324  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1906,   PART    I. 
Hopkins  states"  that  "the  white  clay  is  the  direct  decomposition 
product  of  light-colored  hydromica  slates  which  occur  intercalated 
in  the  Cambro-Ordovician  limestone  and  in  the  Cambrian  slates, 
sandstones,  and  quartzite,"  and  mentions  several  places  where  lie 
observed  the  change  from  clay  to  slate.  The  places  specifically: 
mentioned — Latimore  and  Hensingersville — were,  unfortunately,  not 
visited  by  the  writer.  Latimore  is  located  on  the  southern  edge  of 
South  Mountain,  in  the  belt  of  old  volcanic  rocks,  and  the  clay  there 
is  undoubtedly  not  at  the  same  horizon  as  that  in  the  vicinity  of 
Mount  Holly  Springs,  which  will  be  referred  to  later. 
Clear  exposures  showing  a  change  from  clay  to  hard  rock  were  not 
seen  by  the  writer,  but  in  several  of  the  deeper  tunnels  the  white  clay 
is  hard  and  lias  a  handed,  lamellar  structure,  resembling  slate,  and  a 
faint  greenish  tint.  The  section  in  the  Philadelphia  Clay  Com- 
pany's tunnel  positively  demonstrates  the  interbedded  relations  of 
the  day.  Beyond  the  loose  rock  and  wash  at  the  entrance  the  tunnel 
passes  through  several  hundred  feet  of  gray  shaly  clay  and  less 
altered  hydromica  schist  seamed  with  limestone,  ocher  beds,  hard 
brownish  altered  dolomite,  soft  yellow  (day  containing  iron  ore,  a 
thick  stratum  of  white  (day,  to  the  quartzitic  wall  rock.  It  is  con- 
cluded, therefore,  that  the  larger  deposits  of  white  (day  are  original 
sedimentary  beds  directly  overlying  the  Antietam  sandstone  and 
underlying  the  limestone  of  the  valley.  The  "soapstone"  used  at 
Pine  Grove  Furnace  in  the  manufacture  of  brick  is  a  light  greenish 
sericite  schist  with  quartz  phenocrysts,  an  altered  rhyolitic  rock 
similar  to  other  schist  seen  in  the  volcanic  belt,  which  crushes  readily 
to  a  line  white  clayey  powder  and  is  probably  the  same  as  the  beds  at 
Latimore, from  which  the  local  white  (day  described  by  Hopkins  was 
derived.  These  volcanic  schists  may  he  the  ultimate  source  of  all 
the  white  (day.  in  which  case  the  later  sedimentary  white  beds  were 
derived  from  their  decomposition  and  were  deposited  near  the  vol- 
canic schist  outcrops  in  early  Cambrian  time.  The  (day  sediments 
are  certainly  variable  in  thickness  and  irregular  in  distribution  but 
they  are  thickest  in  Mountain  Creek  valley,  near  the  volcanic  belt. 
Analyses  -how  that  the  white  clays  are  very  siliceous,  very  low  in 
iron,  and  high  in  alkalies.  Hopkins  compared  analyses  of  white 
(days  with  those  of  hydromica  slates  from  which  they  were  derived, 
the  samples  having  been  taken  in  adjoining  districts,  and  found  that 
the  most  noticeable  difference  was  in  the  greater  amount  of  potash 
contained  in  the  slates.  This  difference  is  due  to  leaching  out  of  the 
potash  in  the  process  of  disintegration.  Analyses  of  two  white  clays 
from  this  area  and  of  the  volcanic  sericite  schist  from  Pine  drove 
Furnace  are  given  below. 
■  Ann.  Rept.  Pennsylvania  State  College  for  1899-1900,  appendix,  offic.  doc.  No.  21,  p.  11. 
