MAGNETITE    DEPOSITS   OF    THE    CORNWALL 
TYPE    IN    BERKS   AND    LEBANON 
COUNTIES,    PA. 
By  Arthur  C.  Spencer. 
In  the  autumn  of  1906  the  writer  began  a  systematic  study  of 
those  magnetite  ores  in  eastern  Pennsylvania  that  are  associated 
with  intrusive  masses  of  diabase  at  or  near  the  edges  of  the  belt  of 
Mesozoic  rocks  which  enter  the  State  along  Delaware  River  above 
Trenton  and  continue  in  a  general  southwesterly  direction  to  the 
Maryland  line.  Observations  were  extended  over  the  eastern  por- 
tion of  this  belt  in  Bucks,  Montgomery,  Berks,  Lebanon,  and  Lan- 
caster counties,  Pa.,  with  the  special  view  of  acquiring  personal 
knowledge  of  the  stratigraphy  and  structure  of  the  Mesozoic  sedi- 
mentary rocks  and  their  relations  to  the  igneous  masses  inclosed  by 
them.  It  was  rightly  expected  that  this  general  study  would  con- 
tribute to  a  better  understanding  of  the  geologic  relations  of  the 
known  ore  deposits  and  that  it  might  reveal  data  which  would  show 
where  practical  exploration  might  be  undertaken  for  ore  bodies  that 
do  not  appear  at  the  surface. 
To  complete  the  investigation  as  planned,  certain  parts  of  the 
region  already  covered  must  be  studied  in  greater  detail,  and  obser- 
vations must  be  carried  across  western  Lancaster,  Dauphin,  York, 
and  Adams  counties,  Pa.,  and  thence  along  the  continuation  of  the 
Newark  belt  into  Maryland.  It  is  hoped  that  this  work  may  be 
done  early  in  the  season  of  1907. 
In  this  preliminary  report  no  attempt  is  made  to  set  forth  in 
detail  the  scientific  basis  for  the  conclusions  that  have  been  reached 
concerning  the  origin  of  the  deposits  of  magnetite  ore,  the  present 
object  being  to  present  practical  suggestions  that  seem  warranted 
by  the  investigation  in  its  present  state  of  progress.  Though  full 
consideration  of  the  origin  of  the  magnetites  may  wait  until  the 
entire  field  has  been  studied,  the  writer's  general  conclusions  on  this 
point  are  given,  since  the  manner  and  conditions  of  ore  formation 
must  form  a  starting  point  for  all  practical  suggestions  looking  to 
the  discovery  of  deposits  as  yet  unknown. 
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