PHOSPHORUS    ORE    AT    MOUNT    HOLLY    SPRINGS,  PA.  477 
mountain  front.  Since  the  uppermost  fossiliferous  sandstone  of  the 
(Cambrian  (Antietam)  forms  the  front  of  the  mountain  at  Mount  Holly 
(Springs  and  there  are  no  data  concerning  the  age  or  attitude  of  the 
limestone  immediately  adjacent,  it  is  assumed  that  the  relations  there 
are  the  same  as  those  observed  farther  south;  that  is  to  say,  that  the 
sequence  is  normal  or  unbroken  by  a  fault  of  appreciable  magnitude. 
ORIGIN. 
The  wavellite  and  the  manganese  and  iron  ores  are  secondary  depos- 
its in  the  surface  gravels,  sands,  and  clays  which  cover  the  rock  out- 
crops at  the  foot  of  the  mountain.  These  surface  deposits  are  in  part 
[residual,  in  part  transported.  White  sands,  next  to  the  mountain,  are 
followed  by  beds  of  pure  white  siliceous  clays,  and  these  by  colored 
plastic  clays.  The  sand  is  derived  from  sandstones  which  have  been 
leached  of  their  calcareous  cement,  and  in  the  quarries  the  loose  sand 
merges  into  the  unaltered  rock.  The  white  clay  is  a  decomposed 
hydromica  slate,  transition  into  which  has  been  described  in  the 
reports  of  the  Second  Geological  Survey  of  Pennsylvania  and  by  Hop- 
kins.0 Similar  relations  were  observed  in  the  clay  mines  by  the  writer^ 
but  the  exposures  are  not  so  clear  and  definite  as  they  were  when 
the  iron  mines  were  in  active  operation.  The  colored  plastic  clay  is 
apparently  derived  from  impure  limestone.  Generally  these  beds  are 
steeply  inclined  or  vertical,  like  the  undecomposed  rocks,  but  in  places 
they  have  moved  down  the  slope,  lie  flat,  and  have  become  covered 
over  by  and  mixed  with  the  quartzite  debris  from  the  mountain  above. 
In  this  heterogeneous  mass  the  mineral  deposits  occur.  Through- 
out the  South  Mountain  district  the  iron  ore  that  is  associated  with 
the  basal  part  of  the  limestone  is  at  the  horizon  of  the  hydromica 
slate  at  the  contact  with  the  sandstone,  and  is  usually  found  in  the 
highly  colored  clay,  the  limestone  residuum,  overlying  the  white  clay 
derived  from  the  hydromica  slate.  In  many  places  the  ore  dips 
steeply  into  the  hill  parallel  to  the  bedding  and  appears  to  be  inter- 
bedded  with  the  rocks. 
It  seems  reasonable  to  conclude,  therefore,  that  the  original  deposi- 
tion of  iron  was  in  some  way  a  feature  of  the  change  of  sedimentation 
from  shore  detritus  to  calcareous  silt,  probably  not  as  a  massive  bed 
of  ore  but  as  ferruginous  sediments.  The  solution  of  the  limestone 
and  the  decomposition  of  the  other  rocks  has  left  the  iron  and  clay 
residuum,  and  the  iron  has  been  further  concentrated  in  the  clay  by 
solution  and  redeposition. 
The  wavellite  undoubtedly  had  a  similar  history,  for  it  is  a  com- 
mon constituent  of  the  iron  and  manganese  ores.  Analyses  show 
its  presence  to  a  greater  or  less  extent  in  all  the  iron  ores  of  the  region. 
a  Op.  cit.,  pp.  11-13.  b  See  pp.  323-324  of  this  bulletin. 
Bull.  315—07 31 
