WHITE   CLAYS   OF   SOUTH   MOUNTAIN, 
PENNSYLVANIA. 
By  George  \Y.  Stose. 
INTRODUCTION. 
The  deposits  here  described  are  very  siliceous  clays  that  are  asso- 
ciated with  the  wash  and  decomposition  products  of  certain  Cam- 
brian strata  of  South  Mountain.  They  nave  been  actively  mined  in 
the  vicinity  of  Mount  Holly  Springs,  Pa.,  6  miles  south  of  Carlisle, 
where  they  arc  of  unusual  thickness  and  purity.  These  deposits 
have  been  previously  described  by  T.  C.  Hopkins  in  an  exhaustive 
report  on  the  clays  and  (day  industries  of  Pennsylvania,"  from  which 
some  of  the  data  in  this  paper  are  taken.  Several  new  mines  have 
been  opened  since  that  report  was  "prepared,  and  the  mine  and  plan! 
of  the  Philadelphia  Clay  Company,  which  is  the  largest  mine  in  the 
region  and  was  formerly  (dosed  to  visitors,  was  thoroughly  inspected 
by  the  writer. 
TOPOGRAPHY  AND   GEOLOGY. 
South  Mountain  forms  a  rude  an-  of  a  circle  curving  from  Potomac 
River  at  Weverton  north  and  east  to  Susquehanna  River  south  of 
Barrisburg.  It  is  composed  of  several  parallel  ridges  and  interven- 
ing steep-sided  longitudinal  valleys.  The  larger  part  of  the  drainage 
of  the  mountain  area  passes  westward  through  rugged  water  gaps 
into  the  Cumberland  Valley,  a  low  rolling  limestone  valley  10  to  15 
miles  wide,  part  of  the  great  Appalachian  Valley  that  extends  from 
northern  Pennsylvania  to  central  Alabama. 
The  mountain  ridges  are  composed  largely  of  hard  sandstone  or 
quartzite  and  the  longitudinal  valleys  of  interbedded  shale.  These 
rocks  are  of  Georgian  (Lower  Cambrian)  age,  fossils  of  that  age  having 
been  found  by  C.  D.  Walcott6  in  their  uppermost  beds — the  Antie- 
tam  sandstone.  They  form  an  anticlinorium,  with  vertical  or  over- 
turned dips  on  the  northwest  side.     In  the  core  of  this  anticline  pre- 
a  Ann.  Rept.  Pennsylvania  State  College  for  1899-1900,  appendix,  offic.  doc.  No.  21  i 
b  Bull.  C.  S.  Geol.  Survey  X...  134,  p.  24. 
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