188         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    I. 
Diabase  intrusions  are  characteristic  of  the  Mesozoic  Newark  system 
throughout  practically  its  whole  extent,  from  Nova  Scotia  to  Rich! 
mond,  Ya.,  and  occur  at  but  few  places  outside  of  this  Mesozoic  belli 
Where  they  do  so  occur  they  are  found  mainly  at  no  great  distance 
from  the  edge  of  the  Mesozoic  belt,  and  few  of  them  are  masses  of 
considerable  size. 
At  most  places  within  the  Mesozoic  area  the  diabase  occurs  as 
extensive  and  relatively  thin  sills  between  strata  of  sandstone  and 
shale.  Local  cross-cutting  may  be  observed,  and  in  certain  place! 
the  invading  rock,  though  following  a  group  of  strata,  cuts  hack  and 
forth  across  the  beds  of  the  group.  It  is  believed  that  when  masses 
of  essentially  like  size  are  compared  intrusions  of  this  sort  oiler  con- 
ditions that  arc  distinctly  less  favorable  to  intense  metamorphisii 
than  those  presented  by  extensively  cross-cutting  bodies.  A  deep- 
seated  source  for  the  molten  rock  must  be  admitted,  and  in  order  to 
reach  the  position  it  now  occupies  the  diabase  must  have  cut  througl 
the  rocks  that  form  the  basement  of  the  Mesozoic  system  and  across 
the  lower  si  rat  a  of  the  latter  as  well.  Lateral  spreading  of  t  he  invad- 
ing  magma  could  have  occurred  only  at  relatively  shallow  depths, 
where  the  weight  of  overlying  strata  would  be  not  too  great  to  he 
lifted  by  the  pressure  act ing  upon  t he  molten  rock.  Masses  of  molten 
rock  injected  between  the  strata  to  considerable  distances,  especially 
if  the  strata  lie  at  low  angles,  become  isolated  from  the  main  source' 
of  intrusion.  It  is  evident,  therefore,  that  the  degree  of  alteration  of; 
the  inclosing  rocks  depends  wholly  on  the  amount  of  energy  residinl 
in  the  heated  mass  at  the  time  of  its  injection.  This  would  of  neces- 
sity he  a  limited  source  of  met  amorphisin.  On  the  other  hand,  the 
more  steeply  inclined  fissur.es  through  which  the  sills  are  supposed  to 
have  been  \'^\  would  afford  direct  connection  with  the  original  source 
of  energy,  so  that  transfer  of  heat  and  the  movement  of  mineralizing 
waters  would  here  he  continued  through  a  much  longer  period. 
Undoubtedly  different  masses  of  diabase  within  the  same  general 
held  may  have  differed  greatly  in  respect  t<>  efficiency  in  producing 
me tamorphism, but, other  things  being  equal,  the  amount  of  variation 
in  the  direct  nc--  of  their  paths  from  the  original  reservoir  must  have 
been  a  very  important  factor  in  determining  the  relative  amount  of 
alteration  which  different  masses  could  have  produced.  The  forego- 
ing considerations  indicate  certain  practical  suggestions  to  searchers 
I'm-  iron  ores  in  this  field: 
(1)  Ore  bodies  should  he  sought  only  at  or  near  the  walls  of  masses 
of  diabase. 
(2)  Large  masses  of  diabase  are  more  favorable  for  ore  deposits 
1  han  smaller  masses. 
(3)  Cross-cutting  intrusions  and  highly  inclined  sills  are  more 
favorable  than  sills  of  low  inclination. 
