186  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
Magnetite  ores  of  the  same  general  type  have  been  worked  on   , 
considerable  scale  in  Berks  County  at  Boyertown;  at  two  localities 
south  of  Reading;  near  Fritztown,  about  7  miles  southwest  of  Read- 
ing,  and  near  Joanna  station,  2\  miles  northeast  of  Morgantown;  i 
Lebanon  County  at  the  Cornwall  ore  banks,  and  in  York  County  a 
various  points,  but  especially  near  Dillsburg.     Of  these  deposits,  onl 
one,  that  at  Cornwall,  is  now  worked  in  a  large  way,  though  th 
several  former  interests  at  Boyertown  have  recently  been  acquire' 
by  the  Boyertown  Ore  Company,  and  it  is  expected  that  these  mine 
at  an  early  date  will  again  soon  be  in  operation.     Mining  in  a  sniaj 
way  is  in  progress  at  the  Wheatfield  group  near  Fritztown,  for  th 
purpose   of   procuring   surface    ore.     The    Island    and    Raudcnbusc 
mines,  near   Reading,   have  been  abandoned,  the  former  for  nearl  j 
twenty   years   and   the   latter  for   a   longer   time.     All   the   deposf 
named  are  situated  at  the  northern  edge  of  the  Mesozoic  Newark  b 
and  the  ore  at  all  places  appears  to  lie  in  limestones  or  limy  shaj 
that  belong  to  the  Paleozoic  scries  rather  than  in  strata  of  the  Mesoz 
system   itself.      The  Jones  mine,   near  Joanna   station,  is  situated 
the  southern  edge  of  the  Mesoxoie  belt  and  the  deposit  is  in  Paleozoi 
strata  capped  by  Mesozoic  beds. 
The  above  statement  concerning  the  stratigraphic  position  of  th 
deposits  is  practically  in  accord  with  the  opinion  expressed  by  Lesle 
and  dlnvilliers  in  their  very  clear  description  of  the  Cornwall  mine. 
All  authorities  seem  to  agree  that  in  York  County  the  ores  are  cor 
tained  in  Mesozoic  strata,  and  it  may  he  remarked  that  in  at  leas 
three  places  in  Berks  County  minor  deposits  of  magnetite  have  bee: 
noted  at  the  contact  of  invading  masses  of  diabase  with  sandstone 
and  shales. 
A  natural  deduction  to  he  drawn  from  the  conclusion  that  all  tl 
known  large  ore  deposits  of  the  Cornwall  type  are  associated  wi 
Paleozoic  limestones  or  limy  shales  is  that  exploration  for  oth 
deposits  of  ;i  similar  nature  is  most  likely  to  he  rewarded  if  confined 
to  localities  where  similar  rocks  occur.  Though  this  is  a  practicr 
point  taken  by  itself,  it  i>  so  indefinite  as  to  be  of  no  great  value  t 
the  prospector,  for  Paleozoic  limestones  are  of  wide  occurrence  hot 
north  and  south  of  the  Newark  belt.  Some  other  criterion  is  there* 
fore  required,  and  such,  it  is  believed,  is  found  in  the  fact  that  intru 
sive  diabase  at  all  places  lies  close  to  and  at  most  places  in  actuj 
contact  with  ore  deposits  of  the  Cornwall  type.  This  fact  is  brougl 
out  by  every  published  description  of  the  magnetite  deposits,  an 
the  present  investigation  has  served  to  emphasize  both  the  size  ( 
these  igneous  masses  and  the  importance  of  their  relation  to  oi 
deposits. 
It  is  the  writer's  view  that  the  intrusive  masses  of  diabase  hav> 
been  active  agents  in  segregating  the  iron  contained  in  the  variou 
a  Ann.  Rept.  Second  Geol.  Survey  Pennsylvania,  1885,  pp.  491-570. 
