay  has  a  definite  relation 
to  the  upper  bed  of  the  mountain  sandstone  throughout  the  region 
is  very  significant.  Almost  invariably  the  sequence  in  the  old  mine 
pits  and  clay  prospects  is  (1)  colored  plastic  clay  containing  the  iron 
ore,  (2)  white  clay,  and  (3)  sandstone.  At  many  places  the  sandstone 
is  decomposed  to  loose  sand,  which  is  quarried  for  use  as  building 
material,  but  is  traceable  in  depth  to  hard  sandstone,  the  outermost 
bed  of  the  mountain  rock.  No  rock  exposures  occur  for  several  hun- 
dred feet  from  the  foot  of  the  mountain  in  the  vicinity  of  the  clay 
mines,  but  limestone  was  reported  in  mine  pits  close  to  the  white 
clay  heds,  and  beyond  much  doubt  the  colored  plastic  clays  in  which 
the  iron  ore  occurs  is  a  residuum  of  argillaceous  limestone.  The  sur- 
face relations  therefore  suggest  very  strongly  that  the  white  clay  is 
also  a  residual  product  of  some  argillaceous  sedimentary  rock.  The 
fragments  of  sandstone  contained  in  the  clay  were  probably  thin 
quartzitic  layers  or  lenses  in  the  original  rock. 
