328  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
reached  by  a  spur  from  the  Hunters  Run  and  Slate  Belt  Railroad. 
The  main  tunnel  enters  on  tlie  level  of  the  mill  and  extends  abouj 
1 ,000  feet  into  the  mountain  to  the  sandstone  "wall"  rock.  Beyond 
the  several  hundred  feet  of  surface  wash  at  the  entrance  there  are 
200  to  300  feet  of  gray  shaly  clay  and  altered  sericite  schist,  30  to 
10  feet  of  yellow  ocher,  10  to  15  feet  of  hard  yellow  dolomite,  65  feet 
of  iron  ore  in  clay,  and  about  200  feet  of  white  clay,  streaked  with 
iron  near  the  ore,  but  largely  pure  white.  The  clay  contains  grains 
and  scattered  masses  of  white  sandstone  up  to  2  feet  across  and 
occasional  beds  and  streaks  of  white  sand.  The  mountain  sand- 
stone lies  back  of  the  day,  but  was  not  seen  by  the  writer  because 
the  part  of  the  tunnel  that  reached  it  had  been  mined  out  and 
caved   in. 
The  white  clay  in  this  mine  is  at  most  places  saturated  with  water 
and  as  soon  as  it  is  encountered  yields  water  freely,  which  carries 
the  clay  with  it.  The  (low  of  water  here  is  incessant,  hut  the  grade1 
of  the  tunnel  is  steep  enough  —  1  in  12— to  carry  it  off  quickly.. 
When  ponds  in  old  ore  pits  are  tapped  the  sudden  Hooding  of  the 
mine  is  a  source  of  danger  to  the  workmen.  It  is  impossible  to  keep 
a  drift  in  the  white  clay  open  for  any  length  of  time,  because  the 
irresistible  pressure  of  the  semifluid  clay  breaks  the  timbers  and  a 
cave-in  results.  It  is  said  thai  the  life  of  timbers  in  the  white  clay 
is  but  three  weeks.  Consequently  the  permanent  drifts  for  tram 
ways  arc   maintained   in  the  yellow    clay  just    outside  the  white  clay. 
The  main  tunnel  enter-  normal  to  the  mountain  with  a  grade  of 
1  in  12,  and  at  the  outer  (^\i:c  of  the  clay  lateral  drifts  branch  off  in 
both  directions.  As  these  are  to  be  permanent  drifts  for  tramways 
the\  are  driven  in  the  yellow  clay  outside  the  white.  From  these 
lateral  drift-  crosscuts  or  working  entries  are  driven  at  intervals  of 
<i()   feet    through   the  clay   to  the  wall   rock. 
The  Philadelphia  Company's  method  of  mining-  is  different  from 
that  in  use  in  the  other  [nines  visited,  because  the  deposits  are 
peculiarly  situated.  They  outcrop  high  up  on  the  steep  slopes  of 
the  mountain  am!  stand  nearly  vertical,  so  that  they  can  be  mined 
like  veins,  by  stoping  up  along  the  bedding  from  a  tunnel  b< 
neath.  Stoping  is  begun  at  the  farther  end  of  one  of  these  entries 
for  about  30  feet  on  either  side  until  a  large  room  is  opened,  when 
the  props  and  pillars  are  removed  and  the  clay  soon  crumbles  or 
flows  in.  It  usually  continues  to  flow  in  until  the  clay  overhead  is 
exhausted  and  the  wash  from  the  surface,  several  hundred  feet 
above,  comes  down,  but  sometimes  further  stoping  is  necessary. 
About  1,000  mine  carloads  are  obtained  from  a  single  stope  in  this 
way,  and  as  high  as  4,000  are  reported. 
AVhen  the  stope  is  exhausted  the  drift  is  robbed  back  and  another 
stope  is  opened  and  worked  in  the  same  way,  and  this  is  continued 
