MAGNETITE    IN    BERKS    AND    LEBANON    COUNTIES,  PA.  187 
deposits;  or,  in  other  words,  that  the  masses  of  ore  are  products  of 
contact  metamorphism.  Briefly  stated,  the  theory  of  their  origin, 
which  seems  to  be  indicated  by  all  the  facts  that  have  been  noted 
concerning  their  geological  relations,  is  that  the  magnetite  ore  bodies 
of  the  Cornwall  type  have  been  formed  by  the  more  or  less  complete 
metasomatic  replacement  of  sedimentary  rocks,  mainly  limestones 
and  limy  shales,  by  iron  minerals  precipitated  from  percolating  heated 
solutions  set  into  circulation  by  invading  diabase. 
Diabase  is  associated  with  each  of  the  known  ore  bodies,  but  the 
counter  proposition  that  each  mass  of  diabase  should  furnish  one  or 
more  ore  deposits  is  neither  warranted  by  the  observed  facts  nor 
required  by  the  theory  here  proposed.  As  a  matter  of  observation, 
it  is  to  be  noted  that  in  Lebanon,  Lancaster,  Berks,  and  Montgomery 
counties  no  industrially  important  magnetite  deposits  have  been  dis- 
covered within  the  Mesozoic  area.  Such  properties  as  the  Esterly 
mine,  situated  4  miles  northwest  of  Birdsboro;  the  Wren  mine,  1 
mile  southwest  of  Boyertown;  and  certain  prospects  near  the  north- 
west fork  of  Perkiomen  Creek,  about  5  miles  northeast  of  Boyertown, 
exhibit  relatively  small  masses  of  magnetite,  more  or  less  mixed  with 
pyrite,  segregated  at  the  contact  between  greatly  baked  shales  and 
intrusive  diabase.  At  two  of  these  places  the  dip  of  the  contact  is 
greater  than  45°,  while  at  the  third  place — the  locality  northwest  of 
Boyertown — the  dip  of  the  contact  may  be  as  low  as  20°,  and  the 
deposit  here  is  smaller  than  the  deposits  at  the  other  places.  Here 
it  is  notable  that  the  diabase  mass  next  to  which  the  ore  occurs  is 
one  of  the  larger  masses  of  the  region  and  one  around  which  there  is 
more  than  the  usual  amount  of  alteration  in  the  invaded  sandstones. 
On  comparing  these  unimportant  deposits  with  those  in  the  more 
or  less  extensively  productive  mines  where  Paleozoic  strata  are 
involved,  it  must  be  concluded  that  the  sandstones,  conglomerates, 
and  shales  of  the  Mesozoic  system  have  been  very  much  less  suscep- 
tible to  replacement  by  iron-bearing  solutions  than  the  calcareous 
strata,  mainly  limestones  and  limy  shales,  of  the  older  formations. 
The  unequal  effects  of  intrusive  masses  in  causing  the  metamorphism 
of  siliceous  and  calcareous  rocks  invaded  by  them,  and  especially  the 
greater  frequency  of  metalliferous  segregations  of  contact  origin  in 
limestones  than  in  shales  and  sandstones,  are  facts  of  common  observa- 
tion' in  many  fields.  Large  ore  bodies  composed  of  magnetite, 
pyrite,  and  chalcopyrite  that  were  evidently  formed  by  the  replace- 
ment of  limestone  at  or  near  igneous  contacts  are  worked  at  several 
places  in  California,  British  Columbia,  and  Alaska  for  the  copper 
which  they  contain,  and  throughout  this  region  the  presence  of  lime- 
stone seems  to  be  essential  for  the  existence  of  a  valuable  ore  deposit, 
contacts  of  intrusive  rocks  with  other  sediments  being  far  less  favor- 
able for  the  occurrence  of  ore  bodies. 
